Phenytoin Increases Gene Expression for Platelet-derived Growth Factor B Chain in Macrophages and Monocytes

The mechanism by which phenytoin (PHT) induces gingival overgrowth remains unclear. We hypothesized that PHT increases macrophage production of platelet-derived growth factor (PDGF), an important cytokine in connective tissue growth and repair, and that excessive production PDGF in gingiva could lead to redundant growth. To test the hypothesis, rat peritoneal macrophages and human blood monocytes were cultured in the presence of PHT (5 to 20 micrograms/ml medium) or an equal volume of its solvent for 3 days and tested for expression of PDGF-B mRNA by in situ hybridization. Approximately 300 cells/culture well were examined (3 wells/drug level) for positive indication of PDGF-B mRNA. Data were compared by chi square test. All levels of PHT in both cell types induced a 2- to 8-fold increase in PDGF-B mRNA positive cells, significant in all cases at P < 0.001. Northern blot analysis of RNA from similarly cultured rat macrophages confirmed these findings. Cells treated with 10 micrograms PHT/ml medium or solvent revealed 2.2 +/- 0.3 and 1.0 +/- 0.2 (mean +/- SEM) arbitrary units PDGF mRNA respectively (t tests, P < 0.05). Additionally, rat macrophages were cultured in presence of 5 micrograms PHT/medium or its solvent and medium was analyzed for PDGF secretion by radioimmunoassay. Mean values (+/- SEM) were 1.28 +/- 0.49 and 0.78 +/- 0.07 ng/mg protein respectively (t test, P < 0.05). These data showed that PHT augmented the expression of c-sis, the gene for PDGF-B, and offered a possible explanation for PHT-induced gingival overgrowth.