LOW-DENSITY-LIPOPROTEIN RECEPTOR MESSENGER-RNA IN HUMAN BREAST-CANCERCELLS - INFLUENCE BY PKC MODULATORS

It was reported previously that low-density lipoproteins (LDL) differe ntially stimulate cell growth of hormone-responsive (ER+) and hormone- unresponsive (ER-) mammary tumor cell lines. Here we examined the mRNA levels of the LDL-receptor (LDL-R) gene with RNAse protection analysi s in ER- (MDA-MB-231 and HBL-100) and ER+ (MCF-7 and ZR75-1) cells, an d compared them with the estrogen receptor (ER) status. Measurable amo unts of ER mRNA were only found in ER+ cells as expected. LDL-R mRNA a bundance was 3-5 fold higher in ER- cells as compared to ER+ cells. In cubation with phorbol-12-myristate-13-acetate led to a significant inc rease (p < 0.005) of LDL-R mRNA in ER+ cells, whereas in ER- cells LDL -R mRNA levels remained merely unchanged. Incubation of cells with dio ctanoylglycerol, a synthetic homolog of diacylglycerol, increased LDL- R mRNA in ER+ but not in ER-. Inhibition of protein kinase C (PKC) by H7 resulted in a highly significant reduction of LDL-R mRNA both in ER + and ER- cells. PKC seems to be an important regulator of LDL-R mRNA abundance in mammary tumor cells. It is hypothesized that in human-bre ast cancer the process of conversion from hormone-responsive to hormon e-unresponsive status is accompanied by a change in PKC activity and P KC might exert cell specific differences on the regulation of LDL-R mR NA levels, which in turn influences the delivery of exogenous choleste rol to cancer cells.