Activities of arginase I and II are limiting for endothelial cell proliferation

Polyamines are essential for cell proliferation; therefore, we hypothesized that arginase I or arginase II activities, via production of ornithine for polyamine synthesis, may be limiting for proliferation of endothelial cells (EC). Bovine coronary venular EC stably transfected with a lacZ gene (lacZ-EC, control), rat arginase I cDNA (AI-EC), or mouse arginase II cDNA (AII-EC) were utilized to test this hypothesis. Cell-proliferation assays showed that EC proliferation was markedly increased in AI-EC and AII-EC compared with lacZ-EC. Expression of proliferating cell nuclear antigen was also enhanced in AI-EC and AII-EC. DL-alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase, was used to establish that increased polyamine synthesis was involved in mediating the enhanced growth of AI-EC and AII-EC. Addition of 5 mM DFMO to the culture medium completely abolished the differences in cellular putrescine concentrations and reduced the differences in spermidine concentrations among AI-EC, AII-EC, and lacZ-EC. The DFMO treatment also prevented an increase in AI-EC and AII-EC proliferation compared with lacZ-EC. Addition of 10 and 50 microM putrescine dose-dependently increased AI-EC, AII-EC, and lacZ-EC growth to the same extent. These results demonstrate that either arginase isoform can potentially play a role in modulating EC proliferation by regulating polyamine synthesis.