Mechanisms of hedgehog, calcium and retinoic acid signalling pathway inhibitors: Plausible modes of action along the MLL–EZH2-p53 axis in cellular growth control

Understanding the molecular mechanism(s) of small compounds in cellular growth control are essential for using those against the disease(s). Oral cancers exhibit a very high mortality rate due to higher metastatic potential. Aberrant EGFR, RAR, HH signalling, enhanced [Ca2+] and oxidative stress are some of the important characteristics of oral cancer. So, we target these for our study. Herein, we tested the effect of fendiline hydrochloride (FH) as an LTCC Ca2+-channel inhibitor, erismodegib (a SMO inhibitor of HH-signalling) and all-trans retinoic acid (RA) inducer of RAR signalling that causes cellular differentiation. OCT4 activating compound (OAC1) counters differentiation and induces stemness properties. Cytosine β-D arabinofuranoside (Cyto-BDA), a DNA replication inhibitor was used to reduce high proliferative capacity. Treatment of FaDu cells with OAC1, Cyto-BDA and FH increase G0/G1 population by 3%, 20% and 7% respectively, and lead to reduction of cyclin D1, CDK4/6 levels. Erismodegib arrests the cells in S-phase with reduced cyclin-E1&A1 levels, whereas RA-treatment causes G2/M phase arrest with reduced cyclin-B1. There was a decrease in the expression of EGFR and mesenchymal markers, Snail/Slug/Vim/Zeb/Twist, and increased E-cadherin expression in all the drug treatments, indicating a reduction in proliferative signal and EMT. Enhanced MLL2 (Mll4) and reduced EZH2 expression associated overexpression of p53 and p21 were traced out. We conclude that these drugs impact expression of epigenetic modifiers by modulating signalling pathways and the epigenetic modifiers then controls the expression of cell cycle control genes, including p53 and p21.