| Summary: | Retinoic acid (RA), a bioactive metabolite of vitamin A, inhibits growth in a variety of cancer cells including liver cancer. It is thought that this function of RA is achieved by modulating gene expression through complexing with retinoic acid receptors (RARs) and retinoid X receptors (RXRs), two groups of zinc-finger proteins. Zinc deficiency has been shown to affect gene expression and to impair DNA binding ability of zinc-finger proteins. The hypotheses of my thesis research were: 1) sufficient cellular zinc level is important for the effectiveness of RA-induced growth inhibition in hepatocarcinoma HepG2 cells; and 2) the influence of zinc on RA-induced growth inhibition is through modulating expression or function of RARs and RXRs, which in turn affects the expression of their target genes, CYP26a1 and RARβ. The overall objective was to examine the effects of zinc on RA-induced growth inhibition in HepG2 and the possible mechanisms involved.
Zinc manipulation was achieved by culturing HepG2 cells for 6 d in low-zinc media supplemented with 0, 5, and 10 μmol/L zinc to mimic low-, adequate-, and high-zinc conditions. Growth in low-zinc media for 6 d reduced total cellular zinc and the labile intracellular pool of zinc by 29 and 86%, respectively. Treating the cells with 35 µM of RA for 12 h following zinc manipulation significantly reduced cell proliferation in all zinc-treatment groups compared to their corresponding RA control, with the greatest reduction in the high-zinc group. Cell cycle analysis showed that the proportion of cells in the S-phase was reduced by RA treatment at 24 and 72 h at all zinc levels, with the greatest reduction in cells cultured in high-zinc medium. Following growth in low-, adequate- and high-zinc medium, RA treatment elevated the abundance of RARβ and Cyp26a1 mRNA equally in all zinc-treatment groups compared to their correspondent RA controls. Growth in low zinc medium increased mRNA abundance of RXRα while RARα, RARβ, RARγ, RXRβ and RARγ were not affected. In conclusion, these results showed that increasing zinc appeared to sensitize HepG2 cells to RA-induced growth inhibition, but had no effect on RA-induced gene expression of CYP26a1 and RARβ.
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