Hormonal regulation of the steroidogenic acute regulatory protein (StAR) in gonadal tissues of the Atlantic croaker ( Micropogonias undulatus)

Abstract

The steroidogenic acute regulatory protein (StAR), a member of the StAR-related lipid transfer domain (START) family, is critical to regulated steroidogenesis in vertebrates. We have isolated a cDNA encoding StAR from a well-studied model of teleost physiology, the Atlantic croaker Micropogonias undulatus. This cDNA (1204 nucleotides total length) contains an open reading frame of 858 nucleotides encoding a protein of 286 amino acids. Molecular phylogenetic analysis indicates the putative Atlantic croaker StAR protein is more closely related to StAR proteins (62–85% identity) than to the related START protein MLN-64 (28–31% identity). Green monkey kidney cells (COS-1) cotransfected with Atlantic croaker StAR and human cholesterol side chain cleavage (SCC) expression constructs are able to produce significantly more pregnenolone than cells transfected with SCC alone. StAR mRNA is detected in the Atlantic croaker head kidney by reverse transcriptase-polymerase chain reaction (RT-PCR) and in the kidney and hypothalamus in some individuals. Gonadal StAR gene expression is below the level of detection by RT-PCR in most individuals, but can be detected using fluorescent probes in quantitative RT-PCR. StAR mRNA is not detected in the Atlantic croaker brain. Six hour in vitro treatment of Atlantic croaker ovarian follicles with human chorionic gonadotropin (hCG) is insufficient to significantly alter StAR mRNA levels; however, 24 h hCG treatment induces StAR mRNA levels 17-fold over untreated controls. Neither 6 nor 24 h treatment with hCG significantly alters StAR mRNA levels in Atlantic croaker testicular minces. Likewise, 6 h in vitro treatment with estradiol, testosterone or the maturation-inducing steroid 17,20β,21-trihydroxy-4-pregnen-3-one is without effect on gonadal StAR mRNA levels.