| To study clinically advanced breast cancer, our laboratory previously generated a model system through fractionated radiation of MDA-MB 231 breast cancer cells. Subsequently, radio-resistant (R) and -sensitive (S) subclones were isolated from the fractionated population. Clonogenic assays showed that the R clone is approximately 10-fold more radioresistant than the S clone at 5 Gy. To better understand causal mechanisms through which the R clone gains more radioresistance, we analyzed the levels of mRNA using cDNA microarray assays. Our data suggests that the intrinsic gene expression patterns of the R and S clones are very similar, except for four genes, CCAAT/enhancer-binding protein delta (C/EBP delta), collagen type XI alpha-2 ( COL11A2), TAR RNA-binding protein 2 (TARBP2) and phosphatidylinositol 3 kinase regulatory 1 (PI3KR1), the gene encoding for p85alpha, the regulatory subunit of phosphatidylinositol-3 kinase (PI-3K). We have since established that the PI-3K signal transduction pathway is rapidly activated in the R clone in response to 8 Gy, which is abrogated by the LY294002 PI-3K inhibitor. In addition, our data showed that the progression of the R clone cells through the cell cycle is affected by inhibition of the PI-3K/Akt pathway. We found evidence of an increase in differential gene expression between R and S clones following IR, suggesting the small intrinsic difference in gene expression between the R and S clones is further "amplified" following ionizing radiation treatment. |
