Suppression of c-Myc enhances p21^WAF1/CIP1-mediated G1 cell cycle arrest through the modulation of ERK phosphorylation by ascochlorin

Numerous anti-cancer agents inhibit cell cycle progression via a p53-dependent mechanism; however, other genes such as the proto-oncogene c-Myc are promising targets for anticancer therapy. In the present study, we provide evidence that ascochlorin, an isoprenoid antibiotic, is a non-toxic anti-cancer agent that induces G1 cell cycle arrest and p21^WAF1/CIP1 expression by downregulating of c-Myc protein expression. Ascochlorin promoted the G1 arrest, upregulated p53 and p21^WAF1/CIP1, and downregulated c-Myc in HCT116 cells. In p53-deficient cells, ascochlorin enhanced the expression of G1 arrest-related genes except p53. Small interfering RNA (siRNA) mediated c-Myc silencing indicated that the transcriptional repression of c-Myc was related to ascochlorin-mediated modulation of p21^WAF1/CIP1 expression. Ascochlorin suppressed the stabilization of the c-Myc protein by inhibiting ERK and P70S6K/4EBP1 phosphorylation, whereas it had no effect on c-Myc degradation mediated by PI3K/Akt/GSK3β. The ERK inhibitor PD98059 and siRNA-mediated ERK silencing induced G1 arrest and p21^WAF1/CIP1 expression by downregulating c-Myc in p53-deficient cells. These results indicated that ascochlorin-induced G1 arrest is associated with the repression of ERK phosphorylation and c-Myc expression. Thus, we reveal a role for ascochlorin in inhibiting tumor growth via G1 arrest, and identify a novel regulatory mechanism for ERK/c-Myc.