Epidemiological studies suggest a causal relationship of dietary polyunsaturated fatty acids (PUFA's) with the morbidity and mortality from breast cancer. In order to reveal possible underlying mechanisms of these findings, we studied the influence of n-3 and n-6 PUFA's in comparison to oleic acid on the proliferation of well characterized estrogen dependent (MCF-7, ZR-75, T-47-D) and estrogen independent (MDA-MB-231, HBL-100) breast cancer cells in culture. The cell growth inhibitory effect was related to the formation of lipid peroxidation products. Normal human skin fibroblasts served as a control. In fibroblasts, the addition of 20 micrograms/ml of exogenous fatty acids either had no effect or caused an insignificant increase of proliferation. Similar results were obtained with MCF-7 cells. In all other breast cancer cell types, n-3 long-chain PUFA's, eicosapentaenoic and docosahexaenoic acids, were the most effective fatty acids in arresting the cell growth. Alpha-linolenic and gamma-linolenic acid exerted a variable effect on cell proliferation depending on the cell line investigated. Oleic acid significantly stimulated the proliferation of hormone-independent breast cancer cells while it had no effect on the proliferation of hormone-dependent cells. Viability studies by trypan blue excretion indicated that the arrest in cell growth was not due to major cytotoxic effects. The addition of PUFA's to breast cancer cells caused a significant increase in the formation of conjugated dienes and lipid hydroperoxides in the cellular lipids; their content was significantly correlated with the capacity of arresting cell growth. In contrast, the addition of PUFA's to fibroblasts did not increase lipid hydroperoxide formation. The addition of Vitamin E to cancer cells at a concentration of 10 microM to the PUFA-supplemented medium almost completely restored cell growth. Our data indicate that PUFA's significantly interfere with cell proliferation of breast cancer cells in vitro due to the formation of oxidation products. In addition to that, there must be other factors involved, most probably related to the differential metabolism of PUFA's in tumor cells. Our findings may have some impact on treatment and prevention of breast cancer.

It was reported previously that low-density lipoproteins (LDL) differentially 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 analysis in ER- (MDA-MB-231 and HBL-100) and ER+ (MCF-7 and ZR75-1) cells, and compared them with the estrogen receptor (ER) status. Measurable amounts of ER mRNA were only found in ER+ cells as expected. LDL-R mRNA abundance was 3-5 fold higher in ER- cells as compared to ER+ cells. Incubation with phorbol-12-myristate-13-acetate led to a significant increase (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 dioctanoylglycerol, 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-breast cancer the process of conversion from hormone-responsive to hormone-unresponsive status is accompanied by a change in PKC activity and PKC might exert cell specific differences on the regulation of LDL-R mRNA levels, which in turn influences the delivery of exogenous cholesterol to cancer cells.