Lovastatin, a cholesterol biosynthesis inhibitor, has recently been shown to inhibit mitogenesis and tumor growth. We have investigated the effects of lovastatin on the activation of MAP kinase by insulin using as a model HIRcB cells, a rat fibroblast cell line that overexpresses the human insulin receptor. Treatment with lovastatin (1-30 microM) for 24 h decreased the level of activation of MAP kinase by insulin by as much as 60%. Immunoblotting experiments using a specific anti-MAP kinase monoclonal antibody demonstrated that the amount of MAP kinase protein in the cells was not altered by lovastatin treatment. Likewise, lovastatin had no apparent effects on the expression of the insulin receptor. Treatment with lovastatin (20 microM) reduced the percentage of farnesylated Ras by 50%. Immunoprecipitation of tyrosine phosphorylated proteins from HIRcB cell lysates followed by immunodetection of MAP kinase using specific antibodies demonstrated a reduced level of insulin-induced tyrosine phosphorylation levels of MAP kinase in lovastatin-treated cells. Furthermore, immunodetection of the beta-subunit of the insulin receptor in anti-phosphotyrosine immunoprecipitates revealed that treatment with lovastatin reduced the tyrosine phosphorylation levels of the receptor. Lysates obtained from cells treated with increasing concentrations of lovastatin demonstrated a dose-dependent inhibition of the insulin-induced tyrosine phosphorylation of the receptor. Treatment with mevalonic acid prevented the effects of lovastatin demonstrating that the effects of the drug are a consequence of its inhibitory effects on the synthesis of steroids. It is concluded that, in addition to inhibition of Ras farnesylation, lovastatin reduces receptor tyrosine phosphorylation levels which also contributes to the blockade of MAPK activation by the insulin receptor.