Endocrine Abstracts

Kisspeptins (Kp), a peptide family encoded by Kiss1 gene, and their receptor Kiss1r were first identified by their anti-metastatic actions but have emerged as key regulators of the reproductive axis, where they integrate sexual, metabolic and seasonal cues to control hypothalamic GnRH release. Recent data indicates that some actions of Kps may be effected directly at the pituitary (PIT), since Kissr1 is expressed in the PIT and Kp stimulate luteinizing hormone (LH) secretion directly at the pituitary level in various species (mouse, rat, pig, cow). Additionally, Kp have unexpectedly been found to moderately, albeit significantly, stimulate growth hormone (GH) release directly from rat and cow somatotropes. However, the potential role of Kiss1/Kiss1r in PIT cells from normal adult primates, a model very close to humans at the physiologic and genomic levels, has not yet been addressed. Here, primary PIT cell cultures from female baboons (Papio anubis) were treated for 4 h with kp-10 and release for all PIT hormones was assessed. In this model, kp-10 (10 nM) increased LH and GH release in a dose-dependent fashion; however, it did not affect the release of other hormones (FSH, ACTH, PRL, TSH). Use of specific inhibitors of distinct intracellular signaling pathways showed that Kp-10 signals through PLC, PKC, MAPK and intracellular Ca²⁺ influx, but not AC, PKA, extracellular Ca²⁺ influx or NOS to stimulate LH and GH release. Interestingly, blockade of mTOR and PI3K activity fully abolished the stimulatory effect of kisspeptin on LH, but not GH, release. Further, since sex steroids are key modulators for gonadotrope and somatotrope function, we tested the influence of estradiol (E₂) on the responsiveness of these cell types to kp-10. Although, preincubation with 2 (10 nM, 36 h pre-exposure) increased or decreased baseline LH or GH respectively, it served to enhance the relative LH- and GH-releasing effect of kp-10 alone and in combination with classical hypothalamic stimulatory peptides of LH and GH release (GnRH and GHRH), as compared to E₂-free control cultures. Taken together, our results provide the first evidence that kisspeptins may play a relevant role in stimulating LH and GH from primate somatotropes through a direct effect mediated by distinct signaling pathways, and that sex steroids may sensitize gonadotrope/somatotrope responsiveness to the direct action of kisspeptins. Support: BIO139&CTS1705; BFU2007-60180.