Growth hormone synthesis, secretion and action - Ursus maritimus (polar bear)

Growth hormone (GH) is a peptide hormone secreted from the anterior pituitary and plays a critical role in cell growth, development, and metabolism throughout the body. GH secretion occurs episodically and is primarily under the control of two hypothalamic neuroendocrine hormones: GH-releasing hormone (GHRH), which stimulates GH secretion, and somatostatin, which inhibits GH secretion. GHRH interacts with a G protein-coupled receptor (GHRHR) in somatotroph cells to activate the cAMP signaling pathway, which leads to increased GH mRNA transcription and release. Once released into the circulation, GH binds to cell-surface GHR in target tissues such as liver, muscle, bone, and adipose tissue. Binding leads to activation of JAK2 that in turn triggers an array of signaling cascade including Raf-MEK-ERK, PI3K-Akt, and STAT5. ERK, Akt, and STAT5 all promote growth via the synthesis and secretion of IGF-1.