Distinctive Roles for Prolactin and Growth Hormone in the Activation of Signal Transducer and Activator of Transcription 5 in Pancreatic Islets of Langerhans

Abstract

Although the beta-cells of the pancreatic islets of Langerhans express both prolactin (PRL) and GH receptors, we have observed that PRL is considerably more effective than GH in the up-regulation of islet function in vitro. This study examined whether differences in the activation of the Janus kinase 2/signal transducer and activator of transcription (STAT) 5 signaling pathway by these closely related receptors may be involved in this disparity. The activation of STAT5B by PRL was biphasic, with an initial peak within 30 min, a nadir between 1 and 3 h, and prolonged activation after 4 h. In contrast, the response to GH was transient for 1 h. The importance of the long-term activation of STAT5B by PRL was supported by the similar dose response curves for STAT5B activation and the PRL-induced increases in insulin secretion and islet cell proliferation. Because the pulsatile secretion of GH affects its actions in other target tissues, the ability of pretreatment with either hormone to affect subsequent stimulation was also examined. Surprisingly, the response to PRL was inhibited by prior exposure for less than 3 h to either PRL or GH and disappeared with a longer pretreatment with either hormone. Similar to other tissues, the response to GH was inhibited by any length of prior exposure to GH. However, pretreatment with PRL had no effect. These experiments are the first demonstration of the transient desensitization of the PRL receptor by either PRL or GH pretreatment in any tissue and the desensitization of GH stimulation in islet cells. These observations provide insight into the mechanisms that regulate the desensitization of these receptors and, more importantly, allow the long-term activation of STAT5B by the PRL receptor. These results may apply to other members of the cytokine superfamily of receptors. We also demonstrate that the increase in islet cell proliferation required continuous stimulation with PRL, whereas the smaller effect with GH occurred with either continuous or pulsatile stimulation. In summary, this study demonstrates that islets are sensitive to the temporal pattern of stimulation by these hormones and provides a new basis for understanding their physiological roles in the regulation of islet function.

STAT5 Activity in Pancreatic β-Cells Influences the Severity of Diabetes in Animal Models of Type 1 and 2 Diabetes

“…In rat insulinoma cells, stimulation by growth hormone or prolactin leads to activation of STAT5a and -5b (11,12), which are encoded by two different genes that show 96% sequence similarity at the protein level (13). Induction of insulin gene transcription and ␤-cell proliferation by growth hormone or prolactin is mediated through STAT5 activation in rat insulinoma cells (11,14).…”

Regulation of glucokinase in pancreatic islets by prolactin: a mechanism for increasing glucose-stimulated insulin secretion during pregnancy

“…The most important changes are lowering of the threshold for glucose-stimulated insulin secretion and expansion of islet mass (Parsons et al 1992). The primary factors responsible for these adaptive changes are the lactogenic hormones placental lactogen and/or prolactin (PRL; Brelje et al 1993Brelje et al , 2004. We have shown these hormones induce, both in vitro and in vivo, similar changes in islets as those observed during pregnancy (Sorenson & Brelje 1997.…”

“…PRL was used at 200 ng/ml with insulin 1 (INS-1) cells and 500 ng/ml with islets -concentrations that induce maximal activation of STAT5 (Brelje et al 2004).…”

“…The rat insulinoma INS-1 was provided by Claes Wollheim (University of Geneva, Geneva, Switzerland) and grown as previously described (Stout et al 1997, Brelje et al 2004. Experiments were done in a serum-free medium consisting of RPMI 1640 (Stout et al 1997) with 10 mM glucose supplemented to give a final concentration of 6 mM glutamine, 0 .…”

“…Western blots were prepared from INS-1 cells and islets as previously described (Brelje et al 2004). The following primary antibodies were used at a 1:2000 dilution: sheep antiglucokinase (provided by Dr Mark Magnuson, Vanderbilt University, Nashville, TN, USA), rabbit anti-PRL-r (Brelje et al 2002), rabbit anti-glucose transporter 2(Glut-2; provided by Dr Bernard Thorens, Institute of Pharmacology and Toxicology, Lausanne, Switzerland), guinea pig anti-insulin, mouse anti-a-actin (Sigma), and rabbit anti-STAT5a and STAT5b (Santa Cruz Biotechnology, Santa Cruz, CA, USA).…”