Ultraviolet (UV)-caused photodamage is the primary etiological factor for non-melanoma skin cancers (NMSCs), the most common malignancy in the United States. Better preventive strategies are urgently required to reduce UV-caused photodamage as well as the incidence of NMSCs. The tumor suppressor gene p53 is considered the most important cellular transcriptional factor for preserving genomic stability, and plays a major protective role against UVB-induced photodamage in epidermal cells. Mutated p53 patches are an early molecular biomarker and considered one of the most important characteristics of UVB-induced skin carcinogenesis. Earlier, in vitro and in vivo studies have shown that the natural flavonolignan silibinin activates p53 as well as reduces photodamage and NMSCs; but whether silibinin channels its protective effects primarily through p53 remains unknown. Herein, we generated p53 heterozygous (p53+/−) and p53 null (p53−/−) mice on SKH-1 hairless mouse background to investigate this issue. In wild-type SKH-1 mice, topical application of silibinin (9 mg/200 μl acetone) pre- or post- UVB (180 mJ/cm2) irradiation significantly reduced the formation of CPDs (cyclobutane pyrimidine dimers) after 12 h, while CPD repair by silibinin was significantly compromised in p53−/− mice. In the skin carcinogenesis study, wild type (p53+/+) and heterozygous (p53+/-) SKH-1 hairless mice were exposed to UVB (90 mJ/cm2, 5 times per week), while a subset was administered silibinin topically (9 mg/200 μl acetone) pre- or post-UVB irradiation. Mice were sacrificed at various time-points (4, 8, and 25 weeks) and skin samples were collected and analyzed. Analyses for proliferation marker (PCNA) in the early time-points (4 and 8 weeks) showed that the proliferation rate was higher in p53+/- mice compared to p53+/+ mice; furthermore, the strong inhibitory effect of silibinin on PCNA was evident in p53+/+ mice but compromised in p53+/- mice. At the end of the 25 weeks, p53+/- mice developed skin tumors earlier than p53+/+ mice; and p53+/- mice showed higher tumor number, multiplicity, and volume as compared to p53+/+ mice. Results also showed that silibinin treatment significantly reduced the tumor number, multiplicity and volume in p53+/+ mice but its protective efficacy was significantly compromised in p53+/- mice. In the case of p53+/+ mice, silibinin treatment (pre- or post-UVB) significantly reduced tumor volume in irradiated animals (5.3 and 18.6 mm3/per mouse compared to 109.5 mm3/per mouse in UVB-alone irradiated group; P<0.001) accounting for 95% and 83% reduction in tumor volume respectively. While in p53+/- mice, UVB irradiation induced significantly larger tumors (503.7 mm3), in the case of silibinin pre- and post-treatment the tumor volume was reduced to 293 mm3 (P<0.05 versus UVB-alone) and 425 mm3 accounting for only 42% and 15% reduction respectively. These results not only reestablished previous observations that absence of p53 allele significantly predisposes animals to photodamage and photocarcinogenesis, but also showed for the first time that silibinin mediates its protection against UVB-induced photodamage and photocarcinogenesis in a p53-dependent manner. These mechanistic studies support the use of silibinin as chemopreventive agent in NMSCs.

Citation Format: Cynthia Tilley, Srirupa Roy, Gagan Deep, Chapla Agarwal, Rajesh Agarwal. Silibinin exerts its protective effects against UVB-induced photodamage and skin carcinogenesis through p53. [abstract]. In: Proceedings of the Eleventh Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2012 Oct 16-19; Anaheim, CA. Philadelphia (PA): AACR; Cancer Prev Res 2012;5(11 Suppl):Abstract nr A85.