BACKGROUND.
Normal human prostate accumulates the highest levels of zinc of any soft tissue in the body. In contrast, the zinc level in prostate cancer is markedly decreased from the level detected in nonprostate tissues. Despite these relationships, the possible role of zinc in the growth of normal and malignant prostate has not been determined. METHODS. Growth inhibition and various regulatory responses were investigated in two human prostate carcinoma cell lines (LNCaP and PC-3), treated with or without zinc. RESULTS. Incubation of the prostate carcinoma cell lines with physiological levels of zinc resulted in the marked inhibition of cell growth. A lower 50% inhibition of cell growth (IC 50 ) value for zinc (about 100 ng/ml) was detected in LNCaP cells, which are androgenresponsive, whereas androgen-independent PC-3 cells exhibited a higher IC 50 for zinc (about 700 ng/ml). Incubation with 1 g/ml zinc resulted in maximum inhibition of growth in both cell lines. These inhibitory effects of zinc correlated well with the accumulation of zinc in the cells. Simultaneously, cell flow cytometric analyses revealed a dramatic increase of the cell population in G2/M phase, in both LNCaP (2.3-fold vs. control) and PC-3 (1.9-fold vs. control), and a decreased proportion of cells in S phase (LNCaP, -51.4%; PC-3, -23%), indicating a G2/M phase arrest. The cell growth inhibition and G2/M arrest in these cells were accompanied by an increase in apoptosis, as demonstrated by the characteristic cell morphology and further confirmed by cellular DNA fragmentation. The specificity of zinc-induced apoptosis was identified by ethylenediamine-tetraacetic acid (EDTA)-chelation, which abolished the zinc effect on cellular DNA fragmentation. The zinc-induced G2/M phase arrest and apoptosis were accompanied by increased mRNA levels of p21 Waf1/Cip1/Sdi1 in both LNCaP (p53+/+) and PC-3 (p53-/-) cells. CONCLUSIONS. These results suggest that zinc inhibits human prostatic carcinoma cell growth, possibly due to induction of cell cycle arrest and apoptosis. There now exists strong evidence that the loss of a unique capability to retain high levels of zinc is an important factor in the development and progression of malignant prostate cells.