Abstract 1010: Synergistic antitumor effects of luteolin and silibinin with overexpression of miR-7-1-3p inhibited autophagy and promoted apoptosis in glioblastoma

Glioblastoma is the most common and lethal brain tumor that mostly occurs in adults. Although the exact etiology of glioblastoma is unknown and its cure remains elusive, researchers are actively involved in understanding the biology of glioblastoma and exploring the most effective therapeutic options. Traditional therapeutic strategy against glioblastoma includes surgery, radiation, and chemotherapy. But concern regarding high systemic toxicity of conventional chemotherapy has recently prompted the search for newer drugs and plant-derived natural compounds that can prevent glioblastoma growth with least side effects and no reoccurrence. Luteolin (LUT) and silibinin (SIL) are plant-derived potent flavonoids and they are well regarded for their chemopreventive activities. LUT manifests anti-tumor effects in inhibition of autophagy, cell proliferation, metastasis, and angiogenesis and also induction of apoptosis. SIL is known to cause Bid cleavage and activate mitochondria mediated caspase cascade for inducing apoptosis in tumor cells. Transfection of tumor suppressor microRNA (miR) could enhance the efficacy of anti-tumor agents. In current investigation, we explored the synergistic anti-tumor effects of LUT and SIL in human glioblastoma cells and enhancement of their therapeutic efficacy with miR-7-1-3p transfection in vivo. We determined that combination of 20 μM LUT and 50 μM SIL could most synergistically induce cell death in human glioblastoma U87MG and T98G cells and this combination therapy was more effective than 10 μM BCNU or 100 μM temozolomide (TMZ), the commonly used chemotherapy for glioblastoma. Combination of LUT and SIL induced significant amount of apoptotic death, inhibited invasion, network formation, and spheroid formation. Further, combination of LUT and SIL most effectively inhibited rapamycin (RAPA)-induced autophagy in both glioblastoma cell lines. We found that LUT alone and also in combination with SIL could inhibit protein kinase C alpha (PKCα) signaling for suppression of RAPA-induced autophagy. Treatment with SIL alone and in combination with LUT effectively down regulated inducible nitric oxide synthase (iNOS) to activate caspase cascades for apoptosis in RAPA pre-treated cells. Our real-time qRT-PCR analysis indicated the most significant upregulation of miR-7-1-3p among the three well-known tumor suppressor miRs examined after RAPA pre-treated glioblastoma cells were subjected to combination therapy with LUT and SIL. Most interestingly, our in vivo studies confirmed the enhancement of efficacy of combination of LUT and SIL following overexpression of miR-7-1-3p due to degradation of the potent anti-apoptoptic protein XIAP in both tumors. In conclusion, our results indicated that the anti-tumor efficacy of LUT and SIL was highly augmented by miR-7-1-3p overexpression for controlling growth of human glioblastoma in vivo. Citation Format: Mrinmay Chakrabarti, Walden Ai, Swapan K. Ray. Synergistic antitumor effects of luteolin and silibinin with overexpression of miR-7-1-3p inhibited autophagy and promoted apoptosis in glioblastoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1010. doi:10.1158/1538-7445.AM2015-1010