Increased expression of NIPP-1 mRNA correlates positively with malignant phenotype in rat hepatomas

In order to elucidate an involvement of protein phosphatase type 1 (PP1) in the malignant transformation, we have isolated rat cDNA for NIPP-1, a nuclear inhibitor for PP1. Rat NIPP-1 protein conserved the PP1 binding domain and the nuclear targeting sequence. The mRNA was ubiquitously expressed with the highest expression in bone marrow and thymus. The rat NIPP1 mRNA levels were slightly, if at all, increased in some primary hepatomas, and significantly increased in rat ascites hepatomas. The levels of NIPP-1 mRNA were closely correlated with malignant phenotype of rat ascites hepatomas, suggesting that NIPP-1 is tightly linked to malignant phenotype in hepatomas.