Calcium antagonists increase the amount of mRNA for the low-density lipoprotein receptor in skin fibroblasts

Abstract

The effects of calcium antagonists on the amount of LDL-receptor (LDL-R) mRNA in cultured human fibroblasts was examined by hybridization with a fragment of LDL-R cDNA. In a 'Northern' blot the fragment hybridized to a 5,3-kilobase RNA, as expected for LDL-R mRNA. The concentration of this RNA was increased in preparations from cells that were treated with Verapamil or Diltiazem. The selectivity of the increase was established by using a probe for beta-actin mRNA. In dot-blot hybridization it was observed that the calcium antagonists cause 2-3-fold relative increase in the amount of LDL-R mRNA. The increase in the amount of LDL-R mRNA is paralleled by elevated high binding and uptake of [125J] LDL by cells pretreated with the calcium entry blocker. Studies in several types of animal models especially cholesterol-fed rabbits, have shown that calcium channel blockers can reduce the accumulation of atherogenic lesion components and thus apparently decrease the progression of lesions (Weinstein, D. B., 1988). Verapamil has been proposed to be effective in early stages of atherogenic lesion formation to decrease aortic lesion area in cholesterol fed rabbits (Sievers R., et al., 1987). There are essentially no effects of calcium antagonists on lipoprotein composition or concentrations (Trost et al., 1987). It does not appear, therefore, that calcium antagonists are working by reducing lipid levels. Interactions of calcium channel blockers, however, with lipoprotein metabolic pathways cannot be ignored as a major potential mechanism for altering cholesterol accumulation and the atherogenic state (Weinstein D. B., 1988). It could be shown that Verapamil and Diltiazem could decrease the rate of low density lipoprotein (LDL) degradation in human skin fibroblast cultures (Ranganathon S., et al., 1982) and that Verapamil increased receptor mediated clearance of LDL from cell surfaces but also caused a delay in lysosomal degradation of LDL (Stein O., et al. 1985). How does an calcium blocker lead to an increase of LDL-receptor activity in fibroblasts? We have suggested that calcium antagonists increase the LDL-receptor activity by eliciting an increase in the mRNA for LDL-receptors. It has now become possible to test this hypothesis through the use of cDNA probes for the LDL-receptor (Ma, et al., 1986). We have shown that both calcium entry blockers Verapamil and Diltiazem increase the apparent rate of the synthesis and the endocytic capacity of the LDL-receptors in human skin fibroblasts (Filipovic et al., 1986a).(ABSTRACT TRUNCATED AT 400 WORDS)