RHOX Homeobox Transcription Factor Regulation of Ins2 in Rodent Granulosa Cells

The <i>Rhox</i> family of homeobox transcription factors comprises established regulators of gonad function, but their downstream targets have been relatively elusive, particularly in the female reproductive tract. Here, we characterize <i>Ins2</i> as a downstream target of the two granulosa cell-specific factors, <i>Rhox5</i> and <i>Rhox8</i>, in the ovary. While INS2 is classically produced by islet cells in the pancreas, we found that <i>Ins2</i> gene expression is present in the mural granulosa cell layer of large antral follicles, and it was not significantly reduced in <i>Rhox5</i>-null mice. This was a surprising finding as we previously validated <i>Ins2</i> as a direct target of RHOX5 in Sertoli cells, the male counterpart to granulosa cells that serves the germ cell nurse function in the testis. In the ovary, RHOX8 appears to be the major driver of <i>Ins2</i> expression, as evidenced from the maximal activity of <i>Ins2</i> promoter reporter plasmids when RHOX8 protein was active within granulosa cells in vitro and the downregulation of endogenous <i>Ins2</i> in mice with the granulosa cell-specific knockdown of RHOX8 in vivo. RHOX5 induces <i>Rhox8</i> expression in pre-antral granulosa cells and then becomes relatively silent in peri-ovulatory follicles. However, <i>Rhox8</i> does not peak until after the ovulatory LH surge. The induction of <i>Rhox8</i> by progesterone, after the normal window of RHOX5 has passed, may explain why <i>Rhox5</i>-null female mice display apparently normal fertility, if RHOX8 is capable of the redundant stimulation of target genes that are essential for ovulation.