The involvement of p53 and p21 signal pathway in the G2/M cell cycle progression of zinc-supplemented normal human bronchial epithelial (NHBE) cells was examined using the small interferring RNA (siRNA) approach. Cells were cultured for one passage in a different concentration of zinc: <0.4 microM (ZD) as zinc deficient; 4 microM as normal zinc level (ZN) in culture medium; 16 microM (ZA) as normal human plasma zinc level; and 32 microM (ZS) as the high end of plasma zinc attainable by oral supplementation. Nuclear p21 protein and mRNA levels as well as promoter activity in ZS cells, but not in ZD cells, were markedly elevated to almost twofold compared with ZN control cells. G2/M blockage in ZS cells was coupled with the observation of elevated p21 gene expression. In ZS cells, the abrogation of p21 protein induction by the transfection of p21 siRNA was shown to alleviate the G2/M blockage, demonstrating the positive linkage of p21 elevation and G2/M blockage. Abolishment of the increase in p53 protein in ZS cells with transfection of p53 siRNA normalized the elevated p21 protein to a similar level as in ZN control cells, which demonstrated that the p21 induction is p53 dependent. Furthermore, the normalization of p53 protein by siRNA treatment in ZS cells alleviated cell growth depression and G2/M blockage, which demonstrated that p53 was involved in the high zinc status-induced G2/M blockage and growth depression. Thus high zinc status in NHBE cells upregulates p53 expression which in turn elevates p21 that eventually induces G2/M blockage.