While intracellular signaling pathways regulating tumor growth have been well characterized, the pathways regulating endothelial cell proliferation and new blood vessel formation remain poorly understood. Several studies suggest that the calcium-calcineurin-NFAT pathway may be important in endothelial cells. The best-characterized growth factor for endothelial cells is vascular endothelial cell growth factor (VEGF), which exerts its effects primarily through VEGF receptor 2 (VEGFR-2). Activation of VEGFR-2 leads to activation of the calcium-calcineurin-NFAT pathway with subsequent expression of endothelial cell specific genes.

Cyclosporin A (CsA), a well known inhibitor of calcineurin activity, inhibits endothelial cell migration and tube formation in vitro as well as corneal neovascularization in mice, but CsA would not be a useful anti-angiogenic agent in human cancers given its potent immunosuppressive effects. More specific inhibition of the calcium-calcineurin-NFAT pathway may be possible with other types of calcineurin inhibitors. Endogenous inhibitors of calcineurin, termed Down syndrome candidate region 1 (DSCR1) and DSCR1-like 1 (DSCR1-L1), have been recently described by our collaborator, Dr. Sandra Ryeom (Children’s Hospital, Boston).  DSCR1 and DSCR1-L1 have been shown to directly interact with calcineurin and inhibit its activity.

Our laboratory has generated retroviral and lentiviral vectors carrying the DSCR1 or DSCR1-L1 gene and transduced endothelial cells with these retroviral vectors. Overexpression of these calcineurin inhibitors in endothelial cells leads to decreased endothelial cell proliferation, migration, and capillary tube formation in vitro. Transgenic mice that overexpress have been generated by Dr. Ryeom. Tumors implanted in transgenic mice that overexpress DSCR1 grow more slowly than tumors implanted into normal mice, and analysis of tumors reveals decreased tumor angiogenesis as demonstrated by decreased blood vessel density. Current studies are underway to further elucidate the calcineurin-NFAT pathway in endothelial cells and to determine if DSCR1 or DSCR1-L1 can be used as effective anti-angiogenic agents to inhibit metastastic tumor growth.

Sam S. Yoon, MD
2006