Expression of a HDL Receptor Scavenger Receptor Class B, Type I in Prostate Cancer Cells.

Abstract

The objectives of this study were to determine whether (a) prostate cancer (PCa) cells express a physiological high density lipoprotein (HDL) receptor scavenger receptor class B, type I (SR-BI), (b) SR-BI takes up cholesterol from circulating lipoproteins, and (c) vitamin E succinate, taken up by SR-BI, reduces cell proliferation. PCa is the most frequent cancer among men in the developed countries and the second leading cause of cancer death in the United States. In 2007, PCa was estimated to affect 218,890 new patients (29%) and cause 27,050 cancer deaths (9%) in the United States. Malignant tumors are reported to have greater cholesterol content than normal surrounding tissues. The adenoma has about twice of cholesterol than the remainder of the prostate. Prostate cells produce cholesterol de novo and higher contents of cholesterol in PCa indicate the deregulation of cholesterol homeostasis. PCa cells may take up cholesterol from circulating lipoproteins for cell proliferation. Breast cancer cells are known to express SR-BI, which takes up HDL cholesterol, leading to increased cell proliferation. However, role of SR-BI in PCa has not been studied extensively. To determine the expression of SR-BI protein and its localization in the normal prostate epithelial cells (PrEC) and PCa cells including LNCaP, PC-3, and DU 145 cells, western blotting and immunofluorescence was performed, respectively. To test cholesterol uptake from HDL in PrEC and PCa cells, we incubated these cells with fluorescent DiI-labeled HDL. To determine the effect of vitamin E succinate (VES) on cell proliferation, 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was performed after treating DU 145 cells with varying concentrations of VES for 2 days. MTT data were analyzed for homogeneity of variance, followed by one-way ANOVA. Western blotting results indicated that SR-BI expression was approximately 2- and 3- fold higher in LNCaP, PC-3 or DU 145 cells, compared to PrEC. SR-BI expression appeared to be localized on the plasma membranes. Fluorescence was apparent after DiI-HDL incubation in PrEC, LNCaP, and PC-3 cells, indicating that SR-BI is functional. When DU 145 cells were treated with 6.25, 12.5, 25, 50 or 100 μM VES for 2 days, cell proliferation was 97, 86, 81, 55, and 13% of the control, respectively. Cell proliferation in all groups treated with VES was significantly lower than control, except cells treated with 6.25 μM VES, and cell proliferation was not different between groups treated with 12.5 and 25 μM VES (significant level =0.05). In conclusion, elevated expression of SR-BI in the PCa than PrEC suggests that PCa cells may use SR-BI as one of the mechanisms for taking up cholesterol from circulating lipoproteins to be used for cell proliferation. Targeting SR-BI mediated cholesterol uptake may provide ways of therapeutic intervention in reducing cell proliferation of PCa. In addition, differential expression SR-BI in prostate cancer may be considered in treating with a therapeutic reagent like VES.