Selective blockade of the intermediate-conductance Ca2+-activated K+ channel suppresses proliferation of microvascular and macrovascular endothelial cells and angiogenesis in vivo

Objective - Ca2+-activated K+ (K-Ca) channels have been proposed to promote mitogenesis in several cell types. Here, we tested whether the intermediate-conductance K-Ca channel (IKCa1) and the large-conductance K-Ca channel (BKCa) contribute to endothelial cell (EC) proliferation and angiogenesis. Material and Results - Function and expression of IKCa1 and BKCa/Slo were investigated by patch-clamp analysis and real-time RT-PCR in human umbilical vein ECs (HUVECs) and in dermal human microvascular ECs 1 (HMEC-1). HMEC-1 expressed IKCa1 and BKCa/Slo, whereas HUVECs expressed IKCa1. A 48-hour exposure to basic fibroblast growth factor ( bFGF) augmented IKCa1 current amplitudes and induced a 3-fold increase in IKCa1 mRNA expression in HUVECs and HMEC-1. Vascular endothelial growth factor ( VEGF) was also effective in upregulating IKCa1. BKCa/Slo expression and current amplitudes in HMEC-1 were not altered by bFGF. bFGF- and VEGF-induced EC proliferation was suppressed by charybdotoxin, clotrimazole, or the selective IKCa1 blocker 1-[(2-chlorophenyl) diphenylmethyl]- 1H-pyrazole (TRAM-34), whereas inhibition of BKCa/Slo by iberiotoxin was ineffective. In the Matrigel plug assay in mice, administration of TRAM-34 for 2 weeks significantly suppressed angiogenesis by approximate to 85%. Conclusions - bFGF and VEGF upregulate expression of IKCa1 in human ECs. This upregulation of IKCa1 seems to be required for mitogen-induced EC proliferation and angiogenesis in vivo. Selective IKCa1 blocker might be of therapeutic value to prevent tumor angiogenesis.