Insulin-like growth factor-I rapidly activates multiple signal transduction pathways in cultured rat cardiac myocytes

In response to insulin-like growth factor-I (IGF-I), neonatal rat cardiac myocytes exhibit a hypertrophic response, The elucidation of the IGF-I signal transduction system in these cells remains unknown, We show here that cardiac myocytes present a single class of high affinity receptors (12,446 +/- 3,669 binding sites/cell) with a dissociation constant of 0.36 +/- 0.10 nM. Two different beta-subunits of IGF-I receptor were detected, and their autophosphorylation was followed by increases in the phosphotyrosine content of extracellular signal-regulated kinases (ERKs), insulin receptor substrate 1, phospholipase C-gamma 1, and phosphatidylinositol 3-kinase. IGF-I transiently activates c-Raf in cultured neonatal cardiac myocytes, whereas A raf is activated much less than c-Raf, Two peaks of ERK activity (ERK1 and ERK2) were resolved in cardiac myocytes treated with IGF-I by fast protein liquid chromatography, both being stimulated by IGF-I (with EC50 values for the stimulation of ERK1 and ERK2 by IGF-I of 0.10 and 0.12 nM, respectively), Maximal activation of ERK2 (12-fold) and ERK1 (8.3-fold) activities was attained after a 5-min exposure to IGF-I. Maximal activation of p90 S6 kinase by IGF-I was achieved after 10 min, and then the activity decreased slowly, Interestingly, IGF-I stimulates incorporation of [H-3]phenylalanine (1.6-fold) without any effect on [H-3]thymidine incorporation. These data suggest that IGF-I activates multiple signal transduction pathways in cardiac myocytes some of which may be relevant to the hypertrophic response of the heart.