Purpose: Resistance to anticancer drugs is a common cause of failure in chemotherapy. We recently found that adherent cells growing under prolonged periods (7-12 days) of serum starvation become highly resistant to conventional anticancer drugs such as Paclitaxel, Hydroxyurea and Colchicine but retain sensitivity to therapeutic levels of the cardiac glycoside Digitoxin.
Objectives: The main goal of this study is to characterize the biological properties of cells growing under prolonged periods of serum starvation (PPSS), and to elucidate the underlying mechanism of chemoresistance to Paclitaxel and sensitivity to Digitoxin with particular emphasis on Sox2 and the PI3K/AKT signaling pathway.
Experimental setup: Distribution of cell cycle was characterized by flow cytometry. Cell viability was assessed using the MTT assay. Western blotting was employed to evaluate the expression of several key protein markers of cancer stemness (Sox2, Wnt), multidrug resistance (MDR1, ABCG2), epithelial mesenchymal transition (Vimentin), and cell survival (Bcl-2, XIAP, PI3K/AKT).
Results: Cells grown under PPSS showed a higher proportion of cells in G1/S and G2 compared to cells growing under routine culture conditions (RCCs). Western blot analysis showed that these cells expressed higher levels of Sox2, MDR1, ABCG2, Bcl-2 compared to cells growing under RCCs. Pretreatment of cells for 3 h with Verapamil or Sorafenib (MDR1 and ABCG2 inhibitors, respectively) followed by co-incubation with Paclitaxel for 72 h did not overcome Paclitaxel resistance. Obatoclax (a Bcl-2 inhibitor), Wortmannin or LY294002 (PI3K inhbitors) also failed to overcome Paclitaxel resistance. It was interesting to observe that cells growing under PPSS were highly resistant to Obatoclax, Wortmannin and LY294002 compared to cells growing under RCCs. However, these cells were highly sensitive to treatment with Digitoxin.
Conclusions: It is likely that multiple mechanisms (increased stemness, overexpression of multidrug resistance protein and prosurvival proteins) contributes to the resistance of cells under PPSS to Paclitaxel, Obatoclax and PI3K inhibitors. Preliminary results indicates that Digitoxin exerts its antiproliferative effect in part by downregulating Sox2. However, the lack of effect of Wortmannin and LY294002 on Paclitaxel resistance suggests that this effect is not via the PI3K/AKT pathway. Adherent cells growing under PPSS provide a simple yet powerful system to screen for drugs targeting cancer stemness.
Citation Format: Rajkumar Venkatadri, Juan Sebastian Yakisich, Neelam Azad, Anand Krishnan V. Iyer. Lung cancer cells growing under prolonged period of serum starvation display increased stemness. [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 254.