Multiple endocrine regulation by bone morphogenetic protein system

Activities of bone morphogenetic proteins in prolactin regulation by somatostatin analogs in rat pituitary GH3 cells

Involvement of the pituitary BMP system in the modulation of prolactin (PRL) secretion regulated by somatostatin analogs, including octreotide (OCT) and pasireotide (SOM230), and a dopamine agonist, bromocriptine (BRC), was examined in GH3 cells. GH3 cells are rat pituitary somato-lactotrope tumor cells that express somatostatin receptors (SSTRs) and BMP system molecules including BMP-4 and -6. Treatment with BMP-4 and -6 increased PRL and cAMP secretion by GH3 cells. The BMP-4 effects were neutralized by adding a BMP-binding protein Noggin. These findings suggest the activity of endogenous BMPs in augmenting PRL secretion by GH3 cells. BRC and SOM230 reduced PRL secretion, but OCT failed to reduce the PRL level. In GH3 cells activated by forskolin, BRC suppressed forskolin-induced PRL secretion with reduction in cAMP levels. OCT did not affect forskolin-induced PRL level, while SOM230 reduced PRL secretion and PRL mRNA expression induced by forskolin. BMP-4 treatment enhanced the reducing effect of SOM230 on forskolin-induced PRL level while BMP-4 did not affect the effects of OCT or BRC. Noggin treatment had no significant effect on the BRC actions reducing PRL levels by GH3 cells. However, in the presence of Noggin, OCT elicited an inhibitory effect on forskolin-induced PRL secretion and PRL mRNA expression, whereas the SOM230 effect on PRL reduction was in turn impaired. It was further found that BMP-4 and -6 suppressed SSTR-2 but increased SSTR-5 mRNA expression of GH3 cells. These findings indicate that Noggin rescues SSTR-2 but downregulates SSTR-5 by neutralizing endogenous BMP actions, leading to an increase in OCT sensitivity and a decrease in SOM230 sensitivity of GH3 cells. In addition, BMP signaling was facilitated in GH3 cells treated with forskolin. Collectively, these findings suggest that BMPs elicit differential actions in the regulation of PRL release dependent on cellular cAMP-PKA activity. BMPs may play a key role in the modulation of SSTR sensitivity of somato-lactotrope cells in an autocrine/paracrine manner.

The function of bone morphogenetic proteins in the human ovary

The gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH), are of particular importance in ovarian physiology. However, FSH receptors and LH receptors are not expressed until the secondary follicle stage, indicating that initiation of follicular growth is independent of the gonadotropins. Among many intra-ovarian growth factors, many studies have shown that bone morphogenetic proteins (BMPs) play pivotal roles in regulating the early phases of follicular growth. The BMP system induces the gonadotropin system by modulating gonadotropin receptors in early-stage follicles. Interestingly, the BMP system also prevents precocious maturation of the follicle by suppressing luteinization. Signals provoked by the preovulatory LH surge eliminate BMPs, enabling luteinization to progress. Thus, the BMP system and the gonadotropin system seem to cooperate in regulating follicular development, maturation, and luteinization.

A novel antagonistic effect of the bone morphogenetic protein system on prolactin actions in regulating steroidogenesis by granulosa cells

To investigate the mechanism by which prolactin (PRL) regulates follicular steroidogenesis in the ovary, we examined the functional roles of PRL in steroidogenesis using rat oocyte/granulosa cell coculture and focusing on the bone morphogenetic protein (BMP) system. The expression of long and short forms of PRL receptor (PRLR) were detected in both oocytes and granulosa cells, and PRL effectively up-regulated PRLR expression in granulosa cells in the presence of FSH. PRL suppressed FSH-induced estradiol production and increased FSH-induced progesterone production in granulosa cells. The PRL effects on FSH-induced progesterone were blocked by coculture with oocytes, implying roles of oocyte-derived factors in suppression of progesterone production in PRL-exposed granulosa cells. In accordance with the data for steroids, FSH-induced aromatase expression was suppressed by PRL, whereas FSH-induced steroidogenic acute regulatory protein, P450scc (P450 side-chain cleavage enzyme), and 3β-hydroxysteroid dehydrogenase type 2 levels were amplified by PRL. However, forskolin- and N(6),O(2)-dibutyryl cAMP-induced steroid levels and FSH- and forskolin-induced cAMP were not affected by PRL, suggesting that PRL action on FSH-induced steroidogenesis was not due to cAMP-protein kinase A regulation. Treatment with a BMP-binding protein, noggin, facilitated PRL-induced estradiol reduction, and noggin increased PRL-induced progesterone production in FSH-treated granulosa cells cocultured with oocytes, suggesting that endogenous BMPs reduce progesterone but increase estradiol when exposed to high concentrations of PRL. PRL increased the expression of BMP ligands in oocyte/granulosa cell coculture and augmented BMP-induced phosphorylated mothers against decapentaplegic 1/5/8 signaling by reducing inhibitory phosphorylated mothers against decapentaplegic 6 expression through the Janus kinase/signal transducer and activator of transcription (STAT) pathway. In addition to STAT activation, PRL enhanced FSH-induced MAPK phosphorylation in granulosa cells, in which ERK activation was preferentially involved in suppression of FSH-induced estradiol. Furthermore, noggin treatment enhanced PRLR signaling including MAPK and STAT. Considering that BMPs suppressed PRLR in granulosa cells, it is likely that the BMP system in growing follicles plays a key role in antagonizing PRLR signaling actions in the ovary exposed to high concentrations of PRL.

Estrogen and glucocorticoid regulate osteoblast differentiation through the interaction of bone morphogenetic protein-2 and tumor necrosis factor-alpha in C2C12 cells

Imbalanced functions between osteoclasts and osteoblasts are involved in inflammatory bone damage. The clinical effectiveness of blocking TNF-alpha in treatment of active rheumatoid arthritis established the significance of TNF-alpha in the pathogenesis. In the present study, we investigated the cellular mechanism by which estrogen and glucocorticoid interact in osteoblastic differentiation regulated by BMP and TNF-alpha using mouse myoblastic C2C12 cells. The expression of estrogen receptors, (ER)alpha and ERbeta, and glucocorticoid receptor (GCR) was significantly increased by BMP-2 treatment regardless of the presence of estradiol and dexamethasone. Estradiol, but not dexamethasone, enhanced BMP-induced Runx2 and osteocalcin expression in C2C12 cells. In addition, TNF-alpha suppressed BMP-2-induced Runx2 and osteocalcin expression, and estradiol and dexamethasone reversed the TNF-alpha effects on BMP-2-induced Runx2 expression. Dexamethasone also abolished osteocalcin expression induced by BMP-2. Interestingly, BMP-2-induced Smad1/5/8 phosphorylation and Id-1 promoter activity were enhanced by estradiol pretreatment. On the other hand, dexamethasone suppressed BMP-2-induced Smad1/5/8 activation. TNF-alpha-induced SAPK/JNK activity was suppressed by estradiol, while NFkappaB phosphorylation was inhibited by dexamethasone. Of note, the inhibitory effects of TNF- on BMP-2-induced Runx2 and osteocalcin expression were reversed by SAPK/JNK inhibition regardless of the presence of estradiol. The estradiol effects that enhance BMP-2-induced Runx2 and osteocalcin mRNA expression were restored by antagonizing ER, and moreover, membrane-impermeable estradiol-BSA failed to enhance the BMP-2-induced osteoblastic differentiation. Thus, estrogen and glucocorticoid are functionally involved in the process of osteoblast differentiation regulated by BMPs and TNF-alpha. BMP-2 increases the sensitivities of ERs and GCR, whereas estrogen and glucocorticoid differentially regulate BMP-Smad and TNF-alpha signaling.

Does bone morphogenetic protein 6 (BMP6) affect female fertility in the mouse?

Bone morphogenetic protein 6 (BMP6) is a transforming growth factor beta superfamily member produced by mammalian oocytes as well as other cell types. Despite well-characterized effects of recombinant BMP6 on granulosa cells in vitro, the function of BMP6 in vivo has been ill-defined. Therefore, the effects of genetic deletion of the Bmp6 gene on female mouse fertility were assessed. The mean litter size of Bmp6(-/-) females was reduced by 22% (P < 0.05) compared to Bmp6(+/+) controls. Not only did Bmp6(-/-) females naturally ovulate 24% fewer eggs, but competence of in vitro-matured oocytes to complete preimplantation development after fertilization in vitro was decreased by 50%. No apparent effect of Bmp6 deletion on either the morphology or the dynamics of follicular development was apparent. Nevertheless, levels of luteinizing hormone (LH)/human chorionic gonadotropin (hCG)-induced transcripts, which encode proteins required for cumulus expansion (HAS2, PTGS2, PTX3, and TNFAIP6), and of epidermal growth factor-like peptides (AREG, BTC, and EREG) were lower in Bmp6(-/-) mice than in controls after administration of a reduced dose of hCG (1 IU) in vivo. LH receptor (Lhcgr) transcript levels were not significantly lower in Bmp6(-/-) granulosa cells, suggesting that BMP6 is required for processes downstream of LH receptors. To assess whether another oocyte-derived BMP, BMP15, could have BMP6-redundant functions in vivo, the fertility of Bmp15/Bmp6 double mutants was assessed. Fertility was not significantly reduced in double-homozygous mutants compared with that in double-heterozygous controls. Therefore, BMP6 promotes normal fertility in female mice, at least in part, by enabling appropriate responses to LH and normal oocyte quality. Thus, Bmp6 probably is part of the complex genetic network that determines female fertility.