REGULATION OF 3-HYDROXY-3-METHYLGLUTARYL COENZYME-A REDUCTASE GENE-EXPRESSION IN FRTL-5 CELLS .2. DOWN-REGULATION BY V-K-RAS ONCOGENE

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity and mRNA levels were significantly reduced in FRTL-5 cells transformed wi th the Kirsten-Moloney sarcoma virus (KiMol); these cells have lost th yrotropin dependence and express high levels of p21(ras). FRTL-5 cells , transformed with a temperature-sensitive mutant of the v-K-ras oncog ene (Ats cells: 33 degrees C, permissive; 39 degrees C, nonpermissive) , showed significant reduction of HMG-CoA reductase expression when ex posed to 33 degrees C. In KiMol cells, as well. as in Ats cells at 33 degrees C, the transcription driven by cAMP-responsive element was pro bed by measuring chloramphenicol acetyl transferase (CAT) levels after transfection with a chimeric plasmid containing the reporter gene lin ked to the rat reductase promoter, Basal CAT activity in KiMol cells t ransfected with wild-type promoter was lower than in FRTL-5 cells but was increased by forskolin to the levels attained in thyrotropin-stimu lated FRTL-5 cells, Forskolin failed to increase CAT activity in KiMol cells transfected with the plasmid harboring a reductase promoter in which the cAMP-responsive element octamer was mutated to a nonpalindro mic sequence. The effect of v-K-ras could be mimicked in FRTL-B cells by tetradecanoyl phorbol acetate and reverted in RiMol and Ats cells, expressing active Ras protein, by increasing intracellular cAMP and/or by protein kinase C inhibition. The data are consistent with the cont ention that v-K-ras, through protein kinase C and depletion of intrace llular cAMP, is inhibitory for the protein kinase A pathway. This is t he first demonstration that active v-K-ras down-regulates HMG-CoA redu ctase expression.