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Introduction: Overexpression of MCT1 (multiple copies in a T-cell malignancy), a novel candidate oncogene, in murine fibroblasts is associated with malignant transformation (Prosniak et al, Cancer Res, 1998; Dierov et al, JCB, 1999). To establish the role of MCT1 in the pathogenesis of breast cancer, we stably transfected MCT1 cDNA into MCF7 breast cancer cells that lack endogenous expression of this protein. Three MCT1-overexpressing MCF7 clones (N1, N4 and N7) and empty-vector control were selected for in vitro and in vivo studies. Results: In vitro studies: Transfection of MCT1 gene into breast cancer cells resulted in slightly increased production of estrogen receptor α (ERα), increased levels of DNA synthesis and a higher growth rate in response to estradiol (E2) compared to empty-vector control and parental MCF7 cells. The pure antiestrogen ICI 182,780 (ICI, fulvestrant) inhibited the E2-stimulated proliferation of MCF7-MCT1 cells further substantiating that E2 stimulation of growth was through the functional ER.To examine invasive capacity of MCF7-MCT1 cells we used a 96-well Cell Invasion Assay. All three clones of MCT1-transfected cells showed increased invasiveness in the presence of 50% FBS compared to empty-vector control. Soft agar colony formation assay, however, did not display statistically significant differences of anchorage independent growth between MCT1-transfectants and empty-vector control. We next examined expression of procathepsin D (pCD) and cathepsin D (CD), both markers for invasive potential of MCF7-MCT1 cells. We found that while the expression of CD in MCF7-MCT1 clones was comparable with levels in MDA-MB-231 cells, the levels of pCD reached levels of those in MDA-MB-231 or higher only after stimulation with E2. ICI was able to block the E2-stimulated enhancement of pCD in MCT1-overexpressing cells. In vivo studies: When inoculated into the mammary fat pad of ovariectomized nude mice (10 mice/group), MCT1-overexpressing cells (N1 and N7) had higher tumor rates and formed tumors that were larger than those produced by empty-vector control cells injected under the same conditions. At week 17, the largest N1 tumor was 0.96 cm2 in size, the largest N7 tumor was 0.88 cm2 and empty-vector tumor was 0.25 cm2. To determine the metastatic potential of MCF7-MCT1 cells, we examined spontaneous metastases in lung and liver tissues isolated from sacrificed mice. No evidence of micrometastases was seen in any of the animals in all groups. Conclusion: MCT1 overexpression in breast cancer cells resulted in more aggressive growth characteristics and invasiveness in vitro, as well as greater tumorigenicity when injected into nude mice. The increase tumorigenesis induced by MCT1 transfection into breast cancer cells suggest that MCT1 overexpression may play a role in the pathogenesis and progression of human breast cancer.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]