Mechanism of Action of Insulin-like Growth Factor

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Insulin-like gowth factor-I (IGF-I) is a potent growth factor which plays a pivotal role in somatic growth. We studied the mechanism of action of IGF-I using Balb/c 3T3 cells as a model system. In these cells, IGF-I promotes cell-cycle progression. In doing so, IGF-I exerts its action in a cell-cycle-dependent manner. Thus, IGF-I promotes cell-cycle progression when quiescent cells are pretreated with platelet-derived growth factor (PDGF) and epidermal growth factor (EGF). We studied the action of IGF-I using IGF-I-responsive cells, which we termed primed competent cells. In these cells, IGF-I induces sustained increase in calcium entry. This is brought about by an activation of IGF-I-sensitive calcium-permeable cation channel. When calcium entry is blocked, IGF-I is not capable of stimulating DNA synthesis. In addition, pharmacological stimulation of calcium entry results in an increase in DNA synthesis. These results let us to propose that calcium entry is an intracellular message of the mitogenic action of IGF-I. It should be mentioned that DNA synthesis is augmented in primed competent cells whereas stimulation of calcium entry does not affect DNA synthesis in quiescent cells. These results indicate that calcium-sensing machinery does not operate in quiescent cells. In addition to stimulation of calcium entry, IGF-I increases diacylglycerol (DAG) in primed competent cells. There are at least three sources of diacylglycerol. First, glycosylphosphatidylinositol, known to be a precursor of insulin-mediator inositolglycan, is hydrolyzed by IGF-I. Second, phosphatidylcholine is hydrolyzed by phospholipase C, and finally, IGF-I stimulates de novo synthesis of diacylglycerol. As a result, IGF-I causes sustained elevation of cellular DAG content. It remains unsolved whether protein kinase C is activated continuously by IGF-I.