DIVERGENT MECHANISMS OF INSULIN-LIKE GROWTH-FACTOR-I AND GROWTH-FACTOR-II ON RAT HEPATOCYTE PROLIFERATION

Insulin-like growth factors I and II are peptides with a structural ho mology for proinsulin, and are involved in hepatocyte proliferation. I GF-I and IGF-II, however, have different metabolic roles, and their me chanisms of action are incompletely known. We hypothesized that IGF-I and IGF-II act by different signal transduction pathways. To test this hypothesis, hepatocytes from 200 g male Sprague-Dawley rats were isol ated by a two-step collagenase perfusion technique and plated at a den sity of 10(5) cells/16 mm Primaria plate. Proliferation was measured b y [H-3]thymidine ([H-3]thy) incorporation into DNA, and an autoradiogr aphic nuclear labeling index (LI). To analyze signal transduction, cyc lic AMP (cAMP) levels were measured 5 min after addition of reagents b y a radioimmunoassay. Reagents (doses) used were: IGF-I (2 nM), IGF-II (2 nM), the inhibitory peptide somatostatin-14 (SS14) (10 nM), and th e adenylyl cyclase antagonist dideoxyadenosine (DDA) (10 mu M) A summa ry of the findings is as follows: (1) IGF-I stimulates [H-3]thy, LI an d cAMP accumulation. (2) IGF-II stimulates [H-3]thy and LI but not cAM P; (3) IGF-I but not IGF-II effects are inhibited by SS14 and DDA. We conclude that the hepatotrophic effects of IGF-I and IGF-II occur by d ifferent mechanisms: IGF-I is cAMP-dependent, IGF-II is cAMP-independe nt