Counter-selection for over-expressed human CD44s in primary tumors versus lung metastases in a mouse fibrosarcoma model

Abstract

Human CD44 standard isoform cDNA (hCD44s) was transfected into sis-transformed Balb/c 3T3 cells and into ras-revertant IIIA4 cells (both tumorigenic but nonmetastatic). Transfectants were injected subcutaneously into athymic nude mice to elucidate the functional role of hCD44s over-expression in progression and metastasis. The transfectants (but not parental cells) were capable of lung micrometastasis and of binding exogenously-added hyaluronan. hCD44s protein expression was conserved in lung micrometastases suggesting that it may have been necessary for their formation. In contrast, no hCD44s protein was detected in large subcutaneous (s.c.) tumors but normal levels of murine CD44 were detected. A second round of tumor development, using these two tumor cell classes, demonstrated that hCD44s-nonexpressing s.c. tumor cells re-expressed it in lung micrometastases. Conversely, hCD44s-expressing lung micrometastatic cells, when injected into a second group of mice, downregulated hCD44s expression in order to grow sizable s.c. tumors. S.c. tumor cells still contained the hCD44s gene but its expression was inhibited by epigenetic mechanisms, one of which was shown to be methylation of the hCD44s gene. These studies demonstrate (a) opposing selective pressures on CD44s over-expression for s.c. tumor growth and for metastatic spread to the lung and (b) further credence for the significance of CD44 for metastatic spread of fibrosarcomas. Therefore, CD44s may be a critical component of the metastatic phenotype induced by specific oncogenes.