Steroidogenic factor-1 (SF-1) and the regulation of expression of luteinising hormone and follicle stimulating hormone b-subunits in the sheep anterior pituitary in vivo

Abstract

Luteinising hormone (LH) and follicle stimulating hormone (FSH) comprise a common a-subunit and hormone-specific β-subunit, are expressed in gonadotroph cells of the anterior pituitary and during the sheep oestrous cycle, are regulated by gonadotrophin releasing hormone (GnRH), ovarian peptides and steroids. Transcription factor steroidogenic factor-1 (SF-1) transactivates the common a-subunit and LHβ subunit in other species. We investigated whether SF-1 regulates β-subunit expression and the regulation of SF-I expression in vivo in sheep. Immunocytochemistry co-localised SF-1, LH and FSH to sheep pituitary gonadotrophs and electrophoretic mobility shift assays (EMSA) demonstrated that SF-1 bound to the LHβ promoter in vitro. No SF-1 DNA binding site was found in the FSHβ promoter. No difference in mean levels of SF-1, FSHβ, LHβ and GnRHr mRNA was measured between the luteal and follicular phases of the oestrous cycle. However SF-1 mRNA levels were correlated to those of LHβ in individual luteal phase animals ( r = 0.88, p < 0.05), when the transcription rate of LHβ was significantly higher ( p < 0.01). GnRH antagonist treatment of luteal animals did not produce a significant reduction in mRNA levels of LHβ, SF-1 and FSHβ. Administering oestradiol benzoate with GnRH antagonist reduced levels of SF-I and FSHβ mRNA from luteal values ( p < 0.005). We conclude that: (1) expression of SF-1 is not solely dependent on GnRH and is downregulated by oestradiol; (2) the LHβ transcription rate is increased during the luteal phase but is not related to levels of LHβ or SF-1 mRNA; and (3) SF-1 does not appear to directly regulate expression of FSHβ, but may regulate expression indirectly.