Regulation of TGFbeta1-mediated growth inhibition and apoptosis by RUNX2 isoforms in endothelial cells

Runx transcription factors regulate viral growth, hematopoiesis, bone formation, angiogenesis, and gastric epithelial development through specific DNA-binding motifs on target gene promoters. Vascular endothelial cells (ECs) express RUNX genes that are activated by angiogenic factors. The RUNX2 gene also activates the vascular endothelial growth factor promoter. Alternatively spliced forms of RUNX genes have been described, but their functions in angiogenesis have not been elucidated. In this study, expression of a novel alternatively spliced variant of RUNX2 (RUNX2Delta8), lacking the region encoded by exon 8, was detected in aortic tissue undergoing angiogenesis in vitro and in ECs. Expression of RUNX2 and RUNX2Delta8 increased in vascular sprouts concomitant with expression of cellular proteases and cytokines known to mediate angiogenesis. RUNX2 DNA-binding activity was expressed in proliferating but not quiescent ECs. Ectopic expression of RUNX2 in ECs increased cell sprouting, cell proliferation, DNA synthesis, and phosphorylation of phosphorylated retinoblastoma relative to control transfectants while RUNX2, but not RUNX2Delta8 transfectants, acquired resistance to growth inhibition by transforming growth factor (TGFbeta1). Furthermore, RUNX2Delta8-transfected cells were more sensitive to TGFbeta1-induced apoptosis than RUNX2 transfectants. Consistent with these data, the RUNX2 gene was a strong repressor of the promoter of the cyclin-dependent kinase inhibitor, p21(CIP1), while RUNX2Delta8 was a competitive inhibitor of RUNX2 and exhibited weak repression activity. These results support the hypothesis that ECs regulate growth and apoptosis, in part, by alternative splicing events in the RUNX2 transcription factor to affect the TGFbeta1 signaling pathway. The exon 8 domain of RUNX2 may contribute to the strong repression activity of RUNX2 for some target gene promoters.