Biphasic Action of Prolactin in the Regulation of Murine Leydig Tumor Cell Functions* * This study was supported by a research grant from the Sigrid Jusélius Foundation.

We investigated in this study the effects of ovine PRL on endocrine functions of cultured murine Leydig tumor cells (mLTC-1). The parameters studied were the activation of signal transduction systems involving cAMP and intracellular free Ca(2+), the expression of Janus kinase 2 (JAK2), expression and function of LH and PRL receptors (R), expression of the steroidogenic acute regulatory (StAR) protein, and stimulation of steroidogenesis. Very similar biphasic dose- and time-dependent responses of all the parameters studied were found upon PRL stimulation, comprising a fast inhibition within 24 h in response to high PRL doses (>/=30 microgram/liter), and a slow stimulation, between 48-72 h, in response to lower PRL doses (1-10 microgram/liter). In addition, extracellular Ca(2+) (1.5 mmol/liter) increased the effect of PRL on human CG (hCG)-stimulated StAR messenger RNA expression and progesterone (P) production. Importantly, the biphasic effects of PRL on LHR gene expression and hCG-mediated P production were abolished in the presence of anti-PRL antiserum, demonstrating specificity of PRL action. The PRL effects on StAR expression, and steroid and cAMP production, apparently reflect its effects on LHR function. The relevance of the PRL effects observed in mLTC-1 cells was supported by demonstration of similar PRL responses in hCG-stimulated testosterone production of isolated mouse Leydig cells. Collectively, these findings clearly demonstrate the biphasic regulatory actions of PRL, and clarify some facets of the controversial role of this hormone in Leydig cell function.