Objective Clinical trials have demonstrated the efficacy of indigo naturalis, a traditional Chinese medicine ingredient, against psoriasis, a skin disease characterized by keratinocyte hyperproliferation and inflammation. The present study investigates the efficacy of tryptanthrin, a bioactive compound in indigo naturalis, against non‐melanoma skin cancer (NMSC) and the signalling events involved. Methods Efficacy of tryptanthrin against NMSC was assessed using DMBA/PMA‐induced skin carcinogenesis model in Swiss albino mice. Immunostaining for PCNA and ki‐67 was used to mark proliferating cells in tissues. Haematoxylin and eosin staining and toluidine staining were employed to assess inflammation, and TUNEL assay was used to detect apoptosis in tissues. The signalling events were evaluated using Western blot, imunohistochemistry and immunofluorescence staining. MTT assay and clonogenic assay were performed to assess the viability and proliferation of cancer cells, in vitro. Results In mice, topical application of tryptanthrin suppressed skin carcinogenesis. It attenuated inflammation, impeded the proliferation of hair follicle (HF) cells and suppressed the activation of β‐catenin, a major driver of HF cell proliferation. Additionally tryptanthrin suppressed the activation of ERK1/2 and p38, both of which promote β‐catenin activation and lowered the expression of c‐Myc and cyclin‐D1. Tryptanthrin suppressed the proliferation of the human NMSC cell line, A431 and abrogated EGF‐induced activation of β‐catenin and subsequent cytoskeletal rearrangement. Conclusion The study demonstrates with molecular evidence that tryptanthrin is an effective suppressor of NMSC.

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