Inhibition of proteoglycan synthesis induces an increase in follicle stimulating hormone (FSH)-stimulated estradiol production by immature rat Sertoli cells

Alteration of cell membrane proteoglycans impairs FSH receptor/Gs coupling and ERK activation through PP2A-dependent mechanisms in immature rat Sertoli cells

BACKGROUND

During the pre-pubertal life, the cessation of Sertoli cell proliferation and the onset of differentiation are associated with a shift in the FSH-mediated signaling leading to inhibition of the ERK-mitogenic pathway and to a concomitant sensitization of cAMP/PKA pathway.

METHODS

To highlight the role of cell proteoglycans (PGs) in the shift of FSH signaling, both FSH-induced cAMP production and ERK1/2 inactivation were studied in untreated and sodium chlorate PG-depleted cultured Sertoli cells from 20day-old rats.

RESULTS

Depletion of cell membrane PGs by sodium chlorate reduced FSH-, but not cholera toxin-stimulated cAMP production as well as basal ERK phosphorylation through an okadaic acid (OA)-sensitive mechanism. Involvement of PP2A was further substantiated by a marked decrease in membrane- associated PP2A activity under SC conditions and by the OA-induced restoration of PKA-dependent ERK inactivation in SC-treated cells.

CONCLUSIONS

In 20-day-old rat Sertoli cells, transmembrane cell PGs, through tethering/activation of PP2A activity exerts regulatory control on both FSH receptor/Gs coupling and ERK phosphorylation.

GENERAL SIGNIFICANCE

Besides their antiproliferative roles, cell PGs such as syndecan-1, could be involved in the increase in cAMP response to FSH occurring in Sertoli cells at the time of transition between proliferative and differentiated states.

Inhibition of glycosaminoglycan incorporation influences collagen network formation during cartilage matrix production

To understand cartilage degenerative diseases and improve repair procedures, we investigate the influence of glycosaminoglycans (GAGs) on cartilage matrix biochemistry and functionality. Bovine articular chondrocytes were cultured in alginate beads with(out) para-nitrophenyl-beta-d-xyloside (PNPX) to inhibit GAG incorporation into newly formed proteoglycans. As expected, GAG deposition in alginate beads decreased with increasing PNPX concentration. Next to GAGs, collagen deposition and cross-linking also decreased. In the presence of PNPX, GAGs and collagen were deposited further away from the chondrocyte than in the control and increased amounts were found in the culture medium. These changes resulted in decreased functional properties of the construct. We conclude that in our culture system, intact proteoglycans play a role in deposition of collagen and thus the formation of a functional matrix. The effect of less proteoglycans on the collagen network could explain why cartilage repair is ineffective in osteoarthritis and help us with development of new therapies.

Alterations in proteoglycan synthesis selectively impair FSH-induced particulate cAMP-phosphodiesterase 4 (PDE4) activation in immature rat Sertoli cells

FSH-induced upregulation of cAMP-PDE4 activities was decreased in cultured Sertoli cells when alteration of cell proteoglycans (PGs) metabolism was simultaneously induced either by para-nitrophenyl beta-d-xyloside (PNPX) or by sodium chlorate. This effect was restricted to the particulate PDE4 activities and its timing was consistent with the half-life of Sertoli cell PGs. It did not result from alterations in Pde4d variants expression, the major FSH-regulated PDE4 in Sertoli cells. Moreover, lack of changes in the particulate levels of major immunoreactive 75 kDa and 90 kDa PDE4D proteins, corresponding likely to short PDE4D1 and long PDE4D3/D8/D9 isoforms respectively, suggested that the decrease in FSH-stimulated of PDE4 activities in chlorate- and PNPX-treated cells at the end of the 24-h incubation period resulted from the increased reversal of the activated particulate PDE4(D) activities back to unstimulated levels. By controlling FSH-stimulated particulate PDE4 inactivation through a still unknown mechanism (sustained activation of PKA or reduction of phosphoprotein phosphatase activities), cell PGs could be involved in the alteration of cAMP response to FSH accompanying the transition of Sertoli cells from proliferative to non-proliferative differentiated state.

Expression of the cAMP-phosphodiesterase PDE4D isoforms and age-related changes in follicle-stimulating hormone-stimulated PDE4 activities in immature rat sertoli cells

Major changes in the cAMP-dependent signal transduction pathway triggered by FSH take place during transition of rat Sertoli cells from proliferative to the quiescent/terminally differentiated state. Using Sertoli cell cultures isolated from 10-, 20-, and 30-day-old rats, we recorded a specific increase in PDE4 activity in both the soluble and particulate subcellular fractions of 20-day-old Sertoli cells, which also displayed the highest cAMP response to FSH and the highest FSH-induced increase in PDE4 activity in both subcellular compartments. RT-PCR and immunoblotting experiments showed that almost all the PDE4D isoforms, known as the main cAMP-regulated rolipram-sensitive PDE in Sertoli cells, were expressed throughout the early postpartum period, whereas only the short PDE4D isoforms (PDE4D1 and PDE4D2) were transcriptionally regulated by FSH. Unexpectedly, the immunoblot data also revealed that the soluble PDE4 activities were mainly related to the long PDE4D isoforms and that short PDE4D1 was predominantly particulate. The subcellular distribution and expression of PDE4D proteins were unaffected by the developmental status of the Sertoli cells. Only the expression of short PDE4D1 appeared to be upregulated by FSH and only in 20-day-old Sertoli cells, which suggests phenotype-dependent differential regulation of Pde4d1 mRNA translation. Resensitization of the cAMP response to FSH in 20-day-old Sertoli cells was also associated with the highest FSH-induced transient increase in both soluble and particulate PDE4 activities, which suggests developmental changes in the PKA-mediated upregulation of the catalytic activities of long PDE4D. Such alterations may be involved in the phenotype-dependent alterations in FSH receptor coupling with its associated G proteins in rat Sertoli cells.

The testicular-derived Sertoli cell: cellular immunoscience to enable transplantation

There is a renewed enthusiasm for the potential of cellular transplantation as a therapy for numerous clinical disorders. The revived interest is largely due to the unprecedented success of the "Edmonton protocol," which produced a 100% cure rate for type I diabetics following the transplantation of human islet allografts together with a modified immunosuppressive regimen. While these data provide a clear and unequivocal demonstration that transplantation is a viable treatment strategy, the shortage of suitable donor tissue together with the debilitating consequences of lifelong immunosuppression necessitate a concerted effort to develop novel means to enable transplantation on a widespread basis. This review outlines the use of Sertoli cells to provide local immunoprotection to cografted discordant cells, including those from xenogeneic sources. Sertoli cells are normally found in the testes where one of their functions is to provide local immunologic protection to developing germ cells. Isolated Sertoli cells 1) engraft and self-protect when transplanted into allogeneic and xenogeneic environments, 2) protect cografted allogeneic and xenogeneic cells from immune destruction, 3) protect islet grafts to reverse diabetes in animal models, 4) enable survival and function of cografted foreign dopaminergic neurons in rodent models of Parkinson's disease (PD), and 5) promote regeneration of damaged striatal dopaminergic circuitry in those same PD models. These benefits are discussed in the context of several potential underlying biological mechanisms. While the majority of work to date has focused on Sertoli cells to facilitate transplantation for diabetes and PD, the generalized ability of these unique cells to potently suppress the local immune environment opens additional clinical possibilities.

Regulation of glypican-1, syndecan-1 and syndecan-4 mRNAs expression by follicle-stimulating hormone, cAMP increase and calcium influx during rat Sertoli cell development

In seminiferous tubules, Sertoli cells provide structural and nutritional support for the developing germinal cells. Cell- to-cell signaling and cell adhesion require proteoglycans expressed at the cell membrane. A preliminary biochemical and structural approach indicated that cell surface proteoglycans are mostly heparan sulfate proteoglycans (HSPG). Glypican-1, syndecans-1 and -4 were identified using a molecular approach. Their differential regulation was demonstrated in immature rat Sertoli cells. Follicle-stimulating hormone (FSH) is the main regulator of Sertoli cell function. Signal transduction triggered by FSH involves both an increased intracellular cAMP synthesis and a calcium influx. This study demonstrates that FSH, through its second messengers (increase in intracellular cAMP and intracellular calcium), downregulated the glypican-1 mRNA expression in Sertoli cells from 20-day-old rats. On the other hand, syndecan-1 mRNA expression is not modulated by FSH as it would result from the antagonistic effects of increased intracellular cAMP and intracellular calcium levels. Finally, syndecan-4 mRNA expression is not regulated by this pathway. The present study was extended during Sertoli cell development. Indeed, Sertoli cells undergo extensive changes during the postnatal period both in structure and function. These important transformations are critical for the establishment of spermatogenesis and development of the adult pattern of testicular function. Our data indicated that the regulation of HSPG mRNA expression is HSPG-specific and depends on the Sertoli cell developmental stage.

Protein kinase C regulation of glypican-1, syndecan-1 and syndecan-4 mRNAs expression during rat Sertoli cell development

Our previous studies indicated that cell surface proteoglycans were mostly heparan sulfate ones (HSPG) in 20 day-old Sertoli cells [Biochim. Biophys. Acta 1510 (2001) 474]. Among these HSPG, glypican-1, syndecans-1 and -4 mRNAs were expressed and differentially regulated. Glypican-1 and syndecan-1 mRNA expression was up-regulated under PKC activation in contrast to syndecan-4 mRNA expression which was not affected [Biochim. Biophys. Acta 1474 (2000) 31]. Rat Sertoli cells undergo extensive changes during the postnatal period both in structure and function, as the hematotesticular barrier establishment occurs at around 20 day-old. The testicular PKCalpha expression in developing Sertoli cells results in (i) a soluble (inactive) form which is maximal at the age of 1 day and declines gradually thereafter and (ii) a particulate (active) form which is low at birth, increases six-fold on days 8-11 of age and declines thereafter. The present study focused on the glypican-1, syndecan-1 and syndecan-4 mRNA expression and regulation under PKC activation by the phorbol myristate acetate (PMA) in 10-30 day-old Sertoli cells. Our data indicated that the regulation of their expression specifically depends on the nature of HSPG and Sertoli cell developmental stage and evidenced a specific PKC regulation of HSPG mRNA expression.

Cell surface heparan sulfate proteoglycans: target and partners of the basic fibroblast growth factor in rat Sertoli cells

Basic fibroblast growth factor (bFGF) regulates diversified biological functions in rat Sertoli cells. This report demonstrates that bFGF inhibits steroidogenesis in developing rat Sertoli cells. Follicle stimulating hormone (FSH)-stimulated estradiol production was reduced by bFGF. Moreover, the amount of cytochrome P450 aromatase, responsible for the irreversible transformation of androgens into estrogens, is decreased by bFGF at the transcriptional level. The bFGF inhibitory effect was also observed in the presence of dibutyryl-cAMP, cholera toxin or RO-20-1724, all inducing high levels of cAMP, the second messenger of FSH. Heparan sulfate proteoglycans (HSPGs) were shown to be required as cofactors for bFGF signaling. Indeed, sodium chlorate, described to drastically decrease proteoglycan sulfation, abolishes the bFGF downregulation of FSH-stimulated estradiol synthesis previously observed. Glypican-1, syndecan-1 and -4, potential bFGF coreceptors, are mainly regulated at the transcriptional level. This report shows that the bFGF regulation of their expression specifically depends on the nature of HSPG and of the Sertoli cell developmental stage. In conclusion, HSPG are partners and the target of bFGF in rat Sertoli cells.

Biochemical characterization of integral membrane heparan sulfate proteoglycans in Sertoli cells from immature rat testis

(35)S-Radiolabeled cultured Sertoli cells from immature rat testis were extracted with detergent and the different proteoheparan sulfate (HSPG) forms of the extract were discriminated and quantified on the basis of their high anionic charge, hydrodynamic size, lipophilic properties, susceptibility to trypsin and phosphatidylinositol phospholipase C (PI-PLC). Trypsin released 50% of total cellular HSPG corresponding to 80% of total hydrophobic HSPG. Trypsin-accessible HSPG were presumed to be integral membrane species. Trypsin-resistant HSPG, probably intracellular, distributed into non-lipophilic (37.5%) and lipophilic (12.5%) populations. Biochemical analysis of PG copurified with plasma membrane confirmed the existence of hydrophobic HSPG integrated into this structure. Among hydrophobic HSPG accessible to trypsin, 35% were PI-PLC released and radiolabeled by [(3)H]inositol indicating that about one third of integral membrane HSPG were intercalated into the plasma membrane through a phosphatidylinositol anchor (glypican type). PI-PLC-resistant forms represented HSPG inserted into the membrane through a hydrophobic segment of the core protein (syndecan type). No lipophilic PG was present in other cell compartments (culture medium, cell periphery, extracellular matrix). (125)I-Iodinated hydrophobic HSPG were deglycanated and submitted to SDS-polyacrylamide gel electrophoresis. In the glypican family, a core protein (64--65 kDa) was detected, whereas in the syndecan family, bands of 60 and 68 kDa were observed which may correspond to self-association of different core proteins. In Sertoli cell, specific functional attributes of different integral membrane HSPG forms remain to be investigated.

Expression of glypican-1, syndecan-1 and syndecan-4 mRNAs protein kinase C-regulated in rat immature Sertoli cells by semi-quantitative RT-PCR analysis

In seminiferous tubules, Sertoli cells provide structural and nutritional support for the developing germinal cells. Cell to cell signalization and cell adhesion require proteoglycans expressed at the cell membrane. A preliminary biochemical and structural approach indicated that cell surface proteoglycans are mostly heparan sulfate (HSPG) in immature rat Sertoli cells. The present study focused on the qualitative and quantitative expression of three membrane HSPG, syndecan-1, syndecan-4 and glypican-1 in Sertoli cells of 20-day-old rat. A semi-quantitative multiplex RT-PCR strategy was developed to appreciate the effect of PKC activation on the mRNA expression of the three HSPG. Our data show that the syndecan-1 and glypican-1 mRNA expression is increased by the phorbol myristate acetate (PMA) suggesting a regulation of their expression by the phosphatidyl inositol pathway, as previously hypothesized (Fagen et al., Biochim. Biophys. Acta, 1472 (1999) 250-261). In addition, a physiological effector of the PKC as ATP gave similar effects. Thus, this over-expression could be related with paracrine factors secreted by germ cells.

Activation of protein kinase C increases proteoglycan synthesis in immature rat Sertoli cells

In order to determine the signal transduction pathways involved in the regulation of proteoglycan (PG) synthesis in immature rat Sertoli cells (SC), we have examined the effect of the tumor promoter phorbol ester PMA (phorbol myristate acetate) on [35S]sulfate and [3H]glucosamine incorporation into PG molecules neosynthesized by cultured rat SC. PMA induced a dose- and time-dependent stimulation of labeled cell-associated PG as determined by quantitative solid phase assay. The overall effect of PMA resulted from enhancement of both glycosylation and catabolism of cell PG, this latter effect leading to a drastic decrease of their residence time in the membrane. Besides these quantitative effects, activation of protein kinase C by PMA induced qualitative changes as reflected by increase in relative proportion of heparan sulfate PG (HSPG) in cell membrane PG. In light of our previous results suggesting an inverse relationship between PG synthesis and FSH responsiveness in immature rat Sertoli cells, the PMA-induced upregulation of cell membrane PG, and particularly HSPG, could constitute one mechanism involved in the repression of FSH-stimulated steroidogenesis induced by PKC activation.

Sources of oestrogen in the testis and reproductive tract of the male

The cytochrome P450 aromatase (P450arom) is the terminal enzyme responsible for the irreversible transformation of androgens into oestrogens and is present in the endoplasmic reticulum of various tissues throughout at least the phylum of vertebrates. The CYP 19 gene is unique and its expression is regulated in a tissue and more precisely in a cell-specific fashion via the alternative use of several promoters located in the first exons. The P450arom has been immunolocalized in germ cells of the mouse, brown bear and rooster. According to age, aromatase activity has been measured in immature and mature rat Leydig cells as well as in Sertoli cells, whereas in the pig, ram and human aromatase is mainly present in Leydig cells. In the adult rat testis, four complementary approaches (RTPCR, in situ hybridization, immunocytochemistry and the tritiated water assay) demonstrate that not only somatic cells but also mature germ cells represent a source of oestrogen synthesis. Taking into account the widespread distribution of oestrogen receptors (ER alpha & ER beta) in testicular cells and the genital tract of the male on the one hand, and the cross-talk between sex steroids and growth factors, and between membrane receptors and nuclear receptors for steroids on the other hand, it is anticipated that understanding of the pathophysiological roles of these 'female' hormones in the male will advance understanding of the hormonal regulation of male reproductive function. One of the future goals is to define oestrogen-targeted genes in the male gonad and indeed, a lot of work is now focused on this specific area in order to clarify the role of oestrogens in the reproductive tract of the male as well as to elucidate the regulation of aromatase gene expression.

Role of proteoglycans on testosterone synthesis by purified Leydig cells from immature and mature rats

In order to characterize an involvement of proteoglycans (PG) in the regulation of Leydig cell function, we have examined the effects of para-nitrophenyl-beta-D-xyloside (PNPX), a specific inhibitor of PG synthesis and para-nitrophenyl-beta-D-galactoside (PNPG), an inefficient structural analogue, on testosterone production by purified Leydig cells from immature and mature rats, in the presence or not of various concentrations of hCG during 24 h. Whatever the age, the addition of PNPX induces a decrease of [35S] and [3H] incorporations into cell layer associated-PG; these latter being less numerous (-50 and -25%, respectively in immature and mature rat), and less sulfated (-40%) when compared to control Leydig cells. In immature Leydig cells, the inhibition of PG synthesis decreases both the basal and weakly stimulable-hCG or -(Bu)2cAMP or -LH testosterone synthesis. In mature Leydig cells, the PG inhibition has no effect on testosterone production both in the absence of hCG and in the presence of weak amounts of hCG but increases it in the presence of subsaturating hCG concentrations. Whatever the age, the inhibition of PG synthesis is ineffective in the presence of saturating amounts of either hCG or (Bu)2cAMP. These effects are maintained in the presence of MIX, PMA, but are not observed in the presence of 22R-hydroxycholesterol. Therefore, our results suggest that in rat Leydig cells, the inhibition of PG synthesis affects the signal transduction at a step distal to cyclic AMP and more precisely, the cholesterol supply to the mitochondria by acting on its cellular distribution (free and esterified cholesterol).

The human follitropin alpha-subunit C terminus collaborates with a beta-subunit cystine noose and an alpha-subunit loop to assemble a receptor-binding domain competent for signal transduction

FSH is a member of the pituitary/placental glycoprotein hormone family including luteinizing hormone, thyroid-stimulating hormone, and chorionic gonadotropin. These heterodimeric hormones share a common alpha-subunit and a highly homologous but distinct beta-subunit. The determinant loop of the FSH beta-subunit acts both as a specificity discriminator and as an essential receptor-binding site. The three-dimensional structure of hCG illustrates the proximity of the determinant loop to the carboxyl-terminal residues of the common alpha-subunit. Thus, site-directed mutagenesis was used to mak high-resolution substitutions at this carboxyl-terminal locus. The effects of those substitutions were studied. Twelve single mutations and one composite mutation were made of the region of hFSH alpha 74-92, each residue substituted by alanine. Side chain replacement in this region of FSH proved to be detrimental to binding. hFSH with mutations of either alpha S85A, alpha T86A, alpha K91A, or alpha S92A only retained 10% or less of the hFSH receptor-binding activity, while compared to these, mutants alpha H79A, alpha Y88A, and alpha Y89A retained slightly more binding activity. The single mutant alpha F74A and composite mutant alpha V76A/E77A binding activity was reduced to half of that of wild-type (WT) hFSH. In contrast, mutations of either alpha K75A, alpha T80A, alpha H83A, or alpha H90A did not adversely affect receptor binding, demonstrating the specificity of observed effects. The hFSH and mutant hormones were tested in an in vitro bioassay for stimulation of progesterone production. Those mutations that did not affect receptor binding (alpha K75A, alpha T80A, alpha H83A, and alpha H90A) did not affect signal transduction, measured by progesterone responses. After comparison of wild-type and mutant hFSH activities determined in radioreceptor assays (ID50) and in vitro bioassays (ED50), it became evident that signal transduction correlated with receptor binding.

Evidence that heparin binding autocrine factors modulate testosterone production by the adult rat Leydig cell

Androgen production by adult rat Leydig cells is stimulated by pituitary LH but can also be modulated in vitro by paracrine stimulatory and inhibitory factors, many of which belong to growth factor families. Their actions are mediated through cell surface or extracellular matrix proteoglycans and the aim of this study was to determine the role of cell surface heparan sulfate proteoglycans in the regulation of testosterone secretion by adult rat Leydig cells. The presence of sodium chlorate (25 mM) and protamine sulfate (10 micrograms/ml) inhibited testosterone production by LH stimulated cells by over 50%, but had no effect on unstimulated cells. The LH responsiveness and testosterone production returned to normal after these agents were removed from the culture media. No significant difference in LH receptor numbers at the end of the culture period was seen between sodium chlorate treated and untreated cells. Testosterone production by dibutryl-cAMP stimulated Leydig cells was also inhibited by sodium chlorate. The addition of heparin inhibited testosterone production by LH stimulated cells in a dose-dependent manner, however, in unstimulated Leydig cells heparin stimulated testosterone production to up to 50% of that seen in LH stimulated cells. These data suggest that cell surface heparan sulfate proteoglycans modulate testosterone production by adult Leydig cells in vitro, and that this may involve the autocrine actions of heparin binding growth factors on the Leydig cells.