Transcription of Steroidogenic Acute Regulatory Protein in the Rodent Ovary and Placenta: Alternative Modes of Cyclic Adenosine 3′, 5′-Monophosphate Dependent and Independent Regulation

Abstract

Steroid hormone synthesis is a vital function of the adrenal cortex, serves a critical role in gonadal function, and maintains pregnancy if normally executed in the placenta. The substrate for the synthesis of all steroid hormones is cholesterol, and its conversion to the first steroid, pregnenolone, by the cholesterol side-chain cleavage cytochrome P450 (CYP11A1) enzyme complex takes place in the inner mitochondrial membranes. Steroidogenic acute regulatory protein (STAR) facilitates the rate-limiting transfer of cholesterol from the outer mitochondrial membrane to CYP11A1 located in the inner organelle membranes. The current study explored the mechanisms controlling transcription of the Star gene in primary cell cultures of mouse placental trophoblast giant cells and rat ovarian granulosa cells examined throughout the course of their functional differentiation. Our findings show that the cis-elements required for Star transcription in the rodent placenta and the ovary are centered in a relatively small proximal region of the promoter. In placental trophoblast giant cells, cAMP is required for activation of the Star promoter, and the cis-elements mediating a maximal response were defined as cAMP response element 2 and GATA. EMSA studies show that placental cAMP-responsive element binding protein (CREB)-1 and activating transcription factor-2 (ATF2) bind to a -81/-78 sequence, whereas GATA-2 binds to a -66/-61 sequence. In comparison, patterns of Star regulation in the ovary suggested tissue-specific and developmental controlled modes of Star transcription. During the follicular phase, FSH/cAMP induced CREB-1 dependent activity, whereas upon luteinization STAR expression becomes cAMP and CREB independent, a functional shift conferred by FOS-related antigen-2 displacement of CREB-1 binding, and the appearance of a new requirement for CCAAT enhancer-binding protein beta and steroidogenic factor 1 that bind to upstream elements (-117/-95). These findings suggest that during evolution, the promoters of the Star gene acquired nonconsensus sequence elements enabling expression of a single gene in different organs, or allowing dynamic temporal changes corresponding to progressing phases of differentiation in a given cell type.

The Involvement of Specific PKC Isoenzymes in Phorbol Ester-Mediated Regulation of Steroidogenic Acute Regulatory Protein Expression and Steroid Synthesis in Mouse Leydig Cells

“…To obtain molecular insight into these mechanisms, the functional involvement of CREB and c-Jun/c-Fos in PKD signaling was assessed, because these transcription factors have been demonstrated to be instrumental in StAR gene transcription (23,28). MA-10 cells were transfected with either 100 nM of a negative control siRNA or 50 nM of two PKD (100 nM total) specific siRNAs (Fig.…”

“…Noteworthy, whereas PKC-mediated induction of steroid synthesis is remarkably low compared with PKA signaling, it modulates gonadotropin and/or cAMP/PKA-stimulated steroidogenic responsiveness (24 -26) and thus may play important roles in various adrenal and gonadal steroidogenic functions. Transcription of the StAR gene is influenced by multiple DNA regulatory elements, including cAMP response-element (CRE)-binding protein (CREB)/CRE modulator and activator protein-1 (AP-1, Fos/Jun) (18,23,27,28). CREB-binding protein (CBP) and its functional homolog, p300, are transcriptional coactivators, which interact with a variety of transcription factors and thus play critical roles in gene regulation (23,29).…”

Effect of the metabolic environment at key stages of follicle development in cattle: focus on steroid biosynthesis

“…The rate-limiting step in steroidogenesis is the transport of cholesterol from the outer to the inner mitochondrial membrane, where it is converted to pregnenolone by cytochrome P450 cholesterol side chain cleavage complex. STAR, a mitochondrial phosphoprotein, has been identified as the critical cholesterol transport protein (14,75) and has been extensively studied across numerous species (10,83,85). In bovine follicles, the expression of STAR is restricted to the theca interna (3,73); however, it is acquired in granulosa cells following luteinization (3,61).…”

The Steroidogenic Acute Regulatory Protein (StAR)

“…In steroidogenic cells, C/EBPα and C/EBPβ isoforms are detected and C/EBPβ expression is increased by cAMP-mediated mechanisms in mouse testicular Leydig and ovarian granulosa cells [136][137][138]. The functional C/EBP element in the StAR promoter is highly conserved and C/EBPβ in MA-10 Leydig cell, rodent granulosa-luteal cell, and human granulosa-luteal cell nuclear extracts was shown to bind to the mouse and human StAR proximal promoter [139][140][141]. Protein-protein interactions between C/EBPβ and SF-1 and C/ EBPβ and GATA-4 have been proposed to contribute to Leydig and granulosa cell StAR gene expression, respectively.…”

“…StAR expression and steroid production are important during both phases and StAR gene regulation switches from a cAMP-dependent to a cAMP-independent mechanism. The switch is mediated by an AP-1 family member replacing CREB at the CRE/AP-1 element and recruitment of C/EBPβ to its functional element upstream of the CRE/AP-1 site [141]. ChIP analysis using mouse granulosa-luteal cells showed that C/EBPβ was recruited to the StAR promoter with no apparent increase in CREB/CREM association, indicating a tissue-specific difference in StAR regulation.…”