As shown in Figure 1a, there were strong associations observed be tween the expression of Mcl 1 and that of Bcl xL in both the lung and colon cancer samples. In the 117 human colon cancer samples we analyzed, 47 specimens stained positively for both proteins and a further 29 samples showed weak co staining for both factors. In the 81 lung cancer samples Tipifarnib myeloid tested in this analysis, 51 samples showed strong positive staining for both proteins and five samples showed co staining at low levels. There were further relationships observed be tween Mcl 1 and Bcl xL protein expression and tumor staging in colon cancer samples. Mcl 1 ex pression was found to increase with the staging grade. Bcl xL expression was also found to be significantly associated with staging, with stage I lesions showing significantly dif ferent levels of this protein compared with stage III and stage IV tumors.

Tumor sta ging data were not available for the lung cancer samples. Tumor cells expressing high levels of Mcl 1 and Bcl xL protein exhibit chemoresistance To test the hypothesis that high Mcl 1 and Bcl xL expression contributes to drug resistance, including re sistance to Bcl xL inhibitors, the baseline protein expres sions of Bcl xL and Mcl 1 in multiple cell lines were examined via western blotting. The results demonstrated the concurrent expression of both Mcl 1 and Bcl xL in most cell lines, corroborating the immu nostaining results in both lung and colon tumor tissues shown in Figure 1.

To evaluate the role of Mcl 1 and Bcl xL in tumor cell survival, knockdowns of each factor alone and in combination were performed with small interfering RNAs in A549, REN and H1299 cell lines that overexpress both Mcl 1 and Bcl xL pro teins. Unilateral Mcl 1 reduction caused cell death at 10%, 45% and 50% levels in A549, REN and H1299 cells, respectively, whilst a Bcl xL knockdown alone caused 50%, 37% and 40% rates of cell death in these cells. How ever, the co inhibition of both proteins by RNAi resulted in low cell survival with an almost 80 90% drop in viabil ity. Bcl xl and Mcl 1 reductions via siRNAs were demonstrated using western blotting. To examine whether Mcl 1 contributes to Bcl xL in hibitor resistance, we next evaluated the viability of vari ous cell lines with different Bcl xL and Mcl 1 expression profiles in the presence of ABT 737. The colon adenocarcinoma cell line DLD 1, which expresses relatively lower Mcl 1 levels, but high Bcl xL expression, was found to be sensitive to Bcl xL inhibition via ABT 737. A549 and H1299 cells, which express relatively high levels of Bcl xL and Mcl 1, and H23 cells, which shows strong Drug_discovery Mcl 1 expression and low Bcl xL expression, all demonstrated resistance to ABT 737.