Role of AXL in invasion and drug resistance of colon and breast cancer cells and its association with p53 alterations

Abstract

Aim: To characterize AXL receptor tyrosine kinase (AXL) expression in relationship to tumor protein P53 (TP53 gene, p53 protein) and its role in tumor invasion and response to therapy.

Methods: We used 14 cell lines, including 3 isogenic pairs carrying mutant/knockout p53, to gain insight into the relationship between AXL and TP53. These included HCT116, HCT116.p53 mutant, RKO, and RKO.p53^-/- lines (all from colon cancers) as well as breast cancer cell lines MCF7 and 1001 (MCF7-p53 mutant clone). HeLa cell line was used as a positive control for epithelial to mesenchymal transition (EMT). AXL expression was determined by Western blotting using rabbit monoclonal antibody clone C89E7. AXL siRNA silencing was performed and followed by collagen invasion assay. Cell viability analysis using the sulforhodamine B assay and the invasion assay were performed after exposure to chemotherapeutic agents (doxorubicin for breast cancer cells; 5FU or irinotecan for colon cancer cells).

Results: We showed that the introduction of p53 mutations or knockout increased expression levels of AXL in isogenic cells compared to the matching p53 wild-type parental cells. Overall, we found a trend for correlation between the potential EMT candidate AXL, p53 alterations, and EMT markers in colorectal and breast cancers. The expression of AXL in RKO cells, a rare colon cancer cell line with inactive Wnt signaling, suggests that the AXL oncogene might provide an alternative genetic pathway for colorectal carcinogenesis in the absence of Wnt signaling activation and TP53 mutation. AXL silencing in the TP53 mutant isogenic cell lines 1001, HCT116.p53 mutant and RKO.P53^-/- was > 95% efficient and the silenced cells were less invasive compared to the parental TP53 wild-type cells. AXL silencing showed a subtle trend to restore colon cancer cell sensitivity to 5FU or irinotecan. Importantly, AXL expressing cells developed more invasive potential after exposure to chemotherapy compared to the AXL-silenced cells.

Conclusion: AXL is influenced by p53 status and could cause the emergence of aggressive clones after exposure to chemotherapy. These findings could have applications in cancer management.

Keywords: AXL; Breast cancer; Chemotherapy; Colon cancer; Invasion.

Figure 1

Western blot analysis of AXL protein levels in cancer cell lines. A: Colon cancer cell lines HCT116, HCT116.p53, RKO, RKO.p53^-/- and SW480. A band of 140 kDa was observed in AXL positive samples. Actin was used as a loading control; B: This graph shows the quantification of band intensity in comparison to Actin using the “ImageJ” program; value analysis was done using MS Excel. Red font indicates p53 mutation; C: Breast cancer cell line MCF7, MCF7-TP53 mutant clone 1001, ZR-75-1, CAL-51, MDA-MB-231, BT-549, MDA-MB-361, and T47D. HeLa was used as positive control for EMT. Actin was used as a loading control; D: This graph shows the quantification of band intensity in comparison to Actin using the “ImageJ” program and value analysis was done using MS Excel. Red font indicates p53 mutation.

Figure 2

Invasion assays of different RKO clones. A: The base line invasion of RKO.p53^-/-, which was not silenced or exposed to treatment, was high, as indicated; B: The same cell line after AXL siRNA silencing shows some decreased invasion; C: RKO.p53^-/- treated with siRNA mock control has increased invasiveness and showed some aggressive cells with increased nuclear size, clumped chromatin, increased cytoplasmic extensions (arrow), after exposure to 5FU treatment; D: Quantitative evaluation of different cells after dye elution and spectrophotometric reading at 560 nm.