Lack of functional alpha-lactalbumin prevents involution in Cape fur seals and identifies the protein as an apoptotic milk factor in mammary gland involution
© Sharp et al; licensee BioMed Central Ltd. 2008
Received: 27 August 2008
Accepted: 06 November 2008
Published: 06 November 2008
The mammary gland undergoes a sophisticated programme of developmental changes during pregnancy/lactation. However, little is known about processes involving initiation of apoptosis at involution following weaning. We used fur seals as models to study the molecular process of involution as these animals display a unique mammary gland phenotype. Fur seals have long lactation periods whereby mothers cycle between secreting copious quantities of milk for 2 to 3 days suckling pups on land, with trips to sea alone to forage for up to 23 days during which time mammary glands remain active without initiating apoptosis/involution.
We show the molecular basis by which alpha-lactalbumin (LALBA), a secreted milk protein, is absent in Cape fur seals and demonstrate an apoptotic function for LALBA when exposed to mammary cells.
We propose that apoptosis does not occur in fur seal mammary glands due to lack of LALBA in fur seal milk, allowing evasion of involution during a foraging trip. Our work identifies LALBA as a milk factor that feeds back on the mammary gland to regulate involution.
The mammary gland represents one of the most dramatic examples of physiological development. The massive changes of form and function of mammary glands over the life span of a female are characterized by extreme changes in cell proliferation, differentiation, secretion and death, which accompanies pregnancy, lactation and involution upon weaning. While milk is sucked from the mammary gland it provides nutrition and immunity to the young. However, upon milk stasis, due to absence of sucking at weaning, the mammary gland regresses and is remodelled by a process known as involution, which cleanses the gland and returns it to a virgin-like state. Although the mammary gland appears vastly regulated it is also highly susceptible to cancer, with mortality associated with breast cancer rating amongst the highest causes of death for women in the western world.
The study of apoptosis in the mammary gland during involution is important for understanding both the normal biology of post-natal regression and the events leading to mammary gland tumorigenesis. Interestingly, some mammals have modified their lactation cycle in order to accommodate and adapt to extreme environmental pressures. Animals such as otariid seals (fur seals and sea lions) exhibit an unusual lactation phenotype  which differs from other members of the Pinnipedia family and other mammals. These animals display resistance to mammary gland apoptosis and involution after cessation of sucking, and provide a unique opportunity to investigate aspects of mammary gland physiology that are present but not readily apparent in other species.
The three families of Pinnipeds, comprising Phocids (true seals), Odobenids (walrus), and Otariids (sea lions, fur seals) evolved from a carnivorous ancestor around 25 million years ago and diverged during the middle Miocene (10 million years ago) . Each family adopted different approaches to lactation. Phocid seals evolved large sizes to reduce heat loss and risk of predation and increase body reserves. This enabled them to adopt a 'fasting strategy' of lactation  whereby amassed body reserves of stored nutrients facilitate fasting on land during continuous milk production over relatively short periods (4 to 42 days, depending on the species).
In contrast, ancestral otariid seals retained smaller body sizes and insulating fur, and bred at rockeries to gain proximity to local prey resources adopting a 'foraging lactation' strategy . The small size of otariid seals made it necessary to feed during lactation in order to replenished body stores required to continue milk production. Reduced prey availability led to lengthening of the lactation period (4 to > 12 months) and otariid seals began exploiting resources farther off shore, increasing the duration but reducing frequency of foraging trips during lactation. The current-day otariid seals produce milk with no detectable lactose [4, 5] and have adopted a lactation strategy which is characterized by alternation between periods of several days of copious milk production on shore and extended periods of maternal foraging at sea . Intersuckling intervals have been recorded in otariid seals of up to 23 days and are among the longest ever recorded for a mammal . The need to increase duration of foraging trips due to distant foraging grounds during lactation has selected for an adapted otariid mammary gland, which remains functional despite sustained interruptions in suckling activity.
For other mammals, accumulation of milk in the mammary gland due to cessation of sucking by the young allows factors present within milk to regulate mammary epithelium, causing downregulation of milk protein gene expression, followed by involution via apoptotic cell loss . During long periods at sea, in the absence of sucking, fur seal mammary glands have been recorded to produce 80% less milk than when lactating on land , and milk protein gene expression decreases , aspects which are common with cessation of sucking in other mammalian species and characteristic of the initiation of involution . In other mammals these events are rapidly followed by involution comprising apoptotic mammary gland cell death , but the fur seal mammary gland does not undergo involution at this time  and remains active in readiness for return to land to continue nursing the young. The fur seal mammary gland evades the effects of engorgement by downregulating milk protein production; however, milk still remains in the gland. Therefore these animals display resistance to mammary gland apoptosis and involution following cessation of suckling and provide a unique opportunity to investigate aspects of mammary gland physiology associated with involution. There are many examples of genetic rearrangements that show strong associations with specific phenotypes that have important evolutionary consequences (see  for examples). In many cases a gene required for the development of a specific trait in one species shows a difference in expression in other species that correlates with a different trait . These relationships provide causal evidence of plausibility.
Here we show the alpha-lactalbumin gene (LALBA), which normally encodes a milk protein involved in lactose synthesis, has undergone a number of cis-acting mutations, which render the protein absent in otariids. We show that LALBA causes apoptosis of mouse and human mammary epithelial cell lines and fur seal primary mammary cells. We propose that lactose synthesis and mammary gland involution does not normally occur in fur seal mammary glands due to absence of LALBA in fur seal milk and show that previous predictions based solely on isolation of cDNA in otariid mammary glands  are incorrect. We discuss and present functional evidence that directly relates to the different lactation phenotypes of otariid and phocid seals.
LALBAgene expression in the fur seal mammary gland
Affymetrix canine arrays were used to examine the pattern of gene expression in mammary tissue from Cape fur seals (Arctocephalus pusillus) during pregnancy, on-shore lactation and off-shore lactation. These results were compared with microarray analysis patterns of genes expressed in human (Homo sapiens), cow (Bos taurus) and mouse (Mus musculus) mammary gland during pregnancy and lactation. Using this approach candidate apoptosis genes were examined for significant differences in expression, and unusually low levels of a major milk protein gene, LALBA, were observed in mammary glands of lactating Cape fur seals.
Further analysis was performed by RT-PCR using RNA extracted from lactating mammary gland tissue from pig, mouse and wallabies using LALBA primers specific for each species. The wallaby was included in this analysis due to the absence of lactose in wallaby milk during peak lactation. Compared with pigs, mice and wallabies, Cape fur seals were found to have a 10 to 100-fold reduction in LALBA expression during lactation (Figure 1C).
In order to determine the abundance of LALBA transcripts relative to all other transcripts within the lactating fur seal mammary gland we undertook a large-scale global transcriptome analysis. 11,232 EST clones from a Cape fur seal lactating mammary gland were sequenced and annotated. A single LALBA gene transcript failed to be detected, whereas mRNAs representing other milk protein genes κ-casein, β-casein, casein αS1-casein and αS2-casein were detected at high levels (Figure 1D).
Comparative promoter analysis
Interspecies comparison of LALBA TATA box sequences
TATA box *
Cape fur seal ‡
Sea lion ‡
1.2 – 1.5%
Antarctic fur seal‡
1.2 – 1.5%
60 – 75%
60 – 75%
60 – 75%
60 – 75%
100 – 172%
60 – 75%
60 – 75%
60 – 75%
60 – 75%
60 – 75%
60 – 75%
60 – 75%
60 – 75%
60 – 75%
60 – 75%
60 – 75%
In vitroexpression analysis
A direct comparison between otariid and phocid LALBA gene expression levels was assayed by use of reporter gene constructs. This was performed in order to determine if low levels of Cape fur seal LALBA transcript detection were due to low levels of LALBA gene transcription or low LALBA transcript stability.
Use of different host cell lines eliminated the probability that the observed expression pattern was not cell-line specific. These results showed that the low level of Cape fur seal LALBA expression observed occurred at the level of transcription and was not due to low LALBA transcript stability. These results also showed that the Cape fur seal LALBA TATA mutation alone is not responsible for the low level of expression observed (Figure 3A to 3C).
Identification of LALBA transcripts
Otariid LALBAtranscripts are not translated into protein
Apoptotic effects of LALBA
Characterization of apoptotic effects
Specific triggers for the process of mammary involution are unknown but autocrine feedback mechanisms have been proposed . The source of this mechanism is thought to be factors in milk that interact with the mammary epithelium and trigger involution after a period of milk stasis. It was therefore proposed that fur seals, which exhibit a phenotype where involution is delayed even after very long periods of milk stasis, may lack these factors in their milk, or may lack the machinery to initiate the involution response.
It has long been established that LALBA, a major milk component, plays a central role in the mammary gland as the regulatory subunit of lactose synthase . We show here that otariid (Cape fur seal) mammary glands express very low levels of LALBA mRNA during the lactation cycle compared with expression levels in other mammals, which show LALBA is one of the most highly expressed genes in the mammary gland during lactation . LALBA has also recently been implicated in the induction of apoptosis of a human colon adenocarcinoma cell line  and RAW264.7 cells . LALBA was therefore a likely candidate for further study in the involution process.
Comparison of LALBA expression levels between otariid and phocid seals showed otariid (Cape fur seal) LALBA expression levels were very low and suggested a negligible amount of transcription occurs from this promoter. The low level of otariid LALBA transcription was predicted to be due to an altered TATA box in otariid seals. All three otariid species displayed a T-G transversion within the third position of the TATA box creating an AAGAAA sequence. This substitution is predicted to lead to poor binding of the TATA binding protein and RNA polymerase which are necessary for transcriptional activity . The consequence of a G to T substitution in the third position has previously been demonstrated, showing transcriptional activity is reduced to 2% . Correction of this mutation in the Cape fur seal LALBA promoter failed to increase levels of transcription, suggesting that another/other mutations also play a role in preventing successful transcription of LALBA. Like most pseudogenes where evolutionary pressures are no longer required to prevent divergence, the otariid LALBA has likely undergone a number of promoter mutations, which are conserved in other lineages where LALBA is functional.
Analysis of LALBA transcript abundance showed one major transcript, LALBA(s) was generated for all otariid species, with LALBA(l) and LALAB(Δ) detected as minor transcripts. Interestingly, LALAB(Δ) was also detected in phocid species. Other species such as dogs, humans, sheep, rats, mice and pigs also have the same cryptic 5' splice sequence in exon 1, suggesting the presence of LALAB(Δ) in species other than seals.
Translation of otariid and phocid LALBA transcripts showed otariid LALBA mRNA failed to be translated into secreted protein. These data are consistent with earlier studies, which have failed to detect LALBA protein in milk of otariid species [26, 27]. These data suggest the predicted translation of two isoforms of truncated LALBA protein (Reich and Arnould ) are incorrect. The authors suggest this is the mechanism for lack of lactose in fur seal milk and that lack of lactose as a major milk osmole has facilitated the fur seal lactation strategy and prevented engorgement while at sea. Contrary to this, it has been postulated elsewhere that other osmoles are present in otariid milk, such as Fuc(α1–2)Gal(β1–4)Glc, which is also found in related species of the order Carnivora . These trisaccharides may provide an alternative osmotic mechanism to move water from the extracellular fluid into the milk. All these species have only small amounts or no lactose relative to oligosaccharides. Myo-inositol and free amino acids are also found at high concentrations in otariid milk  and exceptionally high concentrations of taurine are found in Pinnipeds . Therefore, it is likely myo-inositol and taurine may play a significant role as organic osmolites. There are other examples in nature where lactose is not required for milk production. In tammar wallaby (Macropus eugenii) milk, carbohydrate is low and lactose is absent throughout peak lactation , during which time other unknown factors act as the major osmole, demonstrating that lactose is not necessary for milk production in some species. The high level of milk production by fur seals lactating on-shore ultimately requires the presence of an osmole, be it trisaccharides or organic osmolites, while the absence of engorgement in fur seal mammary glands while at sea is likely the result of reduced milk production by downregulation of milk synthesis at the transcriptional level .
Evidence presented here shows that bLALBA causes apoptosis of mouse and human mammary epithelial cell lines and fur seal primary mammary cells, demonstrating that although LALBA is absent in fur seal milk, the LALBA-mediated apoptotic pathway is still intact in fur seal mammary cells. It is interesting to note that all cell types in the Cape fur seal mammary population responded to bLALBA, suggesting that the apoptotic response to LALBA is not limited to epithelial cells, but also affects other cells types such as fibroblast and myoepithelial cells. The loss of LALBA apoptotic activity by heat treatment confirms the active component is of protein origin and suggests the apoptotic effects observed do not occur when the protein is fragmented by digestion with pepsin. Previously a multimeric form of LALBA (MAL), isolated from the casein fraction of human milk and formed by low pH treatment and ion-exchange, has been shown to reduce cell viability in kidney, intestine, bladder, lung cell lines, lymphocytes and thymocytes, but healthy cells were not affected  and mammary epithelial cells were not tested. HAMLET (human α-lactalbumin made lethal to tumour cells) , which is chemically treated to resemble MAL in structure, is a complex of apo-LALBA combined with oleic acid. The oleic acid co-factor binding of HAMLET is very unstable and is easily displaced by foetal calf serum (FCS). All apoptosis-inducing experiments involving HAMLET need to be performed in the absence of FCS, unlike the experiments presented here, which show LALBA inducing apoptosis in the presence of 10% FCS, suggesting that the LALBA in the current study is different in structure to HAMLET. HAMLET has been shown to localize to the nucleus where it binds to histones, disrupts chromatin structure and leads to cell death . It has also been shown that negatively charged untreated LALBA can also bind to positively charged histones without the aid of oleic acid, causing aggregation  and suggesting that other natural forms of LALBA can induce apoptosis as observed in the current study. Indeed, untreated bLALBA has previously been found to have an apoptotic effect on cell types other than mammary cells. LALBA has previously been shown to induce apoptosis in human colon adenocarcinoma cell lines  and RAW264.7 cells  using the same bLALBA preparations from Sigma . Analysis in RAW2.64.7 cells showed cells exposed to bLALBA exhibited cell shrinkage, disruption of cellular membranes, accumulation of apoptotic bodies, DNA fragmentation, an increase in sub-G1 cells, increased Annexin V expression and activation of caspase 3 . These results are characteristic of the apoptotic process and show that bLALBA indeed induces a programmed cell death response via apoptosis. Similarly, we have also observed cell shrinkage, disruption of cellular membranes, accumulation of apoptotic bodies and DNA fragmentation in this study using mammary cells following exposure to bLALBA. We have also observed an increase in caspase 3 expression following exposure of HC11 cells to bLALBA for 5 hours (Brennan et al, manuscript in preparation), suggesting the same apoptotic pathway is activated in mammary cells as in RAW264.6 cells. In the current study we expressed phocid LALBA in HC11 cells without observing apoptosis of the host cell. Although expression of the transgene was observed to be high due to the presence of a CMV promoter, the amount of protein collected in the conditioned medium was determined to be less than that required for effective LALBA-induced apoptosis. In addition, stably transfected cells have been derived which express bLALBA which are also not detrimentally affected ; we suggest that these cell lines are also not capable of producing the high concentration of LALBA (0.2 to 1.6 mg/ml) necessary for induction of apoptosis.
Our data suggest that low levels of LALBA may not cause apoptosis in the in vivo epithelium. It could be postulated that LALBA acts only at a critical concentration and requires a specific amount of time in contact with the cell, or undergoes a conformational change during milk stasis in order to elicit its apoptotic potential. Involution only occurs following milk stasis and it has been previously demonstrated that LALBA concentrations increase in milk during mammary gland involution, while other milk proteins show decreases in concentration . LALBA may cause limited apoptosis in the in vivo epithelium during lactation, as seen by decreased milk production in cows as lactation proceeds . This gradual reduction in milk production over the lactation period has been referred to as 'gradual involution'. In fur seals milk production does not decline as in other mammals, but has been shown to increase as the pup grows larger. This is presumed to sustain the nutritional needs of a growing pup. The lack of LALBA in fur seals may not only allow these mammals to circumvent involution, but may also aid in avoiding gradual involution observed in other lactating mammals.
We suggest that exposure of mammary epithelial cells to LALBA may be a mechanism for mammary gland involution during milk stasis at weaning, a time when LALBA levels increase in the milk. To support this we have presented a molecular analysis of a relevant lactation model, the fur seal, which avoids involution in the presence of milk stasis and showed that these animals do not produce LALBA in their milk. In addition we have shown that mammary epithelial cells, when exposed to similar LALBA concentrations to those found in milk, undergo an apoptotic response.
We propose that mammary gland involution does not normally occur in fur seal mammary glands due to absence of LALBA in fur seal milk and not due to the presence of a truncated LALBA protein as previously reported . The absence of LALBA has therefore facilitated the divergence of the fur seal species during evolution and enabled this species to adopt a unique lactation phenotype to exploit their environment. This hypothesis is supported by previous observations of LALBA-deficient mice . Milk of LALBA-deficient mice  is high in lipid and protein but very low in lactose; however, mothers are not capable of feeding their young as milk is highly viscous and cannot be removed by normal suckling. Mice, unlike fur seals, rely on lactose as their major osmole and therefore milk in this mouse was devoid of water, making sucking-induced removal of milk from the mammary gland impossible. After 5 days of lactation, alveoli of LALBA-deficient mouse mammary glands were distended and so engorged with milk that tight junctions were compromised and milk was observed to move from the alveolar space and enter into the surrounding mammary tissue. However, there was no evidence of apoptosis or involution which would normally occur by this time in the absence of milk removal . The absence of apoptosis and involution in the LALBA-deficient mouse is consistent with absence of apoptosis and involution in the LALBA-deficient otariid seal.
The tammar wallaby is another model species with an unusual LALBA phenotype. As discussed previously, wallaby milk during peak lactation is devoid of lactose; however, LALBA is secreted in wallaby milk at similar levels to other mammals throughout lactation . Examination of expression levels of wallaby LALBA in the current study showed similar levels to other mammals such as pig and mice during peak lactation. This shows that wallabies have uncoupled secretion of LALBA and lactose production. In contrast to fur seals, wallabies do not have an interrupted pattern of lactation and do not have long periods of non-suckling activity, so we predict that continued secretion of LALBA during wallaby lactation is required solely for rapid initiation of mammary gland involution at weaning.
Here we show that LALBA acts an as apoptotic inducer of mammary epithelial cells and we correlate the lack of LALBA with the absence of apoptosis in fur seal mammary glands. This study has shown how lack of LALBA has had important evolutionary consequences leading to the phenotypic trait of foraging lactation in fur seals that is different from the fasting lactation in phocid seals, and by doing so identifies LALBA as a likely candidate trigger for involution in the mammary gland.
RNA was isolated (QIAGEN RNeasy Lipid Kit, Australia) from Cape fur seal, wallaby, pig and mouse mammary tissue or from cells derived from California sea lion and Antarctic fur seal milk during peak lactation, human milk and colostrum at 24 hours prior to birth and at 3 and 4 months peak lactation, or HEK293T cells. mRNA purification was performed by NucleoSpin kit (BD Biosciences).
RNA isolated from Cape fur seal mammary glands during pregnancy (n = 2), on-shore lactation (n = 2) and offshore foraging (n = 1) or human during pregnancy (n = 2) and lactation (n = 2) was used for gene profiling analysis. Labelling of RNA and microarray processing was contracted to the Australian Genome Research Facility (Melbourne, Australia) using Canine genome 2.0 or Human genome Affymetrix genechips. Cow Affymetrix results were kindly provided by Dr PA Sheehy, Faculty of Veterinary Science, University of Sydney, NSW, Australia. Data was normalized using RMA methods in Bioconductor . Mouse Affymetrix results were obtained from http://breast-cancer-research.com/content/6/2/R92.
Off-shore Cape fur seal EST library
mRNA (1 μg) was used to construct a cDNA library using Creator SMART cDNA Library Construction Kit (BD Biosciences). A total of 11,232 ESTs were sequenced by the Australian Genome Research Facility, Brisbane, Australia.
Total RNA (0.1 μg) was used to create cDNA. 0.2 μl cDNA was used in PCR with 20, 25 and 30 cycles of amplification to determine the linear range. Each PCR was performed at least twice. See additional file 1: supplementary Table 1 for primer sequences. Images of PCR products resolved in ethidium bromide-stained agarose gels were visualized using UVP BIODoc-it System (Pathtech, Australia) and saved by PCTV Vision software. Quantification was performed by densitometry using ImageJ software (NIH). Expression levels were estimated by normalizing expression against GAPDH.
mRNA (1 μg) was electrophoresed in 1.2% agarose gels, blotted to Zeta Probe and hybridized with a 32P-labelled Cape fur seal LALBA cDNA probe (550 bp).
Genomic LALBA expression constructs
DNA was extracted from Cape fur seal liver and California sea lion and Antarctic fur seal milk. DNA from ringed seal, grey seal and harbour seal were gifts from Dr P Johnson (Ohio University, USA), Dr M Walton (University of St. Andrews, Scotland) and Dr N Lehman (Portland State University, USA), respectively. LALBA coding sequences were amplified using specific primers spanning the ATG of exon 1 and 3' UTR (Additional file 1: supplementary Table 1). Amplified products were cloned into pTarget (Promega) and sequenced. Fragments in incorrect orientation were digested using NotI and XhoI restriction sites flanking the LALBA fragment and were cloned into pcDNA3.1 (Promega) and sequenced. Both vectors contain CMV promoters and all constructs contained a Kozak sequence A/GTGATTATGA. Cape fur seal, Antarctic fur seal, and harbour seal LALBA were cloned into pTarget, while ringed seal and sea lion DNA were cloned into pcDNA 3.1.
LALBA 5' promoter sequences were amplified using primers corresponding to -770 bp to -750 bp and -12 bp to +3 bp containing an NcoI restriction site. Amplified DNA was cloned into pGemt-easy (Promega) and sequenced. Mutagenesis of TATA box and STAT5 sites was performed by PCR and sequenced. Plasmids were digested with NcoI/SacI using sites within the vector and 718 bp and 398 bp fragments were subcloned into pGL3-basic and pGL3-enhancer respectively and sequenced. See additional file 1: supplementary Table 1 for primers.
Preparation of Cape fur seal mammary cells
Mammary tissue was obtained from one Cape fur seal, which was confirmed to be pregnant and non-lactating from inspection of the uterus and mammary glands, respectively. Tissue was immediately transferred to 1× Hanks' Balanced Salt Solution (HBSS) (Sigma Aldridge, Sydney, Australia) with 10 μl/ml penicillin/streptomycin (Gibco, USA) and 2.5 μg/ml Fungizone (Gibco, USA) on ice and transported back to the laboratory for enzymatic digestion to harvest mammary epithelial cells. Mammary tissue was dissected free from fat, weighed, sliced finely and digested with collagenase and hyaluronidase (25 g tissue per 100 ml media) at 37°C for 4 hours. Cells were harvested by filtration (Nalgene filter, 53 μm and 200 μm mesh). The suspension was centrifuged 80 g/5 minutes, pellets were washed twice with HBSS. Cell suspensions were centrifuged, resuspended in M199 and 90% FCS/10% DMSO (DMSO-Sigma, Sydney, Australia), and frozen at a density of ~2 × 107 cells/ml.
Cape fur seal mammary cells were cultured in either 25 or 80 cm2 culture flasks in 5 ml or 10 ml respectively of M199/Hams/Hepes media with 1 μg/ml cortisol, 10 ng/ml EGF, 1 μg/ml insulin supplemented with 20% horse serum and 5% foetal bovine serum. The cells were passaged (up to three times) when confluent using 0.1% trypsin-versine solution in phosphate buffered saline (Sigma-Aldrich, Sydney, Australia).
HEK293T and MCF-7 were grown in DMEM/10% FCS, HC11 cells were grown in M199/HAMS/HEPES/10% FCS, CfsMCs were grown in M199/HAMS/HEPES/20% horse serum/5% FCS with 1 μg/ml cortisol, 10 ng/ml EGF, 1 μg/ml insulin (I) and BME-UV1 cells were grown in 50% DMEM/30% RMPI/20% NCTC-135/5% FCS with 1.0% lactose, 0.1% lactalbumin hydrolysate, 1.2 mM glutathione, 5 μg transferrin/ml, 10 μg L-ascorbic acid/ml. For induction, BME-UV1 media was supplemented with I (1 μg/ml), prolactin (P) (1 μg/ml), dexamethazone (Dex) (1 μM).
Prior to transfection BME-UV1 cells were induced for 24 hours with I or I, P, Dex. Transfection was performed using lipofectamine (Invitrogen). Reporter constructs were co-transfected with β-glycosidase vector. Induction or growth media was replaced after 6 hours and incubated for 2 days. For LALBA transcript and protein analysis media was changed to DMEM/2% FCS and incubated for 72 hours. Media was collected at 24 and 72 hours. After 72 hours RNA was isolated.
Luciferase assays were performed using Luciferase Assay reagent (Promega, USA) and β-glycosidase assays were performed according to the β-galactosidase Enzyme and Assay System with Reporter Lysis Buffer Kit (Promega, USA). Luciferase expression values were normalized by comparison with β-glycosidase activity. Each experiment was performed in duplicate and repeated at least twice.
1 ml of conditioned media (24 and 72 hours) was washed free from salts by use of Amicon ultra 1 kD columns (Millipore). Supernatants were concentrated and resuspended in 50 μl dH2O and assayed for protein by BCA assay (Pierce, Rockford, IL). 300 μg protein (~50 μl) was separated on 15% SDS-PAGE gel and transferred to PVDF membrane. Coomassie staining of the gel confirmed an equal amount of protein was loaded per sample. Immunoblotting was conducted by using rabbit polyclonal antibodies specific for LALBA, detected with a goat anti-rabbit HRP-conjugated antibody (Pierce), and visualized using SuperSignal chemiluminescent substrate (Pierce, Rockford, IL). The experiment was performed in triplicate using two time points per sample.
Proliferation assay and cell morphology
Cells were plated (2000 cells/well) in 96-well plates in growth media supplemented with 10% FCS. After one day bLALBA (SIGMA, Australia) was added and cells were grown for 2, 4, 6, 8, 10, 12 and 14 days before fixing and staining with Sulforhodamine B. Cells were visualized by phase contrast microscopy on corresponding days using an Olympus BX40 microscope and photographed using a DigitalSight DSL1 (Nikon) camera. Each experiment was performed on three separate occasions in quadruplicate. Heat treatment was performed at 95°C for 10 minutes. Pepsin digestion was performed with 1.5 mg bLALBA in HCl (pH 2) 1:500 enzyme to substrate for 60, 90 or 120 minutes/37°C. Products were purified using 1 kD Centrifugal device (Amicon Ultra). Products of digestion were analyzed on SDS-page gels.
In situ detection of apoptosis
Mouse mammary epithelial cells, HC11, (1 × 105 cells per glass chamber slide) were plated and allowed to grow overnight in growth media. The following day media was removed and cells were treated in duplicate with 1 mg/ml bLALBA in growth media for 1, 2 and 4 hours or 0 mg/ml bLALBA. Apoptotic cells were detected using Apotag Peroxidase in situ Apoptosis Detection kit (Chemicon, Australia), according to the manufacturer's instructions for staining of cultured cells. Slides were counterstained with 0.5% methyl green and mounted using aqueous mounting medium. The apoptotic cells (brown staining) were viewed and counted using an Olympus BX50 microscope for light microscopy. The apoptotic index was defined by the percentage of brown (dark) cells among the total number of cells in each sample. For each treatment five to eight random fields were used to count apoptotic and live cells. A total of 711 cells (0 mg/ml LALBA), 587 cells (1 mg/ml bLALBA/1 hour), 473 (1 mg/ml bLALBA/2 hours) and 434 cells (1 mg/ml bLALBA/4 hours) were counted for each treatment. Results are represented as percentage of apoptotic and live cells.
For expression analysis unpaired student t-tests using 6 degrees of freedom were used to determine statistical significance. For apoptotic analysis a 2-tailed student t-test was used. Each test incorporated data from two independent experiments performed in duplicate on separate days.
Research was conducted under permits from the Department of Environmental Affairs and Tourism, Republic of South Africa. We wish to thank Dr P Johnson (Department of Biomedical Sciences, Ohio University USA), Dr M Walton (Sea and Mammal Research Unit, University of St. Andrews, Scotland) and Dr N Lehman (Department of Chemistry, Portland State University, USA), Dr J Arnould (Deakin University, Burwood, Australia), Mr H Oosthuizen (Marine and Coastal Management, Roggebaai, South Africa), the Alaskan North Slope people for use of seal material and Mary Wijesinghe for cell culture work. This work was supported by grants from Geoffrey Gardiner Foundation, CRC for Co-operative Research of Innovative Dairy Products and Dairy Australia. The authors declare there is no conflict of interest.
Accession numbers: [Genbank:EU295506–EU295521]
- Bonner WN: Lactation strategies in pinnipeds: problems for a marine mammalian group. Symp Zool Soc London. 1984, 51: 253-272.Google Scholar
- Fordyce RE, (Ed): Fossil Record. 2002, San Diego, California: Academic PressGoogle Scholar
- Oftedal OT, Boness DJ, Tedmam RA: The behaviour, physiology, and anatomy of lactation in the Pinnipedia. Curr Mammal. 1987, 1: 175-245.View ArticleGoogle Scholar
- Urashima T, Arita M, Yoshida M, Nakamura T, Arai I, Saito T, Arnould JP, Kovacs KM, Lydersen C: Chemical characterisation of the oligosaccharides in hooded seal (Cystophora cristata) and Australian fur seal (Arctocephalus pusillus doriferus) milk. Comp Biochem Physiol B Biochem Mol Biol. 2001, 128: 307-323. 10.1016/S1096-4959(00)00327-4.View ArticlePubMedGoogle Scholar
- Dosako S, Taneya S, Kimura T, Ohmori T, Daikoku H, Suzuki N, Sawa J, Kano K, Katayama S: Milk of northern fur seal: composition, especially carbohydrate and protein. J Dairy Sci. 1983, 66: 2076-2083.View ArticlePubMedGoogle Scholar
- Li M, Liu X, Robinson G, Bar-Peled U, Wagner KU, Young WS, Hennighausen L, Furth PA: Mammary-derived signals activate programmed cell death during the first stage of mammary gland involution. Proc Natl Acad Sci USA. 1997, 94: 3425-3430. 10.1073/pnas.94.7.3425.PubMed CentralView ArticlePubMedGoogle Scholar
- Arnould JPY, Boyd IL: Temporal patterns of milk production in Antarctic fur seals (Arctocephalus gazella). J Zool. 1995, 237: 1-12.View ArticleGoogle Scholar
- Sharp JA, Cane KN, Lefevre C, Arnould JP, Nicholas KR: Fur seal adaptations to lactation: insights into mammary gland function. Curr Top Dev Biol. 2006, 72: 275-308. 10.1016/S0070-2153(05)72006-8.View ArticlePubMedGoogle Scholar
- Lund LR, Romer J, Thomasset N, Solberg H, Pyke C, Bissell MJ, Dano K, Werb Z: Two distinct phases of apoptosis in mammary gland involution: proteinase-independent and -dependent pathways. Development. 1996, 122: 181-193.PubMed CentralPubMedGoogle Scholar
- Wray GA, Hahn MW, Abouheif E, Balhoff JP, Pizer M, Rockman MV, Romano LA: The evolution of transcriptional regulation in eukaryotes. Mol Biol Evol. 2003, 20: 1377-1419. 10.1093/molbev/msg140.View ArticlePubMedGoogle Scholar
- Reich CM, Arnould JP: Evolution of Pinnipedia lactation strategies: a potential role for alpha-lactalbumin?. Biol Lett. 2007, 3: 546-549. 10.1098/rsbl.2007.0265.PubMed CentralView ArticlePubMedGoogle Scholar
- Wobbe CR, Struhl K: Yeast and human TATA-binding proteins have nearly identical DNA sequence requirements for transcription in vitro. Mol Cell Biol. 1990, 10: 3859-3867.PubMed CentralView ArticlePubMedGoogle Scholar
- Laird JE, Jack L, Hall L, Boulton AP, Parker D, Craig RK: Structure and expression of the guinea-pig alpha-lactalbumin gene. Biochem J. 1988, 254: 85-94.PubMed CentralView ArticlePubMedGoogle Scholar
- Lenasi T, Kokalj-Vokac N, Narat M, Baldi A, Dovc P: Functional study of the equine beta-casein and kappa-casein gene promoters. J Dairy Res. 2005, 72: 34-43. 10.1017/S0022029905001184.View ArticlePubMedGoogle Scholar
- Soulier S, Lepourry L, Stinnakre MG, Langley B, L'Huillier PJ, Paly J, Djiane J, Mercier JC, Vilotte JL: Introduction of a proximal Stat5 site in the murine alpha-lactalbumin promoter induces prolactin dependency in vitro and improves expression frequency in vivo. Transgenic Res. 1999, 8: 23-31. 10.1023/A:1008851802022.View ArticlePubMedGoogle Scholar
- Sharp JA, Cane KN, Mailer SL, Oosthuizen WH, Arnould JPY, Nicholas KR: Species-specific cell-matrix interactions are essential for differentiation of alveoli-like structures and milk gene expression in primary mammary cells of the Cape fur seal (Arctocephalus pusillus pusillus). Matrix Biol. 2006, 25: 430-442. 10.1016/j.matbio.2006.05.003.View ArticlePubMedGoogle Scholar
- Voigt W: Sulforhodamine B assay and chemosensitivity. Methods Mol Med. 2005, 110: 39-48.PubMedGoogle Scholar
- Goldstein JC, Kluck RM, Green DR: A single cell analysis of apoptosis: ordering the apoptotic phenotype. Mechanism of cell death II: The Third Annual Conference of the International Cell Death Society. Edited by: Zakeri Z, Lockshin RA, Martinez-AC. 2000, New York Academy of Sciences, 326: 132-141.Google Scholar
- Nicholas KR, Hartmann PE: Milk secretion in the rat: progressive changes in milk composition during lactation and weaning and the effect of diet. Comp Biochem Physiol A. 1991, 98: 535-542. 10.1016/0300-9629(91)90443-G.View ArticlePubMedGoogle Scholar
- Lin IC, Su SL, Kuo CD: Induction of cell death in RAW 264.7 cells by alpha-lactalbumin. Food Chem Toxicol. 2008, 46: 842-853. 10.1016/j.fct.2007.10.010.View ArticlePubMedGoogle Scholar
- Wilde CJ, Knight CH, Flint DJ: Control of milk secretion and apoptosis during mammary involution. J Mammary Gland Biol Neoplasia. 1999, 4: 129-136. 10.1023/A:1018717006152.View ArticlePubMedGoogle Scholar
- Lonnerdal B, Lien EL: Nutritional and physiologic significance of alpha-lactalbumin in infants. Nutr Rev. 2003, 61: 295-305. 10.1301/nr.2003.sept.295-305.View ArticlePubMedGoogle Scholar
- Rudolph MC, McManaman JL, Hunter L, Phang T, Neville MC: Functional development of the mammary gland: use of expression profiling and trajectory clustering to reveal changes in gene expression during pregnancy, lactation, and involution. J Mammary Gland Biol Neoplasia. 2003, 8: 287-307. 10.1023/B:JOMG.0000010030.73983.57.View ArticlePubMedGoogle Scholar
- Sternhagen LG, Allen JC: Growth rates of a human colon adenocarcinoma cell line are regulated by the milk protein alpha-lactalbumin. Adv Exp Med Biol. 2001, 501: 115-120.View ArticlePubMedGoogle Scholar
- Juo ZS, Chiu TK, Leiberman PM, Baikalov I, Berk AJ, Dickerson RE: How proteins recognize the TATA box. J Mol Biol. 1996, 261: 239-254. 10.1006/jmbi.1996.0456.View ArticlePubMedGoogle Scholar
- Cane K, Arnould JPY, Nicholas KR: Characterisation of proteins in the milk of fur seals. Comp Biochem Physiol B. 2005, 141: 111-120. 10.1016/j.cbpc.2005.02.003.View ArticlePubMedGoogle Scholar
- Schmidt DV, Walker LE, Ebner KE: Lactose synthetase activity in northern fur seal milk. Biochim Biophys Acta. 1971, 252: 439-442.View ArticlePubMedGoogle Scholar
- Urashima T, Saito T, Nakamura T, Messer M: Oligosaccharides of milk and colostrum in non-human mammals. Glycoconj J. 2001, 18: 357-371. 10.1023/A:1014881913541.View ArticlePubMedGoogle Scholar
- Sarwar G, Botting HG, Davis TA, Darling P, Pencharz PB: Free amino acids in milks of human subjects, other primates and non-primates. Br J Nutr. 1998, 79: 129-131. 10.1079/BJN19980023.View ArticlePubMedGoogle Scholar
- Messer M, Elliott C: Changes in alpha-lactalbumin, total lactose, UDP-galactose hydrolase and other factors in tammar wallaby (Macropus eugenii) milk during lactation. Aust J Biol Sci. 1987, 40: 37-46.PubMedGoogle Scholar
- Hakansson A, Andreasson J, Zhivotovsky B, Karpman D, Orrenius S, Svanborg C: Multimeric alpha-lactalbumin from human milk induces apoptosis through a direct effect on cell nuclei. Exp Cell Res. 1999, 246: 451-460. 10.1006/excr.1998.4265.View ArticlePubMedGoogle Scholar
- Svensson M, Duringer C, Hallgren O, Mossberg AK, Hakansson A, Linse S, Svanborg C: Hamlet-a complex from human milk that induces apoptosis in tumor cells but spares healthy cells. Adv Exp Med Biol. 2002, 503: 125-132.View ArticlePubMedGoogle Scholar
- Duringer C, Hamiche A, Gustafsson L, Kimura H, Svanborg C: HAMLET interacts with histones and chromatin in tumor cell nuclei. J Biol Chem. 2003, 278: 42131-42135. 10.1074/jbc.M306462200.View ArticlePubMedGoogle Scholar
- Permyakov SE, Pershikova IV, Khokhlova TI, Uversky VN, Permyakov EA: No need to be HAMLET or BAMLET to interact with histones: binding of monomeric alpha-lactalbumin to histones and basic poly-amino acids. Biochemistry. 2004, 43: 5575-5582. 10.1021/bi049584y.View ArticlePubMedGoogle Scholar
- Do SI, Lee KY, Kim HN: Novel induction of alpha-lactalbumin-mediated lacdiNAc-R expression in vivo. Biochem J. 2000, 348: 229-234. 10.1042/0264-6021:3480229.PubMed CentralView ArticlePubMedGoogle Scholar
- Hartmann PE, Kulski JK: Changes in the composition of the mammary secretion of women after abrupt termination of breast feeding. J Physiol. 1978, 275: 1-11.PubMed CentralView ArticlePubMedGoogle Scholar
- Capuco AV, Wood DL, Baldwin R, McLeod K, Paape MJ: Mammary cell number, proliferation, and apoptosis during a bovine lactation: relation to milk production and effect of bST. J Dairy Sci. 2001, 84: 2177-2187.View ArticlePubMedGoogle Scholar
- Stinnakre MG, Vilotte JL, Soulier S, Mercier JC: Creation and phenotypic analysis of alpha-lactalbumin-deficient mice. Proc Natl Acad Sci USA. 1994, 91: 6544-6548. 10.1073/pnas.91.14.6544.PubMed CentralView ArticlePubMedGoogle Scholar
- Clarkson RW, Wayland MT, Lee J, Freeman T, Watson CJ: Gene expression profiling of mammary gland development reveals putative roles for death receptors and immune mediators in post-lactational regression. Breast Cancer Res. 2004, 6: R92-R109. 10.1186/bcr754.PubMed CentralView ArticlePubMedGoogle Scholar
- Irizarry RA, Bolstad BM, Collin F, Cope LM, Hobbs B, Speed TP: Summaries of Affymetrix GeneChip probe level data. Nucleic Acids Res. 2003, 31: e15-10.1093/nar/gng015.PubMed CentralView ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.