Unidirectional crosstalk between Bcl-xL and Bcl-2 enhances the angiogenic phenotype of endothelial cells

Abstract

Expression of Bcl-xL correlates with the clinical outcomes of patients with cancer. While the role of Bcl-2 in angiogenesis is becoming increasingly evident, the function of Bcl-xL in angiogenesis is unclear. Here, we showed that epidermal growth factor (EGF) induces in vitro capillary sprouting and Bcl-xL expression in primary endothelial cells. Bcl-xL-transduced human dermal microvascular endothelial cells (HDMEC-Bcl-xL), but not empty vector control cells, spontaneously organize into capillary-like sprouts. Searching for a mechanism to explain these responses, we observed that Bcl-xL induced expression of the pro-angiogenic chemokines CXC ligand-1 (CXCL1) and CXC ligand-8 (CXCL8), and that blockade of CXC receptor-2 (CXCR2) signaling inhibited spontaneous sprouting of HDMEC-Bcl-xL. Bcl-xL led to Bcl-2 upregulation, but Bcl-2 did not upregulate Bcl-xL, suggesting the existence of a unidirectional crosstalk from Bcl-xL to Bcl-2. EGF and Bcl-xL activate the mitogen-activated protein kinase/ERK pathway resulting in upregulation of vascular endothelial growth factor (VEGF), a known inducer of Bcl-2 in endothelial cells. Inhibition of VEGF receptor signaling in HDMEC-Bcl-xL prevented Bcl-2 upregulation and demonstrated the function of a VEGF-mediated autocrine loop. Bcl-2 downregulation by RNAi blocked CXCL1 and CXCL8 expression downstream of Bcl-xL, and markedly decreased angiogenesis in vivo. We conclude that Bcl-xL functions as a pro-angiogenic signaling molecule controlling Bcl-2 and VEGF expression. These results emphasize a complex interplay between Bcl-2 family members beyond their classical roles in apoptosis.