Regulation of JAK2 protein expression by chronic, pulsatile GH administration in vivo: A possible mechanism for ligand enhancement of signal transduction

Growth hormone (GH) is a key factor controlling postnatal growth and development. Despite growth-promoting effects in mammals, GH is not associated with muscle growth in the chicken. Janus kinase 2 (JAK2) has been identified as the first intracellular step in GH receptor (GHR) signaling in many species, however, there is limited knowledge regarding the GH signaling pathway in the chicken. In this study, GH-responsive, JAK2 immunoreactive proteins were first assessed in an avian hepatoma cell line (LMH). Tyrosine phosphorylation of a 120-122 kDa JAK2 immunoreactive protein was GH dose-dependent. In addition to in vitro studies, the timecourse of JAK2 activation in liver and skeletal muscle (Pectoralis superficialis) in response to a single intravenous (i.v.) injection of chicken GH (cGH), and the effect of chronic exposure to GH in a physiologically relevant pattern on JAK2 protein expression and tyrosine phosphorylation in vivo were assessed. At a dose of GH that was previously demonstrated to elicit a maximal metabolic response (6.25 μg/kg BW), maximum tyrosine phosphorylation of JAK2 appeared at 10 min post-GH administration in the pectoralis muscle, but was not detectable in liver. To assess whether chronic enhancement of GH would alter expression of JAK2, we utilized a dynamic model of pulsatile GH infusion that mimicked the early pattern of circulating GH expressed in younger, rapidly growing birds (high amplitude peaks with an inter-peak interval of 90 min). A 120-122 kDa protein in liver and muscle, and a dominant 130-136 kDa protein in the muscle, that was phosphorylated in response to GH, were specifically recognized by the JAK2 antibody. Chronic, pulsatile infusion of cGH into 8-week-old chickens was associated with increased abundance and tyrosine phosphorylation of JAK2 protein in both liver and muscle (P < 0.05), which were GH dose-dependent, and mirrored previously reported biological responses for the same birds [Vasilatos-Younken, R., Zhou, Y., Wang, X., McMurtry, J.P., Rosebrough, R.W., Decuypere, E., Buys, N., Darras, V.M., Van Der Geyten, S., Tomas, F., 2000. Altered chicken thyroid hormone metabolism with chronic GH enhancement in vivo: Consequences for skeletal muscle growth. Journal of Endocrinology 166, 609-620.]. In summary (1) JAK2 immunoreactive proteins that associate with the GHR and are tyrosine phosphorylated in response to GH were identified in an avian hepatoma cell line and expressed in both GH responsive (liver) and "non-responsive" (skeletal muscle) tissues; (2) tyrosine phosphorylation of JAK2 occurred within minutes of exposure to a single i.v. injection of GH in vivo in muscle but not liver of 8-week-old birds; and 3) there were GH dose-dependent increases in abundance of JAK2 protein and tyrosine phosphorylation in both tissues when chronically exposed to GH in a physiologically relevant pattern, that mirrored dose-dependent biological responses, including alterations in the pathway of thyroid hormone metabolism, previously reported. Enhanced JAK2 suggests one possible mechanism whereby chronic, physiologically appropriate exposure to the ligand enhances GH biological action via increased abundance of a key upstream component of the signal transduction pathway.