Submitted to: American Journal of Physiology - Cell Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2008
Publication Date: 2/15/2008
Citation: Wong, S., Zhao, Y., Schoene, N.W., Han, C., Shih, R., Lei, K. 2008. Zinc deficiency depresses p21 mRNA and protein levels in HepG2 cells. American Journal of Physiology - Cell Physiology. 294: C1342-9. Interpretive Summary: Zinc is an essential nutrient that controls the production of many proteins critical in the regulation of cell growth and cell death. This regulation is critical to a wide range of physiological processes that range from tumor cell death to proper function of the immune system. This experiment with cultured tumor cells focused on studying the effects of severe and mild zinc deficiencies on the expression of a protein (p21) that plays an indispensable role in regulating DNA synthesis and growth. Cells were grown in media containing 0, 0.4, 4, 16, and 32 micromoles of zinc. With the two lowest amounts of zinc in the media both messenger RNA and protein for p21 were significantly decreased compared to the values observed in cells grown in 4, 16, and 32 micromoles of zinc. The results provide evidence for the critical amount of zinc required for expression of message and synthesis of the p21 protein. Increasing the available amount of zinc from 4 to 16 and then to 32 micromoles to the cells did not alter the p21 parameters measured. These data contribute important new information about dietary zinc requirements needed for optimal balance of cell proliferation versus cell death.
Technical Abstract: The influence of zinc status on p21 gene expression was examined in human hepatoblastoma (HepG2) cells. Cells were cultured for one passage in a basal medium depleted of zinc to induce severe zinc-deficient(ZD)cells or in the basal medium supplemented with 0.4, 4.0,16,or 32 µM zinc to represent mild zinc deficiency (ZD0.4), the amount of zinc in most normal media (ZN,4.0), the normal human plasma zinc level (ZA,16, zinc-adequate, ZA), or the high end of plasma zinc attainable by oral supplementation (ZS,32), respectively. The nuclear p21 level in ZD and ZD0.4 cells was markedly reduced to 40% of ZN cells. In addition, the p21 mRNA abundance in ZD and ZD0.4 cells was decreased to about 70% of ZN cells. Moreover, the p21 promoter activity in ZD and ZD0.4 cells, as measured by transient transfection of a p21 promoter reporter gene, was reduced to 64 and 66 % of ZN cells, respectively. In contrast, p21 protein and mRNA levels as well as p21 promoter activity were not altered in ZA and ZS cells as compared to ZN cells. Furthermore, the amounts of acetylated histone 4 associated with the proximal and distal p21 promoter regions, as a measure of p21 promoter accessibility, were decreased in ZD (73 and 64 %, respectively) and ZD0.4 (82 and 77 %, respectively) cells than found in ZN cells (100 and 100 %, respectively). Thus, multiple lines of evidence indicate that the transcriptional process of p21 is down-regulated by depressed zinc status in HepG2 cells. 12/21/06