Neuroblastoma is a pediatric tumor that has the highest rate of spontaneous regression. But the metastatic malignant neuroblastomas keep growing and eventually kill the young children. Because immature neuroblasts of the adrenal glands in most cases lead to malignant neuroblastomas, there are ongoing endeavors for finding suitable combination of therapeutic agents for induction of differentiation and apoptosis in malignant neuroblastomas. Retinoids (retinoic acid derivatives) have been shown to inhibit growth and induce neurite outgrowth in human malignant neuroblastoma cell lines. The mechanism by which retinoids inhibit cancer cell growth has not been completely elucidated; however, retinoids have been shown to affect multiple signal transduction pathways. In the present investigation, we compared the capabilities of all-trans retinoic acid (ATRA), 13-cis retinoic acid (13-CRA), and N-(4-hydroxyphenyl) retinamide (4-HPR) at low doses in causing growth inhibition due to induction of differentiation with alterations in expression of retinoic acid receptors (RARs) and retinoid X receptors (RXRs) in human malignant neuroblastoma SK-N-BE2 and SK-N-DZ cells. Expression of RARβ was upregulated in SK-N-BE2 and SK-N-DZ cells following treatment with 1 μM of ATRA or 13-CRA for 7 days. Treatment of neuroblastoma cells with 1 μM ATRA, 1 μM 13-CRA, or 0.5 μM 4-HPR for 7 days induced neuronal differentiation with down regulation of telomerase activity. Thereafter, we examined the effects of various combinations of retinoid and flavonoid in SK-N-BE2 and SK-N-DZ cells. Treatment with a flavonoid such as 25 μM (-)-epigallocatechin-3-gallate (EGCG) or 25 μM genistein (GST) for 24 h induced more apoptosis in differentiated cells than undifferentiated cells. Western blotting showed alterations in the levels of Bax and Bcl-2 proteins resulting in an increase in Bax: Bcl-2 ratio in apoptotic cells. Treatment with the combination of retinoid and flavonoid dramatically down regulated the expression of BIRC proteins so as to promote proteolytic activities of the caspases for increasing apoptosis in neuroblastoma cells. Colorimetric assays using chromogenic synthetic peptide substrates confirmed the contribution of different proteolytic mechanisms to apoptosis. The results of this investigation suggested that retinoids induced neuronal differentiation, downregulated telomerase activity, and potentiated the action of flavonoids for increasing apoptosis in neuroblastoma cells. Therefore, combination of retinoid and flavonoid appears to be an effective therapeutic strategy for controlling the growth of human malignant neuroblastoma cells. This investigation was supported in part by the R01 grants (NS-57811 and CA-91460) from the National Institutes of Health (Bethesda, MD).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 678.