Phosphoprotein phosphatases regulate steroidogenesis by influencing StAR gene transcription

Role of protein phosphatase 2A in PTTH-stimulated prothoracic glands of the silkworm, Bombyx mori

In the present study, the roles of a major serine/threonine protein phosphatase 2A (PP2A) in prothoracicotropic hormone (PTTH)-stimulated prothoracic glands (PGs) of Bombyx mori were evaluated. Immunoblotting analysis showed that Bombyx PGs contained a structural A subunit (A), a regulatory B subunit (B), and a catalytic C subunit (C), with each subunit undergoing development-specific changes. The protein levels of each subunit were not affected by PTTH treatment. However, the highly conserved tyrosine dephosphorylation of PP2A C subunit (PP2Ac), which appears to be related to activity, was increased by PTTH treatment in a time-dependent manner. We further demonstrated that phospholipase C (PLC), Ca²⁺, and reactive oxygen species (ROS) are upstream signaling for the PTTH-stimulated dephosphorylation of PP2Ac. The determination of PP2A enzymatic activity showed that PP2A enzymatic activity was stimulated by PTTH treatment both in vitro and in vivo. Okadaic acid (OA), a specific PP2A inhibitor, prevented the PTTH-stimulated dephosphorylation of PP2Ac and reduced both basal and PTTH-stimulated PP2A enzymatic activity. The determination of ecdysteroid secretion showed that treatment with OA did not affect basal ecdysteroid secretion but did significantly inhibit PTTH-stimulated ecdysteroid secretion, indicating that PTTH-stimulated PP2A activity is involved in ecdysteroidogenesis. Treatment with OA stimulated the basal phosphorylation of the extracellular signal-regulated kinase (ERK) and 4E-binding protein (4E-BP) without affecting PTTH-stimulated ERK and 4E-BP phosphorylation. From these results, we hypothesize that PTTH-regulated PP2A signaling is a necessary component for the stimulation of ecdysteroidogenesis, potentially by mediating the link between ERK and TOR signaling pathways.

Low-dose monobutyl phthalate stimulates steroidogenesis through steroidogenic acute regulatory protein regulated by SF-1, GATA-4 and C/EBP-beta in mouse Leydig tumor cells

BACKGROUND

The ubiquitous use of dibutyl phthalate (DBP), one of the most widely used plasticizers, results in extensive exposure to humans and the environment. DBP and its major metabolite, monobutyl phthalate (MBP), may alter steroid biosynthesis and their exposure may lead to damage to male reproductive function. Low-doses of DBP/MBP may result in increased steroidogenesis in vitro and in vivo. However, the mechanisms of possible effects of low-dose MBP on steroidogenesis remain unclear. The aim of present study was to elaborate the role of transcription factors and steroidogenic acute regulatory protein in low-dose MBP-induced distruption of steroidogenesis in mouse Leydig tumor cells (MLTC-1 cells).

METHODS

In the present study, MLTC-1 cells were cultured in RPMI 1640 medium supplemented with 2 g/L sodium bicarbonate. Progesterone level was examined by I125-pregesterone Coat-A-Count radioimmunoassay (RIA) kits. mRNA and protein levels were assessed by reverse transcription-polymerase chain reaction (RT-PCR) and western blot, respectively. DNA-binding of several transcription factors was examined by electrophoretic mobility shift assay (EMSA).

RESULTS

In this study, various doses of MBP (0, 10(-9), 10(-8), 10(-7), or 10(-6) M) were added to the medium followed by stimulation of MLTC-1 cells with human chorionic gonadotrophin (hCG). The results showed that MBP increased progesterone production and steroidogenic acute regulatory protein (StAR) mRNA and protein levels. However, the protein levels of cytochrome P450scc and 3 beta-hydroxy-steroid dehydrogenase (3 beta-HSD) were unchanged after MBP treatment. EMSA assay showed that DNA-binding of steroidogenic factors 1(SF-1), GATA-4 and CCAAT/enhancer binding protein-beta (C/EBP-beta) was increased in a dose-dependent manner after MBP exposure. Western blot tests were next employed and confirmed that the protein levels of SF-1, GATA-4 and C/EBP-beta were also increased. Additionally, western blot tests confirmed the expression of DAX-1, negative factor of SF-1, was dose-dependently down regulated after MBP exposure, which further confirmed the role of SF-1 in MBP-stimulated steroid biosynthesis.

CONCLUSIONS

In conclusion, we firstly delineated the regulation of StAR by transcription factors including SF-1, GATA-4 and C/EBP-beta maybe critical mechanism involved in low-dose MBP-stimulated steroidogenesis.

Long-term hypoxia enhances cortisol biosynthesis in near-term ovine fetal adrenal cortical cells

This study was designed to determine the potential mechanism/mechanisms of previously observed enhanced fetal cortisol secretion following exposure to long-term hypoxia (LTH). Pregnant ewes were maintained at high altitude (3820 m) for approximately the last 100 days of gestation. Between the gestation days of 138 and 141, adrenal glands were collected from LTH and age-matched normoxic control fetuses. Cyclic adenosine monophosphate (cAMP), cortisol, and steroidogenic acute regulatory (StAR) protein were measured in response to adrenocorticotropic hormone (ACTH) stimulation. Cortisol responses to ACTH were also measured in the presence of the protein kinase (PKA) inhibitor H-89, proopiomelanocortin (POMC), or 22-kDa pro-ACTH. Cortisol output was higher in the LTH group compared to the control (P < .05), following ACTH treatment while the cAMP response was similar in both groups. Although PKA inhibition decreased cortisol production in both groups, however no differences were observed between groups. Western analysis revealed a significant increase in protein expression for StAR in the LTH group (P < .05, compared to control). Proopiomelanocortin and 22-kDa pro-ACTH did not alter the cortisol response to ACTH treatment. Results from the present study taken together with those of previous in vivo studies suggest that the enhanced cortisol output in the LTH group is not the result of differences in cAMP generation or PKA. We conclude that enhanced cortisol production in LTH adrenals is the result of enhanced protein expression of StAR and potential downstream signaling pathways.

Activity of retinoic acid receptor-alpha is directly regulated at its protein kinase A sites in response to follicle-stimulating hormone signaling

Retinoic acid receptor-alpha (RARA) is crucial for germ cell development in the testis, as shown by the degenerated testis in Rara gene knockout mice, which are sterile. Similarly, FSH is known to regulate Sertoli cell proliferation and differentiation, indirectly controlling the quantity of the spermatogenic output. Interestingly, FSH inhibited, via activation of FSH receptor, cAMP, and protein kinase A (PKA), the nuclear localization and transcriptional activity of RARA. Given that retinoic acid, the ligand for RARA, is known to regulate cell proliferation and differentiation, we investigated whether FSH regulates RARA by a direct posttranslational phosphorylation mechanism. Mutagenesis of serine 219 (S219) and S369 at the PKA sites on RARA to either double alanines or double glutamic acids showed that both PKA sites are important for RARA activity. The negative charges at the PKA sites, whether they are from glutamic acids or phosphorylation of serines, decreased the nuclear localization of RARA, heterodimerization with retinoid X receptor-alpha, and the transcriptional activity of the receptor. On the other hand, the double-alanine mutant that cannot be phosphorylated at the 219 and 369 amino acid positions did not respond to cAMP and PKA activation. Wild-type and double-mutant RARA interacted with PKA, but only in the presence of cAMP or FSH. These results together suggest that FSH may regulate cell proliferation and differentiation of Sertoli cells, at least partially, by directly affecting the PKA sites of RARA and controlling the transcriptional function of the receptor.

Transcript levels of five enzymes involved in cortisol synthesis and regulation during the stress response in common carp: relationship with cortisol

In this study the expression of five genes involved in cortisol synthesis and regulation in the head kidneys of common carp (Cyprinus carpio L.) has been investigated in response to 3h net confinement stress, followed by 22h recovery. Cortisol, glucose, lactate and free fatty acid levels were measured in blood plasma. StAR, P450c17a2, 3betaHSD, P450c21 and 11betaHSD2 transcript sequences were identified based on Cyprinidae homologs and quantified by real-time PCR. Results showed that the plasma cortisol level reached a peak at one hour post-stress (85-fold higher than in control) and quickly returned to normal after 4h recovery. 11betaHSD2 transcripts were for the first time identified in interrenals. Changes in cortisol levels during and after confinement were correlated in a time-delayed relationship with increase and decrease in mRNA levels of 11betaHSD2, respectively. These results suggest that cortisol may be involved in the control or activation of 11betaHSD2. StAR and P450c21 mRNA levels did not change during net confinement stress and recovery, but P450c17a2 levels were significantly increased 4 and 22h after recovery. Since plasma cortisol levels increased by 68-fold within 5min net confinement stress, it seems that transcriptional activation of this enzyme is not directly involved in acute cortisol production.

ERKs regulate cyclic AMP-induced steroid synthesis through transcription of the steroidogenic acute regulatory (StAR) gene

Cyclic AMP-dependent expression of the steroidogenic acute regulatory (StAR) protein is thought to be the controlling step for steroid production, but the mechanisms through which external signals are translated into increased transcription of the StAR gene are unknown. We demonstrate that cyclic AMP-induced steroid synthesis is dependent upon the phosphorylation and activation of ERKs and that ERK activation results in enhanced phosphorylation of SF-1 and increased steroid production through increased transcription of the StAR gene. Adenylate cyclase activation with forskolin (FSK) caused a time-dependent increase in ERK activity and translocation from cytoplasm to nucleus, which correlated with an increase in StAR mRNA levels, StAR protein accumulation, and steroidogenesis. Similarly, ERK inhibition led to a reduction in the levels of FSK-stimulated StAR mRNA, StAR protein, and steroid secretion. These effects were attributed to the finding that ERK activity is required for SF-1 phosphorylation, a transcription factor required for the regulation of StAR gene transcription. This conclusion was supported by our demonstration of an ERK-dependent increase in the binding of SF-1 from FSK-treated Y1 nuclei to three consensus double-stranded DNA sequences from the StAR promoter region. These observations suggest that the activation of ERK2/1 by increasing cAMP is an obligatory and regulated stage in the stimulation of steroid synthesis by cyclic AMP-generating stimuli.