Prolactin promotes the secretion of active cathepsin D at the basal side of rat mammary acini

RNA-Sequencing for profiling goat milk transcriptome in colostrum and mature milk

BACKGROUND

In this work we aimed at sequencing and assembling the goat milk transcriptome corresponding at colostrum and 120 days of lactation. To reconstruct transcripts we used both the genome as reference, and a de novo assembly approach. Additionally, we aimed at identifying the differentially expressed genes (DEGs) between the two lactation stages and at analyzing the expression of genes involved in oligosaccharides metabolism.

RESULTS

A total of 44,635 different transcripts, organized in 33,757 tentative genes, were obtained using the goat genome as reference. A significant sequence similarity match was found for 40,353 transcripts (90%) against the NCBI NT and for 35,701 (80%) against the NR databases. 68% and 69% of the de novo assembled transcripts, in colostrum and 120 days of lactation samples respectively, have a significant match with the merged transcriptome obtained using Cufflinks/Cuffmerge. CSN2, PAEP, CSN1S2, CSN3, LALBA, TPT1, FTH1, M-SAA3, SPP1, GLYCAM1, EEF1A1, CTSD, FASN, RPS29, CSN1S1, KRT19 and CHEK1 were found between the top fifteen highly expressed genes. 418 loci were differentially expressed between lactation stages, among which 207 and 122 were significantly up- and down-regulated in colostrum, respectively. Functional annotation and pathway enrichment analysis showed that in goat colostrum somatic cells predominate biological processes involved in glycolysis, carbohydrate metabolism, defense response, cytokine activity, regulation of cell proliferation and cell death, vasculature development, while in mature milk, biological process associated with positive regulation of lymphocyte activation and anatomical structure morphogenesis are enriched. The analysis of 144 different oligosaccharide metabolism-related genes showed that most of these (64%) were more expressed in colostrum than in mature milk, with eight expressed at very high levels (SLCA3, GMSD, NME2, SLC2A1, B4GALT1, B3GNT2, NANS, HEXB).

CONCLUSIONS

To our knowledge, this is the first study comparing goat transcriptome of two lactation stages: colostrum and 120 days. Our findings suggest putative differences of expression between stages and can be envisioned as a base for further research in the topic. Moreover because a higher expression of genes involved in immune defense response, carbohydrate metabolism and related to oligosaccharide metabolism was identified in colostrum we here corroborate the potential of goat milk as a natural source of lactose-derived oligosaccharides and for the development of functional foods.

Vasoinhibins, N-terminal mouse prolactin fragments, participate in mammary gland involution

Vasoinhibins are a family of peptides that act on endothelial cells to suppress angiogenesis and promote apoptosis-mediated vascular regression. Vasoinhibins include the N-terminal fragments from prolactin (PRL), GH, and placental lactogen. One of the vasoinhibins, the N-terminal PRL fragment of 16 kDa, is generated by the lysosomal representative protease cathepsin D (Cath D). Because the normal growth and involution of the mammary gland (MG) are profoundly affected by the expansion and regression of blood vessels and also because PRL stimulates the growth and differentiation of MG, we proposed that intact PRL produced during lactation contributes to MG angiogenesis and increased blood flow, whereas during involution, the N-terminal PRL fragment would have proapoptotic effects on mammary epithelial cells (MECs). Therefore, we investigated the production of the N-terminal PRL fragment and its direct effect on the MG. Mouse PRL (mPRL) was proteolytically cleaved by Cath D between amino acids 148 and 149. N-terminal PRL fragment and Cath D expression increased during MG involution. Furthermore, incubation of MG fragments and MCF7 with recombinant 16 kDa mPRL revealed a proapoptotic effect in MECs. Ectopic mPRL in MECs was cleaved to 16 kDa PRL by Cath D in the MG lysosomal fraction. The majority of PRL derived from pituitary gland was cleaved to 16 kDa PRL in culture medium. Therefore, N-terminal PRL fragment increases during the involution period, has a proapoptotic effect on MECs, and is mainly generated by secreted Cath D in the extracellular space of MG.

Two Faces of Cathepsin D: Physiological Guardian Angel and Pathological Demon

Since its discovery as a lysosomal hydrolase, Cathepsin D (CatD) has been the subject of intensive scrutiny by numerous scientists. Those accumulated efforts have defined its biosynthetic pathway, structure, and companion proteins in the context of its perceived “house keeping” function. However, in the past two decades CatD has emerged as a multifunctional enzyme, involved in myriad biological processes beyond its original “housekeeping” role. CatD is responsible for selective and limited cleavage (quite distinct from non-specific protein degradation) of particular substrates vital to proper cellular function. These proteolytic events are critical in the control of biological processes, including cell cycle progression, differentiation and migration, morphogenesis and tissue remodeling, immunological processes, ovulation, fertilization, neuronal outgrowth, angiogenesis, and apoptosis. Consistent with the biological relevance of CatD, its deficiency, altered regulation or post-translational modification underlie important pathological conditions such as cancer, atherosclerosis, neurological and skin disorders. Specifically, deregulated synthesis, post-translational modifications and hyper-secretion of CatD, along with its mitogenic effects, are established hallmarks of cancer. More importantly, but less studied, is its significance in regulating the sensitivity to anticancer drugs.

This review outlines CatD’s post-translational modifications, cellular trafficking, secretion and protein binding partners in normal mammary gland, and restates the “site-specific” function of CatD which is most probably dictated by its post-translational modifications and binding partners. Noteworthy, CatD’s association with one of its binding partners in the context of drug sensitivity is highlighted, with the optimism that it could contribute to the development of more effective chemotherapeutic agent(s) tailored for individual patients.

Effects of dietary fish oil and corn oil on rat mammary tissue

The effects, on the maternal mammary gland, of diets containing similar lipid percentages but differing in composition of polyunsaturated fatty acids (PUFA) have been assessed in rats during pregnancy and lactation. For this purpose, tuna fish oil (an n-3-PUFA-enriched oil) and corn oil (an n-6-PUFA-enriched oil) were included in diets at ratios such that the caloric inputs were the same as that of the control diet. As expected, the maternal diet affected the tissue composition of dams. Unexpectedly, only the tuna fish oil diet had an effect on pup growth, being associated with the pups being underweight between the ages of 11 and 21 days. The maternal mammary gland of rats fed the tuna fish oil diet displayed two main modifications: the size of cytoplasmic lipid droplets was increased when compared with those in control rats and the mammary epithelium showed an unusual formation of multilayers of cells. These results show that the tuna fish oil diet, during pregnancy and lactation, exerts specific effects on mammary cells and on the formation of lipid droplets. They suggest that this maternal diet affects the functioning of the mammary tissue.

Mammary gland secretion: hormonal coordination of endocytosis and exocytosis

The mammary epithelium coordinates the uptake of milk precursors and the transport of milk components in order to produce milk of relatively constant composition at a particular stage of lactation, as long as the mammary gland is healthy. The mammary epithelial cell controls the uptake of blood-borne molecules at its basal side and the release of products into milk at its apical side, through mechanisms of internalization (endocytosis) and mechanisms of release (exocytosis). These events are strictly dependent on the physiological stage of the mammary gland. This review addresses the mechanisms responsible for these processes and points out new questions that remain to be answered concerning possible interconnections between them, for an optimal milk secretion.

N-terminal prolactin-derived fragments, vasoinhibins, are proapoptoptic and antiproliferative in the anterior pituitary

The anterior pituitary is under a constant cell turnover modulated by gonadal steroids. In the rat, an increase in the rate of apoptosis occurs at proestrus whereas a peak of proliferation takes place at estrus. At proestrus, concomitant with the maximum rate of apoptosis, a peak in circulating levels of prolactin is observed. Prolactin can be cleaved to different N-terminal fragments, vasoinhibins, which are proapoptotic and antiproliferative factors for endothelial cells. It was reported that a 16 kDa vasoinhibin is produced in the rat anterior pituitary by cathepsin D. In the present study we investigated the anterior pituitary production of N-terminal prolactin-derived fragments along the estrous cycle and the involvement of estrogens in this process. In addition, we studied the effects of a recombinant vasoinhibin, 16 kDa prolactin, on anterior pituitary apoptosis and proliferation. We observed by Western Blot that N-terminal prolactin-derived fragments production in the anterior pituitary was higher at proestrus with respect to diestrus and that the content and release of these prolactin forms from anterior pituitary cells in culture were increased by estradiol. A recombinant preparation of 16 kDa prolactin induced apoptosis (determined by TUNEL assay and flow cytometry) of cultured anterior pituitary cells and lactotropes from ovariectomized rats only in the presence of estradiol, as previously reported for other proapoptotic factors in the anterior pituitary. In addition, 16 kDa prolactin decreased forskolin-induced proliferation (evaluated by BrdU incorporation) of rat total anterior pituitary cells and lactotropes in culture and decreased the proportion of cells in S-phase of the cell cycle (determined by flow cytometry). In conclusion, our study indicates that the anterior pituitary production of 16 kDa prolactin is variable along the estrous cycle and increased by estrogens. The antiproliferative and estradiol-dependent proapoptotic actions of this vasoinhibin may be involved in the control of anterior pituitary cell renewal.

Knock-down of cathepsin D affects the retinal pigment epithelium, impairs swim-bladder ontogenesis and causes premature death in zebrafish

The lysosomal aspartic protease Cathepsin D (CD) is ubiquitously expressed in eukaryotic organisms. CD activity is essential to accomplish the acid-dependent extensive or partial proteolysis of protein substrates within endosomal and lysosomal compartments therein delivered via endocytosis, phagocytosis or autophagocytosis. CD may also act at physiological pH on small-size substrates in the cytosol and in the extracellular milieu. Mouse and fruit fly CD knock-out models have highlighted the multi-pathophysiological roles of CD in tissue homeostasis and organ development. Here we report the first phenotypic description of the lack of CD expression during zebrafish (Danio rerio) development obtained by morpholino-mediated knock-down of CD mRNA. Since the un-fertilized eggs were shown to be supplied with maternal CD mRNA, only a morpholino targeting a sequence containing the starting ATG codon was effective. The main phenotypic alterations produced by CD knock-down in zebrafish were: 1. abnormal development of the eye and of retinal pigment epithelium; 2. absence of the swim-bladder; 3. skin hyper-pigmentation; 4. reduced growth and premature death. Rescue experiments confirmed the involvement of CD in the developmental processes leading to these phenotypic alterations. Our findings add to the list of CD functions in organ development and patho-physiology in vertebrates.

The mammary gland vasculature revisited

Concomitant with the extensive growth and differentiation of the mammary epithelium during pregnancy and lactation, and epithelial involution after weaning, the vasculature of the mammary gland undergoes repeated cycles of expansion and regression. Vascular expansion is effected by sprouting angiogenesis, intussusception and conceivably also vasculogenesis. The capacity of the epithelial cells to stimulate vascular growth and differentiation is dependent on the constellation of systemic and local hormones and growth factors as well as the changing demands for oxygenation and nutrient supply. This results in the release of angiogenic factors which stimulate endothelial cell growth and regulate vascular architecture. In contrast to the angiogenic phase of the mammary gland cycle, little is known about the control of vascular regression although this would possibly offer new insights into therapeutic possibilities against breast cancer. In this review we summarize knowledge regarding the mechanisms regulating the vasculature of the mammary gland and delineate the importance of the vasculature in the attainment of organ function. In addition, we discuss the angiogenic mechanisms observed during mammary carcinogenesis and their consequences for breast cancer therapy.

Differentiation of the mammary epithelial cell during involution: implications for breast cancer

That milk secretion is not the final differentiated state of the mammary alveolar cells is a relatively new concept. Recent work has suggested that secreting, mammary epithelial cells (MECs) have another function to perform before they undergo cell death in the involuting mammary gland. That is, they help in the final clearance and breakdown of their neighboring cells (and likely residual milk as well.) They become, for a short time, amateur phagocytes, or efferocytes, and then are believed to die and be cleared themselves. Although relatively little study has been made of this change in the functional state of the MEC, nevertheless we may speculate from the involution literature, and extend findings from other systems of apoptotic cell clearance, on some of the mechanisms involved. And with the finding that involution may represent a unique susceptibility window for the progression of metastatic breast cancer, we may suggest areas for future research along these lines as well.

The role of cathepsins in involution and breast cancer

Cysteine cathepsins are proteolytic enzymes that reside in endolysosomal vesicles. Some are expressed constitutively while others are transcriptionally regulated. However, the expression and subcellular localization of cathepsins changes during cancer progression and cathepsins have been shown to be causally involved in various aspects of tumorigenesis including metastasis. The use of mouse models of breast cancer genetically ablated for cathepsin B has shown that both the growth of the primary tumor and the extend of lung metastasis is reduced by the loss of cathepsin B. The role of cathepsins in involution of the mammary gland has received little attention although it is clear that cathepsins are involved in tissue remodeling in the second phase of involution. We discuss here the roles of cathepsins and their endogenous inhibitors in breast tumorigenesis and post-lactational involution.

Nitration of cathepsin D enhances its proteolytic activity during mammary gland remodelling after lactation

Proteomic studies in the mammary gland of control lactating and weaned rats have shown that there is an increased pattern of nitrated proteins during weaning when compared with controls. Here we report the novel finding that cathepsin D is nitrated during weaning. The expression and protein levels of this enzyme are increased after 8 h of litter removal and this up-regulation declines 5 days after weaning. However, there is a marked delay in cathepsin D activity since it does not increase until 2 days post-weaning and remains high thereafter. In order to find out whether nitration of cathepsin D regulates its activity, iNOS (inducible nitric oxide synthase)(-/-) mice were used. The expression and protein levels of this enzyme were similar to WT (wild-type) animals, but the proteolytic activity was significantly reduced during weaning in knockout compared to WT mice. in vitro treatment of recombinant human cathepsin D or lactating mammary gland homogenates with relatively low concentrations of peroxynitrite enhances the nitration as well as specific activity of this enzyme. Using MS, it has been shown that the residue Tyr168 was nitrated. All of these results show that protein nitration during weaning might be a signalling pathway involved in mammary gland remodelling.

Chemotherapy drug response in ovarian cancer cells strictly depends on a cathepsin D-Bax activation loop

The ovarian cancer cell lines A2780 (wild-type p53) and NIHOVCAR3 (mutated p53) showed, respectively, sensitivity and resistance towards several chemotherapy drugs. We hypothesized that the two cell lines differ in their ability to activate the intrinsic death pathway and have, therefore, dissected the lysosome-mitochondrion signalling pathway by pharmacological inhibition or genetic manipulation of key regulators and executioners. Biochemical and morphological confocal fluorescence studies showed that: (1) In A2780 cells bcl-2 is expressed at an undetectable level, whereas Bax is expressed at a rather high level; by contrast, bcl-2 is highly expressed and Bax is expressed at extremely low levels in NIHOVCAR3 cells; (2) Chemotherapy treatment reduced the expression of bcl-2 in NIHOVCAR3 cells, yet these cells resisted to drug toxicity; (3) Cathepsin D (CD), not cathepsin B or L, mediates the activation of the mitochondrial intrinsic death pathway in A2780 cells; (4) Lysosome leakage and cytosolic relocation of CD occurs in the chemosensitive A2780 cells, not in the chemoresistant NIHOVCAR3 cells; (5) Bax is essential for the permeabilization of both lysosomes and mitochondria in A2780 cells exposed to chemotherapy drugs; (6) CD activity is mandatory for the oligomerization of Bax on both mitochondrial and lysosomal membranes; (7) Bax activation did not occur in the resistant NIHOVCAR3 cells despite their high content in CD. The present data are consistent with a model in which on treatment with a cytotoxic drug the activation of a CD-Bax loop leads to the generalized permeabilization of lysosomes and eventually of mitochondria, thus reaching the point of no return, and culminates with the activation of the caspase cascade. Our data also imply that dysfunctional permeabilization of lysosomes contributes to the development of chemoresistance.