Skin epidermis lacking the c-myc gene is resistant to Ras-driven tumorigenesis but can reacquire sensitivity upon additional loss of the p21Cip1 gene

The target gene(s) required for Myc-mediated tumorigenesis are still elusive. Here we show that while endogenous c-Myc is surprisingly dispensable for skin homeo-stasis and TPA-induced hyperplasia, c-Myc-deficient epidermis is resistant to Ras-mediated DMBA/TPA-induced tumorigenesis. This is mechanistically linked to P21(Cip1), which is induced in tumors by the activated Ras-ERK pathway but repressed by c-Myc. Acute elimination of c-Myc in established tumors leads to the upregulation of p21(Cip1), and epidermis lacking both p21(Cip1) and c-Myc reacquires normal sensitivity to DMBA/TPA-induced tumorigenesis. This identifies c-Myc-mediated repression of p21(Cip1) as a key step for Ras-driven epidermal tumorigenesis.