Conditions associated with increased growth hormone and prolactin sensitivity

Abstract

Growth Hormone (GH) and Prolactin (PRL) are critical regulators of body growth and metabolism. Secretion and actions of GH and PRL are regulated at several levels and by different factors. The biological actions of these hormones are initiated by their binding to the respective membrane bound receptors of GH and PRL (GHR and PRLR). Several hormone systems are characterized by changes in target tissue sensitivity. Key factors in hormone sensitivity include the number of particular receptors and the duration of receptor activated intracellular signals. A common theme concerning this is e.g. that tyrosine phosphorylated intracellular proteins become inactivated by tyrosine phosphatases or by proteasomal breakdown.

In this thesis a particular focus is put on two different proteins, Suppressors of Cytokine Signaling2 (SOCS2) and Tuberous Sclerosis Complex2 (TSC2) that uniquely impinge on JAK-STAT activation and on mTOR activation.

Study I. We explored the influence of SOCS2 on glucose metabolism by using a mouse model of diabetes induced by multiple low dose streptozotocin (MLDSTZ). Pancreatic islets from untreated SOCS2-/- mice appeared larger than in wild-type (WT) controls, which could explain the augmented serum insulin levels observed in SOCS2-/- mice. Pancreatic islets, derived from SOCS2-/- mice showed increased GHR and PRLR staining, which suggest a higher sensitivity to GH/PRL-STAT5 signals in SOCS2-/- than in WT-derived β-cells. Our results suggest that SOCS2 ablation can partly overcome β-cell destruction induced by MLDSTZ.

Study II and III. In these studies we aimed to investigate the relevance of PRL in two different human tumors, i.e. lymphangioleiomyomatosis (LAM) and glioblastoma (GBM), by analyzing features of the PRLR and the effect of a novel PRLR antagonist (PRLRA) in such tumors. Reduction of TSC2 (the disease causing gene in LAM) increased PRLR levels in LAM cells and PRL stimulated LAM cell proliferation; an effect that could be blocked with the PRLRA. In GBM, PRLR was detectable in cultured GBM cells as well as in tissue sections from patients with GBM. In cell culture GBM studies, PRL treatment increased STAT5 phosphorylation as well as cell invasion and both effects could be blocked by the PRLRA.

In summary, our studies indicate that the tissue sensitivity to GH/PRL is regulated by SOCS2 and TSC2 proteins. Since both SOCS2 and TSC2 have links to different disorders, an increased GH/PRL sensitivity in such conditions could play a functional role. To block an increased PRL sensitivity we have developed a novel PRLRA and demonstrated its efficacy in cell cultures.