BRAF associated autophagy exploitation: BRAF and autophagy inhibitors synergise to efficiently overcome resistance of BRAF mutant colorectal cancer cells

Abstract

Autophagy is the basic catabolic mechanism that involves cell degradation of unnecessary or dysfunctional cellular components. Autophagy has a controversial role in cancer – both in protecting against tumor progression by isolation of damaged organelles, or by potentially contributing to cancer growth. The impact of autophagy in RAS induced transformation still remains to be further analyzed based on the differential effect of RAS isoforms and tumor cell context. In the present study, the effect of KRAS/BRAF/PIK3CA oncogenic pathways on the autophagic cell properties and on main components of the autophagic machinery like p62 (SQSTM1), Beclin-1 (BECN1) and MAP1LC3 (LC3) in colon cancer cells was investigated. This study provides evidence that BRAF oncogene induces the expression of key autophagic markers, like LC3 and BECN1 in colorectal tumor cells. Herein, PI3K/AKT/MTOR inhibitors induce autophagic tumor properties, whereas RAF/MEK/ERK signalling inhibitors reduce expression of autophagic markers. Based on the ineffectiveness of BRAFV600E inhibitors in BRAFV600E bearing colorectal tumors, the BRAF related autophagic properties in colorectal cancer cells are further exploited, by novel combinatorial anti-cancer protocols. Strong evidence is provided here that pre-treatment of autophagy inhibitor 3-MA followed by its combination with BRAFV600E targeting drug PLX4720 can synergistically sensitize resistant colorectal tumors. Notably, colorectal cancer cells are very sensitive to mono-treatments of another autophagy inhibitor, Bafilomycin A1. The findings of this study are expected to provide novel efficient protocols for treatment of otherwise resistant colorectal tumors bearing BRAFV600E, by exploiting the autophagic properties induced by BRAF oncogene.

Tumor SQSTM1 (p62) expression and T cells in colorectal cancer

“…One previous study 46 has reported that tumor SQSTM1 expression is not associated with tumor location, but the sample size was much smaller (n D 178) than our current study. In an in vitro experimental study, 61 KRAS mutation in colorectal cancer appeared to attenuate tumor SQSTM1 expression level, and we observed the similar trend. In another in vitro experimental study, 62 F. nucleatum appeared to induce impairment of autophagic activity of host cells; however, our current data do not support such negative effects of F. nucleatum on autophagic activity of tumor cells.…”

Acquired and Intrinsic Resistance to Colorectal Cancer Treatment

“…Recently, Goulielmaki and colleagues reported that PI3K/AKT/MTOR inhibitors induce autophagic tumor properties, whereas RAF/MEK/ERK signaling inhibitors reduce expression of autophagic markers. They showed that pre-treatment of autophagy inhibitor 3-MA followed by its combination with BRAFV600E targeting drug PLX4720 can synergistically sensitize resistant colorectal tumors [129].…”

Novel organometallic chloroquine derivative inhibits tumor growth

“…After 72 h of exposure to drug, cell viability was assessed by the MTS assay. Viability curves like those shown in Figure 2A were fit to an inhibition model to determine the IC 50 . As summarized in Figure 2B, and detailed in Table 1, CMQ exhibited more greater potency than CQ across all cell lines tested, as indicated by lower pairwise IC 50 values in both normal and low pH settings.…”