THE INFLUENCE OF ZINC STATUS ON AKT SIGNALING PATHWAY IN HUMAN NORMAL PROSTATE EPITHELIAL CELLS AND HUMAN MALIGNANT PROSTATE CELLS
Lei, David Kai Y.
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Akt is known for promoting tumorigenesis through cellular proliferation. Supra-physiologic levels of zinc has been shown to stimulate the phosphorylation of Akt (p-Akt), which is frequently detectable in prostate tumors. Zinc content of malignant prostate epithelial cells is substantially lower than that of the surrounding normal epithelial cells. The influence of physiologic level of zinc on cell cycle progression via phosphoinositide-3-OH-kinase (PI3K)/Akt signaling pathway was examined in human normal prostate epithelial cells (PrEC) and human prostate malignant LNCaP cells. These cells were selected because of their susceptibility to zinc uptake and ability to express wild-type phosphatase and tensin homolog (PTEN) gene, the tumor suppressor responsible for blocking PI3K/Akt signaling. As a downstream effector of Akt, Mdm2 can be phosphorylated and translocated into the nucleus, subsequently promoting the ubiquitin-dependent degradation of tumor suppressor p53 protein. p-Akt can also affect cell cycle progression by phosphorylating p21, which restricts p21's nuclear entry to induce cell cycle arrest. Cells were cultured for 6 d in low-zinc growth medium added with 0 (zinc-deficient; ZD), 4 (zinc-normal; ZN), 16 (zinc-adequate; ZA), or 32 (zinc-supplemented; ZS) micromol/L of zinc. The effects of zinc on intracellular zinc status and cell cycle progression were determined by atomic absorption spectrophotometry and flow cytometry, respectively. Cytoplasmic and nuclear levels of p-Akt, p-PTEN, p-Mdm2, p53, and p21 proteins were analyzed by Western blotting. In addition, the dependence of zinc-induced Akt phosphorylation on the modulation of p-Akt, p-Mdm2, p53, and p21 protein levels was ascertained by using a PI3K/Akt inhibitor LY294002. Cellular zinc status of PrEC was more readily altered in a dose-dependent manner than LNCaP cells. In both cells, p-Akt was higher in ZD than ZN cells and both levels were normalized to that of ZN cells by LY294002. p-PTEN was higher in ZD than ZN-PrEC. Nuclear p-Mdm2 was higher in PrEC, while nuclear p53 was depressed in both PrEC and LNCaP cells by zinc deficiency. Nuclear p21 was unaffected in ZD-PrEC, but it was depressed in ZD-LNCaP cells. Nuclear p21 was higher in ZA and ZS than ZN-PrEC which coincided with faster G2/M progression. With LY294002, nuclear p21 protein was elevated in all groups, which correlated with an inhibition of G1/S cell cycle progression. Hence, zinc may affect cell cycle through Akt-Mdm2-p53 signaling axis in normal versus Akt-p21 in malignant prostate cells.