LACTOGENIC HORMONE-INDUCIBLE PHOSPHORYLATION AND GAMMA-ACTIVATED SITE-BINDING ACTIVITIES OF STAT5B IN PRIMARY RAT LEYDIG-CELLS AND MA-10 MOUSE LEYDIG TUMOR-CELLS

The signal transducer and activator of transcription Stat5b has been i mplicated in signal transduction pathways for a number of cytokines an d growth factors, including GH and PRL. Although these lactogenic horm ones have the potential to enhance gonadotropin-induced steroidogenesi s, the role of GH and PRL in the testis has long been and remains the subject of controversy. In this report we provide, to our knowledge, t he first evidence of Stat5b protein expression in the testis and chara cterize the activation of Stat5b by these lactogenic hormones in prima ry rat progenitor, immature and adult Leydig cells, and mouse MA-10 Le ydig tumor cells. In MA-10 cells, both GH and PRL mediate tyrosine pho sphorylation of Janus kinase (JAK) 2 and Stat5b and induce DNA-binding activity of Stat5b. GK enhances both PIE (PRL-inducible element) and Fc gamma RI gamma-activated sites (GAS), but PRL modulates only PIE: G AS. In primary Leydig cells isolated from 18-day-old rats, GH, but not PRL, activates cytoplasmic Stat5b and induces the binding of transloc ated nuclear Stat5b to GAS elements. Although Stat5b protein is expres sed in both Percoll-and elutriator-purified adult rat Leydig cells, ne ither GH nor PRL treatment results in Stat5b-DNA binding. Our studies indicate that the MA-10 cell has the capacity to bind both GH and PRL and provides a useful model system with which to study the distinct te sticular roles of these hormones. Moreover, our findings suggest that progenitor and immature Leydig cells are functional targets for GH in the immature rat, suggestive of a role for GH-Stat5b in testicular dev elopment. Our data indicate that lactogenic hormone-inducible transcri ptional activation may target distinct gene expression in a signaling cascade(s) involving Stat5b but also imply coordinate control by multi ple Leydig cell factors.