Cryptolepine Provokes Changes in the Expression of Cell Cycle Proteins in Growing Cells

Problem statement: Cryptolepine, the antimalarial plant alkaloid is cytotoxic to mammalian cells, but there is rather little understanding of the possible mechanisms involved in the cell-kill action of cryptolepine. Approach: We examined mammalian cells exposed to cryptolepine (0.5-5.0 μM) for effects on the cell cycle and associated alterations in the levels of major regulatory proteins using immunoblotting and flow cytometry. Results: Cryptolepine (0.5-5.0 μM) caused a dose-dependent increase in the sub-G1 from 3.03-31.64% in HepG2 and from 13.80-42.82%) in MCL-5, cell lines with wild p53. Additionally, G1 fractions increased to 69.17 and 37.49% in HepG2 and MCL-5 respectively. In both cell lines, the G1 increase was associated with temporal increases in MDM2 protein, p53 protein and p53’s target cyclin-dependent kinase inhibitor, p21. Consistent with the G1 arrest was also a decrease in cyclin D₁, cyclin A and the hyperphosphorylated form of retinoblastoma protein, the key modulators of the G1 checkpoint in HepG2 cells. In cells with null-p53 (SKOV3) however, a p53-independent G2 arrest was evident. Conclusion: The study showed that, though cryptolepine is cytotoxic irrespective of the p53 status of the cell line, its effect on the cell cycle appears to depend on the p53 status of the cell line. The potent cell-kill action of cryptolepine and its ability to evoke cell cycle arrest in susceptible cells favor its anticancer potential.