Human selenium binding protein-1 (hSP56) inhibits anchorage-independent growth of PC-3 human prostate cancer cells and is down-regulated in primary human prostate tumor cells

Abstract

9564 Background: Reduced dietary selenium is associated with increased cancer risk, and selenium exhibits a chemopreventive effect for malignancies of several organs, including the prostate. Previously, we reported that the human selenium binding protein-1 gene, hSP56, the human homologue of a rodent protein implicated in chemoresistance, was highly expressed by slowly growing, androgen-sensitive LNCaP human prostate cancer cells but not by the more rapidly growing, androgen-insensitive PC-3 human prostate cancer cells (Cancer Research, 58:3150). Recently, Venkateswaran et al. reported that treatment of LNCaP cells with selenomethionine caused growth inhibition by G1 arrest and a marked reduction in S phase. In contrast, they saw no such effect on PC-3 cells, leading them to propose that hSP56 may mediate selenium’s growth inhibiting properties (Cancer Research, 62:540). We have now examined the effect of hSP56 expression on the anchorage-independent growth of PC-3 cells and have quantified the level of hSP56 expression in primary human prostate tumors. Methods: The ability of cells to grow in an anchorage-independent manner is a hallmark of cellular transformation, and it is an excellent in vitro assay for predicting tumorigenicity in vivo. We stably transfected PC-3 cells with an hSP56 expression vector (PC-3/hSP56 cells) or with vector alone (PC-3/vector cells) and grew them in soft agar. Primary human prostate tumors were studied by immunohistochemistry using an affinity purified highly specific rabbit antibody to hSP56. Results: When grown in soft agar, PC-3/vector cells gave rise to a large number of robust colonies, many of which were quite large. In contrast, PC-3/hSP56 cells gave rise to >10-fold fewer colonies, which were very small in size. Anchorage-dependent growth in flasks was similar for both cell lines. Using immunohistochemistry of primary prostate tumors, we found that hSP56 was expressed at high levels in normal epithelial cells but at far lower or undetectable levels in malignant cells. Conclusions: Our results suggest that reduced expression of hSP56 may play a role in the malignant phenotype of prostate cancer. No significant financial relationships to disclose.