CROSSTALK BETWEEN GROWTH HORMONE AND INSULIN SIGNALING

Growth Hormone (GH) is a major growth-promoting and metabolic regulatory hormone. Interaction of GH with its cell surface GH receptor (GHR) causes activation of the GHR-associated cytoplasmic tyrosine kinase, JAK2, and activation of several signaling pathways, including the STATs, ERK1/2, and PI3K pathways. Insulin is also a key hormone regulating metabolism and growth. Insulin binding to the insulin receptor (IR) results in phosphorylation/activation of the IR, and activates the PI3K/Akt and ERK1/2 pathways. Due to their important roles in growth and metabolism, GH and insulin can functionally interact with each other, regulating cellular metabolism. In addition, recent data suggests that GH and insulin can directly interact by signaling crosstalk. Insulin regulation of GH signaling depends on the duration of exposure to insulin. Transient insulin exposure enhances GH-induced activation of MEK/ERK pathway through post-GHR mechanisms, whereas prolonged insulin exposure inhibits GH-induced signaling at both receptor and postreceptor levels. Chronic excessive GH interferes with insulin's activation of the IR/IRS/PI3K pathway and several proteins are involved in the mechanisms underlying GH-induced insulin resistance. (C) 2009 Elsevier Inc.