Prosaposin, a neurotrophic factor: presence and properties in milk

Neurotoxic stimulation alters prosaposin levels in the salivary systems of rats

Prosaposin (PSAP), a potent neurotrophic factor, is found in neuronal and non-neuronal tissues and various biological fluids. Neuropathological conditions often alter PSAP production in neural tissues. However, little is known about its alterations in non-neural tissues, particularly in the salivary glands, which are natural reservoirs of various neurotrophic factors. In this study, we explored whether neurotoxic stimulation by kainic acid (KA), a glutamate analog, altered PSAP levels in the salivary system of rats. The results revealed that KA injection did not alter total saliva production. However, KA-induced neurotoxic stimulation significantly increased the PSAP level in the secreted saliva but decreased it in the serum. In addition, KA-induced elevated immunoreactivities of PSAP and its receptors have been observed in the granular convoluted tubule (GCT) cells of the submandibular gland (SMG), a major salivary secretory organ. Indeed, a large number of PSAP-expressing immunogold particles were observed in the secretory granules of the SMG. Furthermore, KA-induced overexpression of PSAP was co-localized with secretogranin in secretory acini (mostly in GCT cells) and the ductal system of the SMG, suggesting the release of excess PSAP from the salivary glands into the oral cavity. In conclusion, the salivary system produces more PSAP during neurotoxic conditions, which may play a protective role in maintaining the secretory function of the salivary glands and may work in distant organs.

Tracking of Prosaposin, a Saposin Precursor, in Rat Testis

We tracked prosaposin (PSAP), a trophic factor, using an antibody specific to its proteolytic portion and an antibody to sortilin that traffics PSAP only to the lysosome. Immunostaining revealed that PSAP was distributed mainly on the basal side of seminiferous tubules, where many Sertoli cells and pachytene spermatocytes contained PSAP and its distribution differed depending on the stage of the spermatogenic cycle. The PSAP-sortilin complex was sorted to large lysosomes in the basal cytoplasm of Sertoli cells, where it may be processed into saposins. In contrast, in the thinner apical cytoplasm of Sertoli cells, PSAP in small lysosomes was transported to the apical side around sperm heads or into the lumen for secretion. The results of in situ hybridization analyses suggested that immature tubular cells in young animals produce PSAP to self-stimulate proliferation. However, in adults, not only Sertoli cells but also pachytene spermatocytes produce and secrete PSAP around germ cells or into the tubular lumen to stimulate cell proliferation or differentiation in a paracrine or autocrine manner. In summary, PSAP is not only a precursor of lysosomal enzymes but also a pivotal trophic factor in organogenesis in the immature testis and spermatogenesis in the mature testis.

Expression patterns of prosaposin and neurotransmitter-related molecules in the chick paratympanic organ

The paratympanic organ (PTO) is a small sense organ in the middle ear of birds that contains hair cells similar to those found in vestibuloauditory organs and receives afferent fibers from the geniculate ganglion. To consider the histochemical similarities between the PTO and vestibular hair cells, we examined the expression patterns of representative molecules in vestibular hair cells, including prosaposin, G protein-coupled receptor (GPR) 37 and GPR37L1 as prosaposin receptors, vesicular glutamate transporter (vGluT) 2 and vGluT3, nicotinic acetylcholine receptor subunit α9 (nAChRα9), and glutamic acid decarboxylase (GAD) 65 and GAD67, in the postnatal day 0 chick PTO and geniculate ganglion by in situ hybridization. Prosaposin mRNA was observed in PTO hair cells, supporting cells, and geniculate ganglion cells. vGluT3 mRNA was observed in PTO hair cells, whereas vGluT2 was observed in a small number of ganglion cells. nAChRα9 mRNA was observed in a small number of PTO hair cells. The results suggest that the histochemical character of PTO hair cells is more similar to that of vestibular hair cells than that of auditory hair cells in chicks.

Comparative Structural and Compositional Analyses of Cow, Buffalo, Goat and Sheep Cream

Factors affecting milk and milk fraction composition, such as cream, are poorly understood, with most research and human health application associated with cow cream. In this study, proteomic and lipidomic analyses were performed on cow, goat, sheep and Bubalus bubalis (from now on referred to as buffalo), bulk milk cream samples. Confocal laser scanning microscopy was used to determine the composition, including protein, lipid and their glycoconjugates, and the structure of the milk fat globules. BLAST2GO was used to annotate functional indicators of cream protein. Functional annotation of protein highlighted a broad level of similarity between species. However, investigation of specific biological process terms revealed distinct differences in antigen processing and presentation, activation, and production of molecular mediators of the immune response. Lipid analyses revealed that saturated fatty acids were lowest in sheep cream and similar in the cream of the other species. Palmitic acid was highest in cow and lowest in sheep cream. Cow and sheep milk fat globules were associated with thick patches of protein on the surface, while buffalo and goat milk fat globules were associated with larger areas of aggregated protein and significant surface adsorbed protein, respectively. This study highlights the differences between cow, goat, sheep, and buffalo milk cream, which can be used to support their potential application in functional foods such as infant milk formula.

Astaxanthin counteracts the effects of heat shock on the maturation of bovine oocytes

The cellular mechanisms induced by elevated temperature on oocytes are not fully understood. However, there is evidence that some of the deleterious effects of heat shock are mediated by a heat-induced increase in reactive oxygen species (ROS). In this context, carotenoid antioxidants might have a thermoprotective effect. Therefore, the objective of this study was to determine the role of astaxanthin (AST) on oocyte ROS production and on the redox profile and developmental competency of cumulus-oocyte complexes (COCs) after 14 h heat shock (41°C) during in vitro maturation (IVM). Exposure of oocytes to heat shock during IVM increased ROS and reduced the ability of the oocyte to cleave and develop to the blastocyst stage. However, 12.5 and 25 nM astaxanthin rescued these negative effects of heat shock; astaxanthin counteracted the heat shock-induced increase in ROS and restored oocyte developmental competency. There was no effect of astaxanthin on maturation medium lipid peroxidation or on glutathione peroxidase and catalase activity in oocytes and cumulus cells. However, astaxanthin stimulated superoxide dismutase (SOD) activity in heat-shocked cumulus cells. In conclusion, direct heat shock reduced oocyte competence, which was restored by astaxanthin, possibly through regulation of ROS and SOD activity in oocytes and COCs.

A saposin deficiency model in Drosophila: Lysosomal storage, progressive neurodegeneration and sensory physiological decline

Saposin deficiency is a childhood neurodegenerative lysosomal storage disorder (LSD) that can cause premature death within three months of life. Saposins are activator proteins that promote the function of lysosomal hydrolases that mediate the degradation of sphingolipids. There are four saposin proteins in humans, which are encoded by the prosaposin gene. Mutations causing an absence or impaired function of individual saposins or the whole prosaposin gene lead to distinct LSDs due to the storage of different classes of sphingolipids. The pathological events leading to neuronal dysfunction induced by lysosomal storage of sphingolipids are as yet poorly defined. We have generated and characterised a Drosophila model of saposin deficiency that shows striking similarities to the human diseases. Drosophila saposin-related (dSap-r) mutants show a reduced longevity, progressive neurodegeneration, lysosomal storage, dramatic swelling of neuronal soma, perturbations in sphingolipid catabolism, and sensory physiological deterioration. Our data suggests a genetic interaction with a calcium exchanger (Calx) pointing to a possible calcium homeostasis deficit in dSap-r mutants. Together these findings support the use of dSap-r mutants in advancing our understanding of the cellular pathology implicated in saposin deficiency and related LSDs.

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Drosophila model of PSD recapitulates neurodegenerative phenotype of human PSD.

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Preferential degeneration of sensory regions correlates with loss of sensory function.

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Sphingosine levels rise with age with an imbalance in sphingosine/ceramide ratios.

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Genetic interaction with the Na +/Ca + exchanger points to a calcium regulation deficit.

Yeast Culture and Vitamin E Supplementation Alleviates Heat Stress in Dairy Goats

This study was conducted to determine and compare the effects of yeast yeast culture (YC) and vitamin E (VE) supplementation on endotoxin absorption and antioxidant status in lactating dairy goats suffering from heat stress (HS). Three first lactation Saanen dairy goats (body weight 30±1.5 kg) were surgically fitted with indwelling catheters in the portal vein, mesenteric vein and carotid artery, and were randomly assigned to a 3×3 Latin square design. Dietary treatments were the basal diet, and the basal diet supplemented with either 100 IU VE or 30 g YC. Goats were kept in temperature and humidity-controlled room at 35°C from 8:00 to 20:00 and at 24°C from 20:00 till the next morning at 8:00. The relative humidity was kept at 55%. HS increased dairy goats' rectum temperature and respiration frequency (p<0.01). HS reduced plasma flux rate of milk goats (p<0.01), but the plasma flux rate increased when the animal was under the conditions of the thermo-neutral period (p<0.01). The VE supplementation lowered dairy goats' rectum temperature during thermo-neutral period (p<0.01). Meanwhile, no significant differences were observed between the control and YC treatment in rectum temperature and respiration frequency (p>0.05). Dietary supplementation of VE and YC reduced heat stressed dairy goats' endotoxin concentration of the carotid artery and portal vein (p<0.01). However, the endotoxin concentration of the YC treatment was higher than that of the VE treatment (p<0.01). Both VE and YC supplementation decreased heat stressed dairy goats' absorption of endotoxin in portal vein (p<0.01). The endotoxin absorption of YC treatment was higher than the VE treatment (p<0.01). The addition of VE and YC decreased dairy goats' superoxide dismutase (SOD) concentration during HS and the whole experiment period (p<0.01). The addition of VE lowered SOD concentration during thermo-neutral period (p<0.01). Likewise, the addition of VE and YC lowered dairy goats' malonaldehyde (MDA) concentration during HS and the whole experimental period, and the MDA concentration in the VE treatment was lower than the YC treatment (p<0.05). The addition of VE decreased MDA concentration during thermo-neutral period. On the contrast, the addition of VE increased dairy goats total antioxidant potential (TAP) concentration during HS, thermo-neutral and the whole experimental period (p<0.01). The addition of YC increased TAP concentration only during HS period (p<0.01). It is concluded that both VE and YC are useful in alleviating HS of dairy goats by weakening endotoxin absorption and promoting antioxidant capacity. Compared with YC, VE is much more powerful in easing dairy goats HS.

Hemato-biochemical and Cortisol Profile of Holstein Growing-calves Supplemented with Vitamin C during Summer Season

Effect of vitamin C (VC) on biochemical, hematological and cortisol profile of growing Holstein calves during summer was investigated. Eighteen calves between 14 and 16 weeks of age were divided equally into two groups. One group received a diet supplemented with VC (20 g/d) for 60 days, while other non-supplemented diet fed group served as a control (CON). The temperature humidity index (THI) was recorded and computed during the experiment. From days 0 to 60, the THI exceeded 70. Blood samples were collected from the jugular vein of each calf at days 0, 15, 30, 45 and 60. Serum albumin and total protein decreased (p<0.05) in CON and VC calves with age. Serum glutamic-oxaloacetic transaminase concentrations were not affected by treatments. Serum creatinine, albumin and glutamic-pyruvic transaminase concentrations were higher in calves in the VC group than the CON group. While red blood cells, hemoglobin and hematocrit were lower (p<0.05) in VC calves, mean corpuscular volume, mean corpuscular hemoglobin, red blood cell distribution width and mean platelet volume were higher (p<0.05) in these VC supplemented calves. Leukocyte parameters including white blood cells and full term for lymphocytes were not affected by the treatments. Also, serum cortisol was not affected by treatments. At day 15, 30 and 45, the total VC in plasma was higher (p<0.05) in calves fed with VC. In conclusion, serum cortisols were not affected by plasma VC concentration, while some blood parameters were positively influenced in calves fed with VC.

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.

Distribution of prosaposin in rat lymphatic tissues

Prosaposin (PSAP) is as a trophic factor and an activator protein for sphingolipid hydrolase in lysosomes. We generated a specific antibody to PSAP and examined the spatiotemporal distribution of PSAP-immunoreactive (PSAP-IR) cells in the lymphatic tissues of Wistar rats. Immunoblots of tissue homogenates separated electrophoretically showed a single band for PSAP in brain but two bands in spleen. PSAP-IR cells were distributed in both the red and white pulp of the spleen, in both the cortex and medulla of the thymus and in mesenteric lymph nodes. Many PSAP-IR cells were found in the dome portion of Peyer's patches and the number of PSAP-IR cells increased with the age of the rat. To identify the PSAP-IR cells, double- and triple-immunostainings were performed with antibodies against PSAP, CD68 and CD1d. The large number of double- and triple-positive cells suggested that antigen-presenting cells contained much PSAP in these lymphatic tissues. Intense expression of PSAP mRNA, examined by in situ hybridisation, was observed in the red pulp and corona of the spleen. In rats, the PSAP gene generates two alternative splicing forms of mRNA: Pro+9 containing a 9-base insertion and Pro+0 without the insertion. We examined the expression patterns of the alternative splicing forms of PSAP mRNA in the spleen. The presence of both types of mRNA (Pro+9 and Pro+0) indicated that the spleen contains various types of prosaposin-producing and/or secreting cells. These findings suggest diverse functions for PSAP in the immune system.

Prosaposin, a regulator of estrogen receptor alpha, promotes breast cancer growth

Prosaposin, a secreted protein, is a well-known pleiotropic growth factor. Although a previous report has indicated that prosaposin is overexpressed in breast cancer cell lines, the role of prosaposin in the development of breast cancer remains to be identified. Here, we first revealed that prosaposin upregulated estrogen receptor alpha expression, nuclear translocation and transcriptional activity by western blot, immunofluorescence assay and dual luciferase reporter gene assay, respectively. Furthermore, we demonstrated prosaposin upregulated estrogen receptor alpha expression through MAPK-signaling pathway using MAPK inhibitor. Proliferation assay and tumor xenograft experiments in nude mice (n = 6 per group) further confirmed prosaposin could promote breast cancer growth significantly in vitro and in vivo. These findings suggested that prosaposin might enhance estrogen receptor alpha-mediated signaling axis and play a role in breast cancer development and progression.

Serum prosaposin levels are increased in patients with advanced prostate cancer

BACKGROUND

We previously cloned prosaposin (PSAP) from metastatic castrate-resistant prostate cancer (mCRPCa) cells and demonstrated its genomic amplification and/or overexpression in metastatic PCa cell lines, xenografts, and lymph node metastases. The clinicohistopathological significance of serum PSAP levels and its tissue expression and association with predictive or prognostic variable in primary or advanced PCa are not known.

METHODS

We examined PSAP expression by immunohistochemical staining during early embryogenic development of the prostate and within a large tissue microarray which included 266 benign and malignant prostate tissues. In addition, serum PSAP levels in the age-adjusted normal male population and in 154 normal individuals and patients with primary or mCRPCa were measured by an ELISA assay.

RESULTS

Univariate and multivariate analyses revealed a significant and inverse association between PSAP expression and clinical stages II and III tumors, dominant Gleason patterns 3 and 4, and seminal vesicle invasion. In the normal male population, the lowest serum PSAP level was detected before puberty, peaked at the most reproductive age group (20- to 39-year old), and then, decreased to a range between the two groups for men above 40-year old. Regardless of age and when compared with normal individuals, serum PSAP levels significantly decreased in primary organ-confined PCa, but increased in those with mCRPCa.

CONCLUSION

Our results show that PSAP has the potential to differentiate between primary and advanced PCa. Additional large-scale studies are needed to define the usefulness of tissue expression or serum PSAP levels as a diagnostic or prognostic marker or as a therapeutic target in PCa.

Chronological changes in prosaposin in the developing rat brain

Prosaposin is the precursor protein of four glycoproteins, saposins A, B, C, and D, which activate sphingolipid hydrolases in lysosomes. Besides this role, intact prosaposin is also known as a potent neurotrophic factor that prevents neuronal cell death and stimulates neurite outgrowth in in vivo and in vitro experiments. In the present study, we examined chronological changes in prosaposin immunoreactivity in the rat brain using immunofluorescence staining and Diaminobenzidine (DAB) immunohistochemistry. In the hippocampal regions CA1, CA3, and dentate gyrus, the strongest staining of prosaposin was observed on postnatal day 1. The prosaposin immunoreactivity then decreased gradually until postnatal day 28. But in the cerebral cortex, prosaposin staining intensity increased from postnatal day 1 to 14, then decreased until postnatal day 28. The prosaposin immunoreactivity co-localized with the lysosomal granules labeled by an anti-Cathepsin D antibody, indicating that prosaposin mainly localized in the lysosomes of the neurons. We also examined the chronological changes in prosaposin mRNA and its two alternatively spliced variants using in situ hybridization. We found that both the mRNA forms, especially the one without a nine-base insertion, increased significantly from embryonic day 15 to postnatal day 7, then decreased gradually until postnatal day 28. Abundant prosaposin expression in the perinatal stages indicates a potential role of prosaposin in the early development of the rat brain.

Proteomic analysis of synaptosomal protein expression reveals that cerebral ischemia alters lysosomal Psap processing

Cerebral ischemia (CI) induces dramatic changes in synaptic structure and function that precedes delayed post-ischemic neuronal death. Here, a proteomic analysis was used to identify the effects of focal CI on synaptosomal protein levels. Contralateral and ipsilateral synaptosomes, prepared from adult mice subjected to 60 min middle cerebral artery occlusion, were isolated following 3, 6 and 20 h of reperfusion. Synaptosomal protein samples (n=3) were labeled using the cleavable ICAT system prior to analysis with nanoLC-MS/MS. Each sample was analyzed by LC-MS to identify differential expressions using InDEPT software and differentially expressed peptides were identified by targeted LC-MS/MS. A total of 62 differentially expressed proteins were identified and Gene Ontology classification (cellular component) indicated that the majority of the proteins were located in the mitochondria and other components consistent with synaptic localization. The observed alterations in synaptic protein levels poorly correlated with gene expression, indicating the involvement of post-transcriptional regulatory mechanisms in determining post-ischemic synaptic protein content. Additionally, immunohistochemistry analysis of prosaposin (Psap) and saposin C (SapC) indicates that CI disrupts Psap processing and glycosphingolipid metabolism. These results demonstrate that the synapse is adversely affected by CI and may play a role in mediating post-ischemic neuronal viability.

Localization of prosaposin in rat cochlea

Prosaposin, the precursor of the sphingolipid hydrolase activator proteins called saposins A, B, C, and D, is abundant in the nervous system and muscles. Besides its role as the precursor of saposins, prosaposin is reported to function as a neurotrophic factor, initiating neural differentiation and preventing neuronal cell death in vivo and in vitro. In this study, we examined the localization and synthesis of prosaposin in the rat cochlea. Intense prosaposin immunoreactivity was observed in the organ of Corti, stria vascularis, and spiral ganglion. In an immuno-electron microscopic study, prosaposin immunoreactivity was found mainly in lysosomal granules of the cells in these regions. In the lysosome, prosaposin does not always colocalize with cathepsin D, but was localized mainly in the dark area of the lysosome. Prosaposin mRNA was observed in these same regions. Our results suggest that prosaposin plays a role in homeostasis in the peripheral auditory system.

A mutation in the saposin A coding region of the prosaposin gene in an infant presenting as Krabbe disease: first report of saposin A deficiency in humans

A six-month-old infant girl presenting with progressive encephalopathy and abnormal myelination in the cerebral white matter was originally diagnosed as suffering from Krabbe disease. The diagnosis was based on a deficiency of galactocerebrosidase activity found in leukocytes isolated from whole blood. When cultured skin fibroblasts did not show a similar enzyme deficiency and sulphatide (stearoyl-1-14C) uptake indicated an abnormal storage of galactosylceramide, a deficiency of an activator was implied. A three base pair deletion was found in the saposin A coding sequence of the prosaposin gene leading to the deletion of a conserved valine at amino acid number 11 of the saposin A protein. This deletion in saposin A is proposed as the cause for the abnormal galactosylceramide metabolism in this infant. This is the first report of a saposin A mutation in humans leading to pathological consequences.

Procathepsin D interacts with prosaposin in cancer cells but its internalization is not mediated by LDL receptor-related protein

The cell surface binding, endocytosis, and lysosomal routing of procathepsin D (procath-D) in cancer cells are mostly independent of the mannose-6-phosphate (M6P) receptors. In an attempt to define the receptor involved, we intracellularly cross-linked procath-D with a 68-kDa protein that we identified with specific antibodies as prosaposin in human breast and ovarian cancer cell lines. In cancer cells, this protein-protein interaction was resistant to ammonium chloride or M6P treatment, indicating that it was independent of the M6P receptors. A similar interaction also occurred in the breast cancer cell culture medium between the secreted prosaposin and procath-D. Since these two precursors can be endocytosed, we then determined whether they were interacting with the same cell surface receptor. In fibroblasts, we confirmed that the endocytosis of these two proteins was different since it was generally mediated by the M6P receptors for procath-D and mostly by LRP (LDL receptor-related protein) for prosaposin. In breast cancer cells, prosaposin endocytosis was not detected, in contrast to procath-D endocytosis, suggesting that the majority of procath-D is not internalized as a complex with prosaposin. Moreover, RAP (receptor-associated protein), a ligand inhibiting LRP-mediated endocytosis, prevented internalization of prosaposin in 49-F rat fibroblasts, but did not affect procath-D M6P-independent internalization in MDA-MB231 cells. We conclude that in breast cancer cells, even though procath-D interacts intracellularly and extracellarly with prosaposin, it is endocytosed independent of prosaposin by a receptor different from the M6P receptors and the LRP.

Insulin-like growth factor-II/cation-independent mannose 6-phosphate receptor mediates paracrine interactions during spermatogonial development

The insulin-like growth factor-II/cation-independent mannose 6-phosphate (IGF-II/M6P) receptor transduces signals after binding IGF-II or M6P-bearing growth factors. We hypothesized that this receptor relays paracrine signals between Sertoli cells and spermatogonia in the basal compartment of the seminiferous epithelium. For these studies spermatogonia were isolated from 8-day-old mice with purity >95% and viability >85% after overnight culture. The IGF-II/M6P receptors were present on the surface of spermatogonia, as detected by indirect immunofluorescence. We determined that both IGF-II and M6P-glycoproteins in Sertoli cell conditioned medium (SCM) modulate gene expression in isolated spermatogonia. The IGF-II produced dose-dependent increases in both rRNA and c-fos mRNA. These effects were mediated specifically by IGF-II/M6P receptors, as shown by studies using IGF-II analogues that are specific agonists for either IGF-I or IGF-II receptors. The SCM treatment also induced dose-dependent increases in rRNA levels, and M6P competition showed that this response required interaction with IGF-II/M6P receptors. The M6P-glycoproteins isolated from SCM by IGF-II/M6P receptor affinity chromatography increased spermatogonial rRNA levels at much lower concentrations than required by SCM treatment, providing further evidence for the paracrine activity of Sertoli M6P-glycoproteins. These results demonstrate that Sertoli cells secrete paracrine factors that modulate spermatogonial gene expression after interacting with cell-surface IGF-II/M6P receptors.

Secretion of prosaposin, a multifunctional protein, by breast cancer cells

Western blotting and immunodetection with three antibodies were used to probe conditioned media of breast cancer cells (MDA231, MDA435, MCF-7) for prosaposin, a lysosomal protein that occurs in milk. It was readily detected in media from these cells, and from that of an sv40-transformed mammary epithelial cell, HBL100, but not from medium of human neural tumor cells (SK-N-MC). In cultures of MCF-7 cells, the prosaposin pattern of secretion over time closely resembled that of procathepsin D, another lysosomal protein occurring in milk. Supplementing medium with 17beta-estradiol (0. 1-100 nM) dose dependently increased secretion of both proteins after 48 h without changes in cell viability. The influence of 17beta-estradiol on secretion could play a role in the trophic activity of prosaposin in cellular differentiation and cell death protection. In concert with other lysosomal proteins in the tumor environment, such as procathepsin D, prosaposin may be a factor in eliminating barriers to tumor metastasis by facilitating hydrolysis of membrane glycolipids. The number of milk proteins known to be secreted by breast cancer cells is growing. There is evidence that at least some of these may be secreted in an endocrine manner in the normal, non-lactating breast.