P21Waf1/Cip1 expression by curcumin in U-87MG human glioma cells:: Role of early growth response-1 expression

Curcumin, a natural compound, is a well-known chemopreventive agent with potent anticarcinogenic activity in a wide variety of tumor cells. Curcumin inhibits cancer cell proliferation in part by suppressing cyclin DI and inducing expression of the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). Both p53-dependent and p53-independent mechanisms regulate p21(Waf1/Cip1) expression, but the mechanism by which curcumin regulates p21(Waf1/Cip1) expression remains unknown. Here, we report that transcription of the p21(Waf1/Cip1) gene is activated by early growth response-1 (Egr-1) independently of p53 in response to curcumin treatment in U-87MG human glioblastoma cells. Egr-1 is a transcription factor that helps regulate differentiation, growth, and apoptosis in many cell types. Egr-1 expression is induced by curcumin through extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK), but not the p38, mitogen-activated protein kinase (MAPK) pathways, which mediate the transactivation of Elk-1. Transient expression of Egr-1 enhanced curcumin-induced p21(Waf1/Cip1) promoter activity, whereas suppression of Egr-1 expression by small interfering RNA abrogated the ability of curcumin to induce p21(Waf1/Cip1) promoter activity. In addition, stable knockdown of Egr-1 expression in U-87MG cells suppressed curcumin-induced p21 expression. Our results indicate that ERK and JNK MAPK/Elk-1/Egr-1 signal cascade is required for p53-independent transcriptional activation of p21(Waf1/Cip1) in response to curcumin in U-87MG human glioblastoma cells.