Differentially expressed protein Pdcd4 inhibits tumor promoter-induced neoplastic transformation

An mRNA differential display comparison of mouse JB6 promotion-sensitive (P +) and -resistant (P-) cells identified a novel gene product that inhibits neoplastic: transformation. The JB6 P+ and P- cells are genetic variants th at differ in their transformation response to tumor promoters; P+ cells for m anchorage-independent colonies that are tumorigenic, and P- cells do not. A differentially displayed fragment, A7-1. was preferentially expressed in P- cells at levels greater than or equal to 10-fold those in P+ cells, mak ing its mRNA a candidate inhibitor of neoplastic transformation. An A7-1 cD NA was isolated that was identical to murine Pdcd4 gene cDNAs. also known a s MA-3 or TIS,and analogous to human H731 and 197/15a. Until now, the funct ion of the Pdcd4 protein has been unknown. Paralleling the mRNA levels, Pdc d4 protein levels were greater in P- than in P+ cells. Pdcd4 mRNA was also expressed at greater levels in the less progressed keratinocytes of another mouse skin neoplastic progression series. To test the hypothesis that Pdcd 4 inhibits tumor promoter-induced transformation, stable cell lines express ing antisense Pdcd4 were generated from parental P- cells. The reduction of Pdcd4 proteins in antisense lines was accompanied by acquisition of a tran sformation-sensitive (Pf) phenotype. The antisense-transfected cells were r everted to their initial P-phenotype by overexpression of a Pdcd4 sense fra gment. These observations demonstrate that the Pdcd4 protein inhibits neopl astic transformation.