Spontaneous metastasis of rat liver epithelial cells transformed with v-raf and v-raf/v-myc: association with different phenotypic properties.

Cloned v-raf, v-raf/v-myc, and spontaneously transformed rat liver epithelial (RLE) cell lines were examined for meastatic capability in nude mice, using the LacZ gene as a marker for quantitation of micrometastases. Six cloned lines (R3611-T lines) derived from nude mouse xenografts of the v-raf transformed R3611-3 cells displayed variable metastatic capabilities. Three of six subcutaneously inoculated R3611-TlacZ lines produced spontaneous lung metastasis in nude mice. One of the lines, R3611-T2lacZ was highly efficient at metastatic conversion and produced more lung colonies than a faster growing v-raf/v-myc-transformed RJ2-14lacZ line. The spontaneously transformed RLElacZ line (C4T) was nonmetastatic, although it produced larger subcutaneous tumors than the metastatic R3611-T2lacZ line. Metastatic conversion correlated with upregulation of urokinase-type plasminogen activator receptor RNA expression and downregulation of plasminogen activator inhibitor-1, collagen alpha1 (I), and cytokeratin 14 (K14) RNA expression. These findings indicate that proteolytic activities associated with plasminogen activation play a role in the metastatic development in this model. Decreased production of extracellular proteins and cytoskeletal changes associated with lack of K14 expression are also likely to have contributed to the metastatic conversion of the RLE transformants.