Inhibition of Growth and Survival of Head and Neck Cancer Cells by Silibinin Involves the Down-Regulation of Erk1/2, AKT and STAT3 Signaling.

Head and neck squamous cell carcinoma (HNSCC) is one of the most prevalent and fatal cancers in India. Silibinin, a naturally occurring small molecule from milk thistle (Silybum marianum), is gaining attention as a potent anticancer agent against various cancers; however, its impact on HNSCC and the associated molecular mechanisms are largely unknown. We checked the effect of silibinin on proliferation, cell viability, and DNA damage in HNSCC cells, and employed immunoblotting to detect the underlined molecular mechanism. Also, we validated silibinin's anticancer efficacy and associated molecular changes in the xenograft mouse model. Silibinin inhibited cell proliferation and viability in HNSCC cells, and enhanced G1-S phase arrest by increasing p53 expression and inhibiting p27, p21, Cyclin D1-CDK4/6, and Cyclin E-CDK2 complexes. Silibinin-induced DNA damage and apoptosis in HNSCC cells were evidenced by comet assay, expression of p-H2AX, Bax, Bcl-2, and cleavage of caspase 3 and PARP proteins. Moreover, silibinin also impaired DNA repair pathways, ATM-Chk2, ATR-Chk1, DNA-PK, Ku70/80 and Rad51, and activated JNK contributing to DNA damage. The strong inhibition of EGFR-mediated Erk1/2, AKT and STAT3 signaling by silibinin was identified. Silibinin augmented PD98059 and LY294002-induced cell death and inhibition of pSTAT3. Silibinin inhibited Cal33 tumor growth in athymic mice model without any adverse effects. Our study revealed anticancer efficacy of silibinin in suppressing cell viability and proliferation, promoting DNA damage, apoptosis and cell cycle arrest in HNSCC. Further, oral silibinin inhibited Cal33 tumor xenograft growth. Hence, silibinin could have promising therapeutic efficacy for HNSCC.