Abstract 718: Significantly strong protective efficacy of silibinin against basal cell carcinoma growth and progression

Basal cell carcinoma (BCC) is the most common skin malignancy accounting for ∼80% of non-melanoma skin cancers (NMSCs). It arises in basal cells of skin that line the deepest layer of the epidermis. The most important risk factor for BCC is solar ultraviolet B (UVB) radiation that results in DNA damage (CPDs formation) in skin epidermis, which, if not repaired, leads to fixation of mutations and initiation of skin carcinogenesis. One of the key molecular features of BCC is sustained activation of Hedgehog signaling (Hh) pathway through inactivating mutations in tumor suppressor gene Patched (Ptch) or activating mutations in Smoothened (Smo). Therefore, extensive efforts have been made to target activated Hh pathway for the treatment of BCC, though with toxic side effects. With regards to chemoprevention of BCC, the agents which target the events associated with UVB-induced DNA damage repair, together with targeting promotion/progression stages, might provide more effective broad-spectrum opportunities to intervene at the earliest. Utilizing the well-established patched (Ptch)+/- mouse model of UVB radiation-induced BCC formation, our approach in this study was to target BCC development and its prevention by employing silibinin (a natural flavonolignan from milk thistle seeds). Previously, we have extensively reported the efficacy of silibinin against UVB-induced photodamage and photo-carcinogenesis, and also recently reported silibinin efficacy against BCC growth in cell culture (including anti-BCC drug-resistant cell lines) and mouse BCC allograft tumors. In this study, both male and female (Ptch)+/- mice were irradiated with 240 mJ/cm2 UVB dose, 3 times per week (M, W, F) for 26 and 46 weeks with or without silibinin. Silibinin (9 mg/200μl of acetone) was applied topically, 30 min post and pre-UVB exposure. Our study shows that chronic UVB exposure induced BCCs in Ptch+/- mice. Treatment with silibinin post and pre-UVB exposure for 26 weeks decreased BCC lesion numbers (39-65%), and cross-sectional area (45-72%) p<0.001, respectively compared to UVB alone. Furthermore, continuous UVB exposure up to 46 weeks increased the BCC lesion number by ∼6 folds and cross-sectional area by ∼3.4 folds (p<0.001) respectively. Notably, silibinin (irrespective of post and pre-UVB treatment) significantly halted the progression of BCC (81-94%, p<0.001), even upon prolonged UVB exposure. Histological analysis showed increased dysplasia, fibro-sarcoma, and squamous cell carcinoma upon UVB exposure, which significantly decreased upon silibinin treatment. In addition, mechanistic studies revealed that silibinin significantly decreased basal cell proliferation (Ki-67) and the expression of cytokeratins (14 and 15), and Hh signaling mediators Smo and Gli1 in the BCC lesions. Together, our findings demonstrate that silibinin has the potential to prevent the growth and progression of UVB-induced BCC.

Citation Format: Sandeep Paudel, Komal Raina, Vasundhara R. Tiku, Akhilendra Maurya, David J. Orlicky, Zhiying You, Cindy M. Rigby, Gagan Deep, Rama Kant, Bupinder Raina, Chapla Agarwal, Rajesh Agarwal. Significantly strong protective efficacy of silibinin against basal cell carcinoma growth and progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 718.

Chemopreventive efficacy of silibinin against basal cell carcinoma growth and progression in UVB-irradiated Ptch+/- mice

The factors (environmental and genetic) contributing to basal cell carcinoma (BCC) pathogenesis are well-established; however, effective agents for BCC prevention are marred by toxic side-effects. Herein, we assessed the efficacy of flavonolignan silibinin against ultraviolet B (UVB)-induced BCC in Ptch+/- (heterozygous patched homolog 1 gene) mouse model. Both male and female Ptch+/- mice were irradiated with a 240 mJ/cm2 UVB dose 3 times/week for 26 or 46 weeks, with or without topical application of silibinin (9 mg/200 µl in acetone, applied 30 min before or after UVB exposure). Results indicated that silibinin application either pre- or post-UVB exposure for 26 weeks significantly decreased the number of BCC lesions by 65% and 39% (P < 0.001 for both) and the area covered by BCCs (72% and 45%, P < 0.001 for both), respectively, compared to UVB alone. Furthermore, continuous UVB exposure for 46 weeks increased the BCC lesion number and the BCC area covered by ~6 and ~3.4 folds (P < 0.001), respectively. Notably, even in this 46 week prolonged UVB exposure, silibinin (irrespective of pre- or post-UVB treatment) significantly halted the growth of BCCs by 81-94% (P < 0.001) as well as other epidermal lesions; specifically, silibinin treated tissues had less epidermal dysplasia, fibrosarcoma, and squamous cell carcinoma. Immunohistochemistry and immunofluorescence studies revealed that silibinin significantly decreased basal cell proliferation (Ki-67) and the expression of cytokeratins (14 and 15), and Hedgehog signaling mediators Smo and Gli1 in the BCC lesions. Together, our findings demonstrate strong potential of silibinin to be efficacious in preventing the growth and progression of UVB-induced BCC.

Dietary Rice Bran-Modified Human Gut Microbial Consortia Confers Protection against Colon Carcinogenesis Following Fecal Transfaunation

Rice bran, removed from whole grain rice for white rice milling, has demonstrated efficacy for the control and suppression of colitis and colon cancer in multiple animal models. Dietary rice bran intake was shown to modify human stool metabolites as a result of modifications to metabolism by gut microbiota. In this study, human stool microbiota from colorectal cancer (CRC) survivors that consumed rice bran daily was examined by fecal microbiota transplantation (FMT) for protection from azoxymethane and dextran sodium sulfate (AOM/DSS) induced colon carcinogenesis in germ-free mice. Mice transfaunated with rice bran-modified microbiota communities (RMC) harbored fewer neoplastic lesions in the colon and displayed distinct enrichment of Flavonifractor and Oscillibacter associated with colon health, and the depletion of Parabacteroides distasonis correlated with increased tumor burden. Two anti-cancer metabolites, myristoylcarnitine and palmitoylcarnitine were increased in the colon of RMC transplanted mice. Trimethylamine-N-oxide (TMAO) and tartarate that are implicated in CRC development were reduced in murine colon tissue after FMT with rice bran-modified human microbiota. Findings from this study show that rice bran modified gut microbiota from humans confers protection from colon carcinogenesis in mice and suggests integrated dietary-FMT intervention strategies should be tested for colorectal cancer control, treatment, and prevention.