RESUMEN
Hyperinsulinemia is a likely cause of the increased cancer incidence and mortality in diabetic patients, but its role is difficult to define in vivo. Previous in vitro studies testing the mitogenic potential of insulin and its analogs provided incomplete and sometimes contradictory results. To better evaluate cancer cell responsiveness to insulin, to its analogs and to IGF-I, we measured under identical experimental conditions cell proliferation, invasiveness, and foci formation in six cancer cell lines with different insulin receptor family expression levels. The cancer cells studied have a different expression of insulin receptor (IR), its isoforms (IR-A and IR-B), and of the IGF-I receptor. The data indicate that insulin stimulates proliferation in all cancer cell lines, invasiveness in some, and foci formation in none. Cancer cell responses to insulin (and IGF-I) are not related to receptor expression levels; moreover, hormone-stimulated proliferation and invasiveness are not correlated. IGF-I is a more potent stimulator than insulin in most but not all cancer cell lines. Insulin analogs including M1 and M2 Glargine metabolites stimulate cancer cells similar to insulin. However, exceptions occur for specific analogs in particular cancer cells. In conclusion, in vitro insulin is an effective growth factor for all cancer cells but the biological response to insulin cannot be predicted on the basis of receptor expression levels. In the clinical setting, these observations should be taken in account when deciding treatment for diabetic patients who are at risk of undiscovered cancer or survivors of oncological diseases.