Recent studies suggested an involvement of
thromboxane A2 in cyclooxygenase-2-dependent inhibition of steroidogenic acute regulatory (StAR) gene expression. The present study further investigated the role of
thromboxane A2 receptor in StAR gene expression and steroidogenesis in testicular Leydig cells. The
thromboxane A2 receptor was detected in several Leydig cell lines. Blocking
thromboxane A2 binding to the receptor using specific antagonist SQ29548 or
BM567 resulted in dose-dependent increases in StAR
protein and
steroid production in MA-10 mouse Leydig cells. The results were confirmed with Leydig cells isolated from rats. StAR promoter activity and StAR
mRNA level in the cells were also increased after the treatments, suggesting an involvement of the
thromboxane A2 receptor in StAR gene transcription. Furthermore study indicated that blocking the
thromboxane A2 receptor reduced
dosage sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene 1
protein, a transcriptional repressor of StAR gene expression. Specific binding of the antagonists to the receptors on cellular membrane was demonstrated by binding assays using (3)H-SQ29548 and binding competition between (3)H-SQ29548 and
BM567. Whereas SQ29548 enhanced cAMP-induced StAR gene expression, in the absence of cAMP, it was unable to increase StAR
protein and steroidogenesis. However, when the receptor was blocked by the antagonist, subthreshold levels of cAMP were able to induce maximal levels of StAR
protein expression, suggesting that blocking the
thromboxane A2 receptor increase sensitivity of MA-10 cells to cAMP stimulation. Taken together, the results from the present and previous studies suggest an autocrine loop, involving
cyclooxygenase-2,
thromboxane A synthase, and
thromboxane A2 and its receptor, in cyclooxygenase-2-dependent inhibition of StAR gene expression.