Cellular and in vivo characterization of the MCR rat mammary tumor model

The slowly growing, transplantable MCR-83 rat mammary tumor is estrogen-dependent and non-metastasizing. A rapidly growing, estrogen-independent, metastasizing subline (MCR-86) was subsequently isolated in vivo. We have established and characterized cell lines from both MCR rat mammary tumors. MCR cell lines and tumors were studied in vivo and in vitro. Analysis of DNA from tumors and cell lines showed that mutations had not occurred in codons 12, 13 and 61 of the Ha-ras and Ki-ras genes. Additionally, dominant transforming activity could not be detected by DNA transfection using NIH 3T3 focus-forming assay. No gene amplification was detected for either the EGF-receptor or c-erbB-2 genes. Differences in the tyrosine phosphorylation patterns were found between the 2 MCR cell lines. Addition of serum to starved cells resulted in the tyrosine phosphorylation of a 120-kDa protein, which was elevated in the MCR-86. The lack of ras activation in the MCR tumors differentiates this model from the widely studied, chemically induced rodent mammary tumors. In addition, the differences in the cellular phosphotyrosine patterns between MCR-83 and MCR-86 suggests the occurrence of alterations in signalling pathways that involve tyrosine protein kinases.