3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase is the major rate-limiting enzyme in sterol and non-sterol isoprenoid synthesis. Isoprenoids are involved in the mechanisms of cell proliferation and transformation leading notably to crucial post-translational maturation of small G-proteins of the Ras superfamily. HMG-CoA reductase is among the most highly regulated enzymes. It is controlled by several feedback regulation mechanisms induced by sterol and non-sterol metabolites. The present results show that tyrosine kinase activity is also involved in the regulation of HMG-CoA reductase activity in the human breast cancer cell line SKBR-3. Incubation of SKBR-3 cells with the tyrosine kinase inhibitor, herbimycin A, induces a concentration-dependent reduction of HMG-CoA reductase activity with an IC50 of 80nM. The inhibition of HMG-CoA reductase activity by herbimycin A is also time-dependent. A similar effect of herbimycin A was obtained on the steady-state level of the HMG-CoA reductase protein. The effect of herbimycin A is probably specific as it abolished the stimulation of reductase activity by epidermal growth factor. To elucidate the molecular basis of the inhibition of HMG-CoA reductase activity and protein level by herbimycin A, we performed experiments to study the metabolic turnover of this enzyme using [35S]methionine and [35]cysteine. Herbimycin A (1 microM) did not have any significant effect on the rate of HMG-CoA reductase protein degradation but did affect its rate of synthesis and mRNA levels. The decrease in protein synthesis rate correlates with the lower reductase protein level but is more pronounced than the decrease in mRNA levels. Taken together, the results reveal a novel pathway of regulation of HMG-CoA reductase expression and activity by cellular tyrosine kinase activities.