Inhibition of deoxyribonucleic acid synthesis by difluoromethylornithine. Role of polyamine metabolism in monocrotaline-induced pulmonary hypertension.

Previously, we have shown that the protection provided by 2-difluoromethylornithine (DFMO) against the development of monocrotaline (MCT)-induced pulmonary hypertension (PH) is associated with inhibition of polyamine biosynthesis in the lungs of MCT-treated rats. Although these studies suggest that prevention of the development of MCT-induced PH is polyamine dependent, no one has demonstrated which cellular events of MCT-induced PH are polyamine dependent. In the present study, using DFMO we tested the hypothesis that inhibition of polyamine biosynthesis may protect against MCT-induced PH by limiting increases in DNA synthesis. We injected rats with MCT (60 mg/kg) or 0.9% NaCl and measured DNA synthesis 7 days after MCT by determining [3H]thymidine incorporation into whole lung DNA. We found that 7 days after MCT treatment DNA synthesis was increased compared to the control (0.9% NaCl). However, DFMO treatment (2% in drinking water) reduced the increase in DNA synthesis following MCT. To confirm that DFMO was acting as a specific inhibitor of polyamine biosynthesis in MCT-induced PH, we administered DFMO concurrently with exogenous ornithine (ORN) (2% in drinking water), the substrate for polyamine biosynthesis, to reverse the protection afforded by DFMO against MCT-induced PH. Twenty-one days after MCT injection we examined right ventricular hypertrophy (RVH), mean pulmonary arterial pressure (MPAP), lung wet weight, and lung polyamine levels. While animals given DFMO (MCT + DFMO) did not increase RVH, MPAP, lung wet weight, or lung polyamine levels, animals given ORN (MCT + DFMO + ORN) did develop increases paralleling those found in animals treated with MCT alone. Our results suggest that suppression of polyamine biosynthesis by DFMO may protect against the development of MCT-induced PH in part by preventing increases in DNA synthesis. This suppression of DNA synthesis may limit the proliferation of key lung cells involved in the inappropriate vascular remodelling associated with MCT-induced PH. These results are consistent with our working hypothesis that elevated lung polyamine levels are essential for the development of MCT-induced PH.