Selective Increases in Isoform PP1 α of Type‐1 Protein Phosphatase in Ascites Hepatoma Cells

The amounts of four isoforms of the catalytic subunit of type‐1 protein phosphatases, PP1 α , PP1γ1, PP1γ2, and PP1δ, have been determined in extracts of rat ascites hepatomas, AH131A, AH13, AH13NMOR, AH143A, and Yoshida sarcoma, and compared to those of rat liver by Western blot analysis. The amount of PP1 α was increased over three times in all five hepatomas. The amount of PP1γ1 was increased over two times in AH13, AH13NMOR, and AH143A. The amount of PP1δ was selectively increased about 4 times in AH131A and AH143A. The PP1γ2 protein was undetectable in both liver and hepatomas. There was good parallelism between the general increase in only PP1 α protein in the hepatomas and the previous data demonstrating the general increase in PP1 α mRNA in numerous ascites hepatomas. These results suggest that PP1 α plays important roles in the expression of malignant phenotype, that its amount is under strict control at the transcription level, and that PP1γ1 and PP1δ play different roles in the expression of some phenotype(s) of the ascites hepatomas.