Extracellular zinc stimulates ERK-dependent activation of p21^Cip/WAF1 and inhibits proliferation of colorectal cancer cells

1. Zinc is an important trace element in the body and is involved in both the proliferation and growth arrest of many kinds of cells including colorectal epithelial cells. The aim of this study was to identify the molecular mechanism of the growth regulation of colorectal cancer cells by extracellular zinc. 2. Zinc-stimulated activation of the mitogen-activated protein kinase (MAPK) cascade was measured by immunoblotting and Elk-1 dependent trans-reporter gene expression, and zinc-stimulated p21^Cip/WAF1 activation by immunoblotting, Northern blot analysis and immunochemistry. Cell proliferation was measured by thymidine and bromodeoxyuridine (BrdU) incorporation. 3. By treating colorectal cancer cells with 100 μM ZnCl₂, MAPKs were activated in two different phases, the initial weak activation occurred within 5 min and this was followed by a stronger and more prolonged activation. 4. Zinc concomitantly activated Raf-1-MEK-MAPK kinases, and induced Elk-1 dependent trans-reporter gene expression. 5. Prolonged activation of MAPKs by 100 μM of ZnCl₂ resulted in the induction and nuclear localization of p21^Cip/WAF1 and was related to the inhibition of both thymidine and BrdU incorporations. 6. These results not only suggest the presence of a mechanism for p21_Cip/WAF1 dependent negative regulation of colorectal cancer cell growth by zinc but also suggest potential usage of zinc to control the growth of colorectal cancer cells.