Synergistic Activation of Steroidogenic Acute Regulatory Protein Expression and Steroid Biosynthesis by Retinoids: Involvement of cAMP/PKA Signaling

Both retinoic acid receptors (RARs) and retinoid X receptors (RXRs) mediate the action of retinoids that play important roles in reproductive development and function, as well as steroidogenesis. Regulation of steroid biosynthesis is principally mediated by the steroidogenic acute regulatory protein (StAR); however, the modes of action of retinoids in the regulation of steroidogenesis remain obscure. In this study we demonstrate that all-trans retinoic acid (atRA) enhances StAR expression, but not its phosphorylation (P-StAR), and progesterone production in MA-10 mouse Leydig cells. Activation of the protein kinase A (PKA) cascade, by dibutyrl-cAMP or type I/II PKA analogs, markedly increased retinoid-responsive StAR, P-StAR, and steroid levels. Targeted silencing of endogenous RARα and RXRα, with small interfering RNAs, resulted in decreases in 9-cis RA-stimulated StAR and progesterone levels. Truncation of and mutational alterations in the 5′-flanking region of the StAR gene demonstrated the importance of the −254/−1-bp region in retinoid responsiveness. An oligonucleotide probe encompassing an RXR/liver X receptor recognition motif, located within the −254/−1-bp region, specifically bound MA-10 nuclear proteins and in vitro transcribed/translated RXRα and RARα in EMSAs. Transcription of the StAR gene in response to atRA and dibutyrl-cAMP was influenced by several factors, its up-regulation being dependent on phosphorylation of cAMP response-element binding protein (CREB). Chromatin immunoprecipitation studies revealed the association of phosphorylation of CREB, CREB binding protein, RXRα, and RARα to the StAR promoter. Further studies elucidated that hormone-sensitive lipase plays an important role in atRA-mediated regulation of the steroidogenic response that involves liver X receptor signaling. These findings delineate the molecular events by which retinoids influence cAMP/PKA signaling and provide additional and novel insight into the regulation of StAR expression and steroidogenesis in mouse Leydig cells.

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Stimulation of steroidogenic acute regulatory protein (STAR) gene expression by D-aspartate in rat Leydig cells

FEBS Letters ◽

10.1016/s0014-5793(99)00840-6 ◽

1999 ◽

Vol 454 (3) ◽

pp. 317-320 ◽

Cited By ~ 70

Keyword(s):

Gene Expression ◽

Leydig Cells ◽

Regulatory Protein ◽

Steroidogenic Acute Regulatory Protein ◽

Steroidogenic Acute Regulatory ◽

Star Gene ◽

Stimulation Of

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

Endocrinology ◽

10.1210/en.2008-0541 ◽

2009 ◽

Vol 150 (2) ◽

pp. 977-989 ◽

Cited By ~ 29

Author(s):

Natalie Yivgi-Ohana ◽

Noa Sher ◽

Naomi Melamed-Book ◽

Sarah Eimerl ◽

Moriah Koler ◽

...

Keyword(s):

Binding Protein ◽

Regulatory Protein ◽

Giant Cells ◽

Steroidogenic Acute Regulatory Protein ◽

Maximal Response ◽

Cis Elements ◽

Tissue Specific ◽

Steroidogenic Acute Regulatory ◽

Trophoblast Giant Cells ◽

Star Gene

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 β 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. Proximal cis-elements in the steroidogenic acute regulatory protein (Star) promoter allow versatile transcriptional regulation in tissue-specific manners and differentiation-dependent patterns of STAR expression in rodent ovary and placenta.

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