Submitted to: American Journal of Physiology - Cell Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/19/2008
Publication Date: 6/11/2008
Citation: Wong, S., Shih, R., Schoene, N.W., Lei, K.Y. 2008. Zinc Induced G2/M Blockage is p53 and p21 Dependent in Normal Human Bronchial Epithelial Cells. American Journal of Physiology - Cell Physiology. Interpretive Summary: Zinc is an essential nutrient necessary for many critical cellular processes, and deficiencies can lead to many health problems. On the other hand, when present in excess, zinc acts as a stressor to produce cellular responses that promote improper cell growth. Experiments described in this report used normal lung cells in culture to study the effects of different amounts of zinc on the expression of two proteins (p21 and p53) that play critical roles in controlling cellular growth. Cells were grown in media containing 0.4, 4, 16, and 32 micromoles of zinc. With the highest amount of zinc (32 micromoles) messenger RNA and protein for these two proteins were significantly increased compared to the values observed in cells grown in 0.4, 4, and 16 micromoles of zinc and these increases correlated with a block in the cell growth cycle. These novel findings provide evidence that supplementation with high amounts of zinc can create imbalances in cellular responses that produce adverse effects on the growth of normal cells. The results contribute critical new information to nutritionists and other researchers in health-related areas as they underline the importance of the potential for problems that can arise from over-exposure to zinc.
Technical Abstract: The involvement of the 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 siRNA approach. Cells were cultured for one passage in different concentrations of zinc: <0.4 microM (ZD) as zinc-deficient; 4 microM as normal zinc level (ZN) in culture medium; 16 microM (ZA) as the 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 2-fold when compared to 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.