Steroidogenesis-inducing protein stimulates protein-tyrosine kinase activity in rat Leydig cells.

Steroidogenesis-inducing protein (SIP) isolated from human ovarian follicular fluid stimulates steroid production in Leydig cells, human luteal cells, and rat adrenal cells. In addition, SIP is a potent mitogen that stimulates the proliferation of Leydig cells from immature rats to a greater extent than do LH/hCG and other known growth factors. We have shown previously that the actions of SIP on Leydig cells are independent of the adenyl cyclase-cAMP pathway. In the present study we have explored the possibility that SIP, like many growth factors, may exert its effects by activation of tyrosine kinase(s). Stimulation of Leydig cells isolated from immature rats with SIP resulted in an increase in the tyrosine phosphorylation of proteins that were detected with phosphotyrosine-specific antibodies. The phosphorylation of a 90-kilodalton (kDa) protein band, a 65-kDa protein band, and a doublet at 140 kDa was apparent after 5 min. After 30 min, additional SIP-induced phosphotyrosine proteins were detected at 42, 44, 50, 80, 100, and 150 kDa. In addition to phosphorylation at tyrosine residues, all of the proteins isolated from SIP-stimulated cells were phosphorylated at threonine and serine residues. SIP-induced phosphoproteins recovered with phosphotyrosine-specific antibodies were found to have associated protein-tyrosine kinase activity. The major substrate for this kinase activity in vitro was a 140-kDa protein, similar to one of the major phosphotyrosine-containing proteins induced by SIP treatment of intact cells. These observations suggest that SIP influences gonadal cell steroidogenesis and proliferation, presumably by activating cellular protein-tyrosine kinase(s) as part of a phosphorylation-based signalling pathway.