Cryptolepine, a Plant Alkaloid, Inhibits the Growth of Non-Melanoma Skin Cancer Cells through Inhibition of Topoisomerase and Induction of DNA Damage

Topoisomerases have been shown to have roles in cancer progression. Here, we have examined the effect of cryptolepine, a plant alkaloid, on the growth of human non-melanoma skin cancer cells (NMSCC) and underlying mechanism of action. For this purpose SCC-13 and A431 cell lines were used as an in vitro model. Our study reveals that SCC-13 and A431 cells express higher levels as well as activity of topoisomerase (Topo I and Topo II) compared with normal human epidermal keratinocytes. Treatment of NMSCC with cryptolepine (2.5, 5.0 and 7.5 mu M) for 24 h resulted in marked decrease in topoisomerase activity, which was associated with substantial DNA damage as detected by the comet assay. Cryptolepine induced DNA damage resulted in: (i) an increase in the phosphorylation of ATM/ATR, BRCA1, Chk1/Chk2 and gamma H2AX; (ii) activation of p53 signaling cascade, including enhanced protein expressions of p16 and p21; (iii) downregulation of cyclin-dependent kinases, cyclin D1, cyclin A, cyclin E and proteins involved in cell division (e.g., Cdc25a and Cdc25b) leading to cell cycle arrest at S-phase; and (iv) mitochondrial membrane potential was disrupted and cytochrome c released. These changes in NMSCC by cryptolepine resulted in significant reduction in cell viability, colony formation and increase in apoptotic cell death.