Abstract 3763: Cucurbitacin B targets EGFR-mutant lung cancer cells via activation of sestrin-3
Authors: Khan Naghma , Jajeh Farah , Khan Mohammad Imran , Mukhtar Eiman , Mukhtar Hasan 
Published: July 2016

The standard therapy for advanced non-small cell lung cancer (NSCLC) is based on the presence of epidermal growth factor receptor (EGFR) mutations with a clinical response to the EGFR tyrosine kinase inhibitors. However, despite the initial efficacy of the treatments, almost all patients acquire drug resistance and develop relapse after variable periods of time. One limiting factor for the use of natural and dietary agents is that they exert their effect at high concentrations which are not physiologically attainable. Cucurbitacins are highly oxidized tetracyclic triterpenoids which are arbitrarily divided into twelve categories and are widely distributed in the plant kingdom. Cucurbitacin B is one of the most common and has the highest content in many plants. We found striking effect of cucurbitacin B (0.2-0.6 μM; 24h) on EGFR-mutant lung cancer cells. There was remarkable decrease in the viability of H1975 and H1650 EGFR-mutant lung cancer cells. However, only modest effect of cucurbitacin B occurred on the viability of normal human bronchial epithelial cells. Treatment with cucurbitacin B also resulted in the inhibition of EGFR-mutant H1650 lung cancer cells colonies in a dose-dependent manner with almost complete inhibition at two weeks of treatment. Besides other signaling pathways, over-expression of phosphatidylinositol 3-kinase (PI3K)/Akt and mammalian target of rapamycin (mTOR) is frequently observed in tumors with EGFR mutations. Therefore, inhibition of the PI3K/AKT/mTOR signaling represents a favorable approach for the treatment of lung cancer patients with EGFR mutations. Treatment with cucurbitacin B caused inhibition of PI3K/Akt/mTOR and STAT-3 signaling alongwith simultaneous activation of AMPKα levels in EGFR-mutant lung cancer cells. Sestrins are highly conserved gene family found in all multicellular organisms of the animal kingdom. It is known that there is one sestrin gene in invertebrates and three genes in vertebrates (sestrins 1, 2 and 3). The inhibition of mTOR by sestrins has an impact on many mTOR-reliant processes such as translation, cell growth and proliferation. We found that treatment with cucurbitacin B caused specific increase in the protein and mRNA expression of sestrin-3 in EGFR-mutant lung cancer cells. Mechanistically, cucurbitacin B targeted mTOR via activating cell intrinsic inhibitor sestrin-3. We found that cucurbitacin B specifically activates sestrin-3, which may have resulted in dramatic mTOR inhibition. These results were further confirmed by using sestrin-3 knockdown in cucurbitacin B-treated cells. Sestrin-3 knockdown significantly reduced the effect of cucurbitacin B on cell-viability in EGFR-mutant lung cancer cells. These findings suggest novel mechanism for the action of cucurbitacin B and suggest that it could be explored further as potential therapy for EGFR-mutant lung cancer.

Citation Format: Naghma Khan, Farah Jajeh, Mohammad Imran Khan, Eiman Mukhtar, Hasan Mukhtar. Cucurbitacin B targets EGFR-mutant lung cancer cells via activation of sestrin-3. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3763.