Ascochlorin, an isoprenoid antibiotic, induces G1 arrest via downregulation of c-Myc in a p53-independent manner.

Numerous anti-cancer agents inhibit cell cycle progression via a p53-dependent mechanism; however, many of these carcinostatic substances are toxic. Here, we show that ascochlorin, an isoprenoid antibiotic, is a non-toxic anti-cancer agent that induces G1 arrest via the induction of p21(WAF1/CIP1) in a c-Myc, but not a p53, dependent manner. Ascochlorin has a broad spectrum of anti-tumor and anti-metastatic activities, but the molecular mechanism by which it inhibits cell cycle progression of cancer cells remains to be elucidated. We demonstrated that cytostatic G1 arrest by ascochlorin is mainly associated with the upregulation of p21(WAF1/CIP1), and the downregulation of c-Myc. Furthermore, we used a chromatin immunoprecipitation assay, RNA interference, and p53-deficient cells to verify that p21(WAF1/CIP1) induction by ascochlorin is related to transcriptional repression of c-Myc. Ascochlorin abolished pRB hyperphosphorylation, which resulted in the inactivation of E2F transcriptional activity. These results suggest that ascochlorin induces G1 arrest via the p53-independent suppression of c-Myc. Thus, we reveal a role for ascochlorin in inhibiting tumor growth via G1 arrest, and identify a novel regulatory mechanism for c-Myc.