Is the Fischer 344/CRJ rat a protein-knock-out model for dipeptidyl peptidase IV-mediated lung metastasis of breast cancer?

Fischer 344/CRJ rats harbor a G633R substitution in dipeptidyl peptidase IV (DPP IV) that leads to retention and degradation of the mutant protein in the endoplasmic reticulum (Tsuji E, Misumi Y, Fujiwara T et al. Biochemistry 1992; 31 (47): 11921-7). However, when these rats were used as a 'protein knock-out' model in further evaluating the previously established role of DPP IV in metastasis, lung colonization of the highly metastatic MTF7 rat breast cancer cell line was reduced by only 33% relative to normal Fischer 344 rats. To examine whether lung endothelia leak expression of mutant DPP IV and whether mutant DPP IV exhibits the same adhesion qualities as wild type DPP IV, detailed immunohistochemical, biochemical, transfection, and FACS analyses were performed to assess the surface expression of mutant DPP IV on lung endothelia and transfected HEK293 cells and adhesion assay to compare the adhesion qualities of wild-type and mutant DPP IV. Both endothelial and transfected HEK293 cells expressed mutant, enzymatically inactive DPP IV on their surfaces, albeit at greatly reduced levels when compared to expression of wild type DPP IV. Purified mutant DPP IV had identical adhesion qualities for lung-metastatic MTF7 cells as wild type DPP IV, and competitive inhibition of MTF7 lung colonization by truncated DPP IV confirmed involvement of mutant DPP IV in lung metastasis of Fischer 344/CRJ rats. Although metastasis appears to be mediated by several, often parallel mechanisms involving multiple tumor and host factors, these data indicate that altered expression of a single component can drastically change the outcome of metastatic disease.