Abstract
The Ras family of small GTPases transmits extracellular signals that regulate cell growth, differentiation, motility and death. Ras signaling is constitutively active in a large number of human cancers. Ras can also regulate autophagy by affecting several signaling pathways including the mTOR pathway. Autophagy is a process that regulates the balance between protein synthesis and protein degradation. It is important for normal growth control, but may be defective in diseases. Previously, we have shown that Ras inhibition by FTS induces autophagy, which partially protects cancer cells and may limit the use of FTS as an anti-cancer drug. Since FTS is a non toxic drug we hypothesized that FTS and chloroquine (an autophagy inhibitor) will synergize in cell growth inhibition and cell death. Thus, in the present study, we explored the mechanism of each individual drug and their combined action. Our results demonstrate that in HCT-116 and in Panc-1 cells, FTS induces autophagy, which can be inhibited by chloroquine. Furthermore, the combined treatment synergistically decreased the number of viable cells. Interestingly, the combined treatment enhanced apoptotic cell death as indicated by increased sub-G1 cell population, increased Hoechst staining, activation of caspase 3, decrease in survivin expression and release of cytochrome c. Thus, chloroquine treatment may promote FTS-mediated inhibition of tumor cell growth and may stimulate apoptotic cell death.
Highlights
The Ras family of small GTPases transmits signals initiated by cell surface receptors, to regulate several cellular processes such as cell growth, differentiation, motility and death [1]
We examined whether inhibition of autophagy by chloroquine will potentiate the effect of FTS on cell growth inhibition and cell death
These results suggest that p62, which is needed for continuation of the autophagic process, may be synthesized de-novo at later stages of autophagy
Summary
The Ras family of small GTPases transmits signals initiated by cell surface receptors, to regulate several cellular processes such as cell growth, differentiation, motility and death [1]. Ras signaling is constitutively activated in a large fraction of human cancers [3]. Activating mutations of the three major Ras isoforms (H, K and N) were found in more than 33% of all human cancers [4,5,6]. Ras signaling is a major junction of various signaling pathways. For this reason, Ras and its effectors serve as important targets for therapeutic intervention. FTS acts as a functional Ras antagonist in cells, affecting Ras-membrane interactions by dislodging the protein from its anchorage domains, facilitating its degradation and, reducing cellular active Ras content [8, 9]. FTS has been shown to inhibit the growth of H-Ras, K-Ras and N-Ras transformed rodent fibroblasts in vitro and to inhibit the anchorage-independent growth of several cancer cell lines [10,11,12,13]
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