Synergistic efficacy of a novel combination therapy controls growth of Bcl-xL bountiful neuroblastoma cells by increasing differentiation and apoptosis

Abstract

Neuroblastoma is the most prevalent extracranial solid tumor mainly in pediatric patients. We explored the efficacy of the combination of 2[(3-[2,3-dichlorophenoxy]propyl)amino]ethanol (2,3-DCPE, a small molecule inhibitor of the anti-apoptotic protein Bcl-xL) and N-(4-hydroxyphenyl) retinamide (4-HPR, a synthetic retinoid) in inducing differentiation and apoptosis in human malignant neuroblastoma cells. Immunofluorescence confocal microscopy and flow cytometry showed that the highest level of Bcl-xL expression occurred in SK-N-DZ cells followed by SH-SY5Y and IMR-32 cells. Combination of 20 μM 2,3-DCPE and 1 μM 4-HPR acted synergistically in decreasing viability of SK-N-DZ and SH-SY5Y cells. In situ methylene blue staining and protein gel blotting showed the efficacy of this combination of drugs in inducing neuronal differentiation morphologically and also biochemically with upregulation of the neuronal markers such as neurofilament protein (NFP) and neuron specific enolase (NSE) and downregulation of the differentiation inhibiting molecules such as N-Myc and Notch-1 in SK-N-DZ and SH-SY5Y cells. Annexin V-FITC/PI staining showed the synergistic action of this combination therapy in increasing apoptosis in both cell lines. Protein gel blotting manifested that combination therapy increased apoptosis with downregulation of the anti-apoptotic proteins Bcl-xL, Bcl-2 and Mcl-1 and upregulation of the pro-apoptotic proteins Bax, p53, Puma (p53 upregulated modulator of apoptosis), and Noxa, ultimately causing activation of caspase-3. In conclusion, our results appeared highly encouraging in advocating the use of 2,3-DCPE and 4-HPR as a novel combination therapy for increasing both differentiation and apoptosis in human malignant neuroblastoma cells having Bcl-xL overexpression.