INHIBITION OF CELL-GROWTH - EFFECTS OF THE TYROSINE KINASE INHIBITOR CGP-53716

The growth factors, platelet-derived growth factor (PDGF) and basic fi broblast growth factor (bFGF) play major roles in enhanced smooth musc le cells growth in rodent blood vessels after vascular injury. Tyrosin e kinase inhibition has been shown to be effective in blocking tyrosin e phosphorylation at the PDGF and bFGF receptors in cultured fibroblas t and vascular smooth muscle cells which in turn inhibits their prolif eration. Our study evaluated the PDGF selective tyrosine kinase inhibi tor, CGP 53716, on serum, PDGF-BB, bFGF or epidermal growth factor-ind uced growth responses in cultured rat aortic smooth muscle cells (RASM C) and Balb/3T3 fibroblasts (3T3). CGP 53716 inhibited serum-induced c ell growth in RASMC, but not in 3T3 cells. CGP 53716 completely blocke d PDGF-BB tyrosine receptor autophosphorylation in RASMC and 3T3 cells , PDGF-SB-induced phosphorylation of mitogen-activated protein kinase at 1 mu M in RASMC and inhibited PDGF-BB-induced c-Fos protein express ion at 1 mu M in RASMC; consistent with inhibition of PDGF-BB-induced DNA synthesis. To examine the selectivity of CGP 53716, PDGF-BB, bFGF or EGF-induced DNA synthesis was measured using thymidine incorporatio n. CGP 53716 inhibited PDGF-BB-, bFGF-and EGF-induced DNA synthesis in a concentration-dependent manner in each cell line. CGP 53716 showed a 2- to 4-fold selectivity for PDGF-BE-stimulated DNA synthesis over b FGF or EGF in RASMC or 3T3 cells. To rule out that bFGF induced the re lease of endogenous PDGF, an antibody to PDGF-AB, which binds to all t hree isoforms of PDGF, was coincubated with bFGF and did not suppress the DNA synthesis induced by bFGF. Based on these results, CGP 53716 i s not selective for the PDGF receptor as previously reported. However, EGF-stimulated receptor autophosphorylation of mitogen-activated prot ein kinase phosphorylation and c-Fos protein expression were not inhib ited by CGP 53716 al 1 or 10 mu M in RASMC. These findings suggest tha t CGP 53716 may inhibit multiple growth factor pathways as indicated b y inhibition of DNA synthesis. However, these effects must be downstre am from the signaling for c-Fos protein expression or use an alternate signaling route. These results further suggest that CGP 53716 may hav e a therapeutic potential for the treatment of Vascular proliferative diseases which are stimulated by not only PDGF but other growth factor s such as bFGF and EGF.