Expression and localization of serum/glucocorticoid-induced kinase in the rat ovary: relation to follicular growth and differentiation

Abstract

Expression of serum/glucocorticoid-inducible kinase (Sgk), one member of an inducible serine/threonine kinase family, is induced by FSH/cAMP in rat granulosa cells cultured in defined medium. The FSH-stimulated pattern of sgk expression is biphasic, and transcriptional activation of the sgk gene depends on an intact Sp1/Sp3 binding site within the proximal promoter. To determine whether sgk was expressed in a hormone-dependent and physiologically relevant manner in vivo, the cellular levels of sgk messenger RNA (mRNA) and protein as well as the subcellular localization of this kinase were analyzed in ovaries containing follicles and corpora lutea at specific stages of differentiation. To stimulate follicular development and luteinization, hypophysectomized (H) rats were treated with estradiol (E; HE) and FSH (FSH; HEF) followed by hCG (hCG; HEF/hCG). To analyze Sgk in functional corpora lutea, PRL was administered to HEF/hCG rats, or ovaries of pregnant rats were obtained on day 7, 15, or 22 of gestation. In situ hybridization indicated that sgk mRNA was low/undetectable in granulosa cells of H and HE rats. An acute injection (i.v.) of FSH to HE rats rapidly increased sgk mRNA at 2 and 8 h. Sgk mRNA was also elevated in granulosa cells of preovulatory follicles of HEF rats and in luteal cells of HEF/hCG and pregnant rats. Northern blots and Western blots confirmed the in situ hybridization data, indicating that the amount and cellular localization Sgk protein were related to that of sgk mRNA. When the subcellular localization of this kinase was analyzed by immunohistochemistry, Sgk protein was nuclear in granulosa cells and some thecal cells of large preovulatory follicles. In contrast, Sgk protein was cytoplasmic in luteal cells as well as some cells within the stromal compartment. Intense immunostaining was also observed in oocytes present in primordial follicles, but not in growing follicles. Collectively, these results show that FSH and LH stimulate marked increases in the cellular content of Sgk, as well as dramatic changes in the subcellular distribution of this kinase. The specific nuclear vs. cytoplasmic compartmentalization of Sgk in granulosa cells and luteal cells, respectively, indicates that Sgk controls distinct functions in proliferative vs. terminally differentiated granulosa cells.