Growth Hormone–Mediated Janus Associated Kinase–Signal Transducers and Activators of Transcription Signaling in the Growth Hormone–Resistant Potassium-Deficient Rat

ABSTRACT. Potassium deficiency (KD) is associated with severe growth failure, in part caused by growth hormone (GH) resistance. This study set out to determine whether the resistance could be caused by a defect in GH-mediated janus associated kinase–signal transducers and activators of transcription (STAT) signaling as occurs in uremia. To this end, rats were fed a K-deficient diet for 8 d and pair-fed controls received a K-replete diet. Animals from each group received GH or vehicle, and during this period, KD rats were GH resistant; GH induced body and liver weight gain and linear body growth were severely attenuated in these rats. In addition, signal transduction was studied in the liver of rats that were killed 10 or 15 min after an intravenous GH bolus or vehicle. When the rats were killed, GH receptor mRNA and protein levels were similar in the two groups. The abundance of STAT5, STAT3, and STAT1, proteins that mediate GH signaling, was significantly increased by 40 to 130% in KD. Furthermore, GH induced a far greater increase in STAT5 and STAT3 phosphorylation in this group. STAT5 phosphorylation was enhanced fourfold even when normalized for total STAT5 content. Phosphorylated STAT5 and STAT3 proteins were also increased in nuclear extracts, suggesting normal nuclear translocation of the activated signaling proteins. DNA binding of nuclear STAT5 was unaltered. Thus, in KD, there is resistance to the growth-promoting action of GH despite hyperactivation of the janus associated kinase–STAT signaling pathway. This suggests the presence of a defect distal to the nuclear binding of STAT or, alternatively, a defect in a STAT-independent GH-activated signaling pathway.