REEXPRESSION OF NEU-ENCODED ONCOPROTEIN COUNTERACTS THE TUMOR-SUPPRESSING BUT NOT THE METASTASIS-SUPPRESSING FUNCTION OF E1A

By transfecting the adenovirus 5 E1A gene into neu-transformed NIH3T3 cells, we previously showed that E1A can dramatically repress neu-enco ded p185 expression and, concomitantly, suppress the features of trans formation and metastasis of neu+E1A transfectants. From these results we concluded that suppression of transformation and metastasis by E1A in neu-transformed cells may be through repression of neu gene express ion. However, E1A has recently been shown to also repress the transfor mation features of other human cancer cells that do not overexpress ne u. This observation raised a possibility that repression of neu gene e xpression in our neu+E1A cells might not be the only mechanism for tra nsformation and metastasis suppression. To study whether other molecul ar mechanisms might be involved in suppression of transformation and m etastasis by E1A in our neu+E1A cells, we reexpressed p 185 oncoprotei n in the neu+E1A cells by transfecting them with a plasmid containing activated rat neu complementary DNA and we examined whether E1A can su ppress transformation and metastasis when the neu-encoded p185 protein is reexpressed. All the features of transformed cells including cell morphology, DNA synthesis rate, colony formation in soft agar, and tum origenicity in nu/nu mice were restored in the cell lines that reexpre ssed neu. In addition, the levels of neu reexpression corresponded to the degree of malignant transformation. However, the in vivo metastati c tumor formation by these p185 reexpressing cells was still significa ntly inhibited by E1A. When metastasis-associated properties were exam ined in the cell lines that reexpressed p185, we found that cell motil ity was recovered by reexpression of p185 to the degree corresponding to the p185 reexpression level, but secretion of membrane-degrading ge latinases and invasion through the basement membrane preparation Matri gel by these cells were still inhibited by E1A. The data demonstrated that reexpression of p185 in neu+E1A cells can counteract the tumor-su ppressing function of E1A but not completely recover the neu-induced m etastatic phenotype. We conclude from these results that (a) repressio n of neu oncogene expression was indeed the molecular mechanism by whi ch E1A suppressed tumor formation in neu-transformed 3T3 cells, and (b ) suppression of metastasis by E1A in neu-transformed 3T3 cells was vi a multiple molecular mechanisms in addition to repressing neu. Our mod el system clearly demonstrated that tumorigenicity and metastasis are related but separable phenomena.