UV-induced ablation of the epidermal basal layer including p53-mutant clones resets UV carcinogenesis showing squamous cell carcinomas to originate from interfollicular epidermis.

Chronic ultraviolet (UV) exposure induces clones of cells overexpressing mutant p53 in the interfollicular (IF) epidermis and subsequently squamous cell carcinomas (SCCs) with similar p53 mutations. Mutated p53 may give cells growth advantage over neighbouring cells by impaired apoptosis. We tested this by UV overexposure of skin laden with p53-mutant clones and assessed the impact on subsequent tumour development. P53-mutant clones were induced in two groups of hairless SKH1 mice by daily exposures (500 J/m(2) UV from TL12 lamps) for 28 days. On day 29, one group was overexposed (to 10 kJ/m(2) UV), whereas the control group received the regular daily dose. After 1 week of recovery, the daily exposures were resumed in both groups to induce SCCs. UV overexposure forced the entire IF basal layer into caspase-3-driven apoptosis while leaving overlying layers with sunburn cells intact. No apparent regions were spared from apoptosis. Pulse-chase BrdU labelling showed the IF epidermis to be repopulated from the hair follicles (remaining p63 positive). One week after overexposure, the p53-mutant clones had virtually disappeared (0.6, 95% confidence interval 0.5-0.8 per mouse versus 102, 59-179, without overexposure). Tumour development was significantly delayed after UV overexposure (P < 0.0001) by an average of 27 days (standard error of the mean 3); a period matching that of daily exposures preceding the overexposure. Thus, we found that UV-induced ablation of the IF epidermal basal layer eliminates p53-mutant clones and resets UV carcinogenesis. Furthermore, and in contrast with earlier reports, our data show that UV-induced p53-mutant clones and SCCs originate from the IF epidermis.

Rapamycin impairs UV induction of mutant-p53 overexpressing cell clusters without affecting tumor onset.

Because of its antitumor effect, the immunosuppressant rapamycin holds great promise for organ transplant recipients in that it may lower their cancer risk. In a mouse model, we showed previously that rapamycin inhibits the outgrowth of primary skin carcinomas induced by UV radiation. However, the tumors that did grow out showed an altered p53 mutation spectrum. Here, we investigated whether this shift in p53 mutations already occurred in the smallest tumors, which were not affected in onset. We found that rapamycin did not alter the mutational spectrum in small tumors and in preceding microscopic clusters of cells expressing mutant-p53. However, rapamycin did reduce the number of these cell clusters. As this reduction did not affect tumor onset, we subsequently investigated whether rapamycin merely suppressed expression of mutated p53. This was not the case, as we could demonstrate that switching from a diet with rapamycin to one without, or vice versa, did not affect the number of existing mutant-p53 expressing cell clusters. Hence, rapamycin actually reduced the formation of mutant-p53 cell clusters. In wild-type and p53-mutant mice, we could not measure a significant enhancement of UV-induced apoptosis, but we did observe clear enhancement in human skin equivalents. This was associated with a clear suppression of HIF1α accumulation. Thus, we conclude that rapamycin reduces the formation of mutant-p53-expressing cell clusters without affecting tumor onset, suggesting that tumors grow out of a minor subset of cell clusters, the formation of which is not affected by rapamycin.