Abstract
Autophagy is a catabolic process for recycling of cellular contents in response to metabolic stress in malignant tumors. We explored efficacy of the synthetic retinoid N-(4-hydroxyphenyl) retinamide (4-HPR) and the isoflavonoid apigenin (APG) in the serum-starved human malignant neuroblastoma cells. Combination of 0.5 μM 4-HPR and 50 μM APG synergistically decreased cell viability in the serum-starved neuroblastoma SH-SY5Y, SK-N-BE2, and IMR-32 cells. Acridine orange (AO) staining and LC3 II upregulation showed that serum-starvation for 12 and 24h progressively increased the formation of acidic vesicular organelles (AVO) and autophagy in SH-SY5Y cells. Further, AO staining and flow cytometry showed blockage of formation of AVO and accumulation of auophagic population, respectively, following the treatment of the serum-starved SH-SY5Y cells with combination of 0.5 μM 4-HPR and 50 μM APG. Combination therapy downregulated autophagy inducing proteins such as Beclin 1, LC3 II, TLR-4, and Myd88 while upregulated autophagy inhibitory p-Akt/mTOR singaling pathway. Consistent with the hypothesis that inhibition of autophagy could induce apoptosis, we noticed inhibition of autophagy and induction of apoptosis in the serum-starved SH-SY5Y cells with the suppression of the survival factor NF-κB, upregulation of pro-apoptotic Bax, downregulation of anti-apoptotic Bcl-2, activation of caspase-3, and degradation of poly(ADP-ribose) polymerase (PARP) after combination therapy. Collectively, combination of 4-HPR and APG worked synergistically to suppress autophagy and promote apoptosis in human malignant neuroblastoma cells.