Calmodulin antagonists stimulate LDL receptor synthesis in human skin fibroblasts

The LDL receptor synthesis of human skin fibroblasts in the presence of the specific calmodulin antagonists trifluoperazine, condensation product of N-methyl-p-methoxyphenethylamine with formaldehyde (compound 48/80) and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide) (W-7) was studied. Labelling of cells with [35S]methionine followed by immunoprecipitation of radioactive LDL receptor protein with monospecific antibodies revealed that calmodulin antagonists caused a 3-fold increase in the radioactivity of the LDL receptor protein as compared with values found in control cells. A corresponding increase of high-affinity binding and internalization of 125I-labelled LDL was observed. The drugs did not influence the overall protein synthesis or the half-life of the LDL receptor. A concomitant suppression of cholesterol synthesis from [14C]mevalonolactone was found to be an independent effect. The calmodulin antagonist-produced stimulation of LDL receptor synthesis could not be simulated by preincubation of cells with cyclic nucleotide analogues, cholera toxin or 3-isobutyl-1-methylxanthine, known as specific effectors of adenylate cyclase and cyclic nucleotide phosphodiesterase, respectively. Modulation of calcium concentration in the incubation medium had no reproducible effect on the rate of LDL receptor synthesis. The results implicate calmodulin as an intracellular suppressor of LDL receptor synthesis in human skin fibroblasts.