Confirmation of alternatively spliced platelet-derived growth factor-A chain and correlation with expression of PDGF receptor-alpha in human cardiac allografts.

PDGF is a potent mitogen for vascular smooth muscle cells (SMC) and may play an important role in the pathogenesis of cardiac allograft vasculopathy (CAV). Two isoforms of PDGF-A chain exist as a result of alternative mRNA splicing that either includes (long-form) or excludes (short-form) exon 6. Short-form PDGF-A is expressed in both resting and activated cells, while the long-form is present predominantly in activated cells. Using RT/PCR, we have found previously that long-form PDGF-A chain was expressed in human cardiac allografts but not in normal human hearts. In the experiments reported here, we studied the cellular distribution of PDGF-A chain isoforms and expression of PDGF receptor-alpha in cardiac allografts. In situ hybridization and immunohistochemistry confirmed the PCR data and demonstrated that expression of long-form PDGF-A chain was diffusely increased in cardiac allografts, predominantly in myocytes and vascular structures. Expression of PDGF receptor alpha also was induced in cardiac allografts and was not detected in any of the normal hearts. Induction of PDGF receptor alpha in cardiac allografts was associated with the presence of long-form PDGF-A chain. In vitro experiments with human endothelial cells demonstrated that aFGF, IL-6, and TGF-beta, which are produced in cardiac allografts in vivo, induced expression of long-form PDGF-A chain. Expression of long-form PDGF-A chain and its receptor was markedly increased in cardiac allografts, predominantly in vascular structures and myocytes. Alterative splicing of PDGF-A chain variants may be mediated by growth factors and cytokines produced in vivo.