β₂-microglobulin promotes the growth of human renal cell carcinoma through the activation of the protein kinase A, cyclic AMP-responsive element-binding protein, and vascular endothelial growth factor axis

Purpose: β₂-Microglobulin (β2M), a soluble protein secreted by cancer and host inflammatory cells, has various biological functions, including antigen presentation. Because aberrant expression of β2M has been reported in human renal cell carcinoma, we investigated the effects of β2M overexpression on cancer cell growth and analyzed its molecular signaling pathway. Experimental Design: We established clonal cell lines that overexpressed β2M in human renal cell carcinoma (SN12C) cells and then examined cell growth in vitro and in vivo and studied the β2M-mediated downstream cell signaling pathway. Results: Our results showed that β2M expression positively correlates with (a) in vitro growth on plastic dishes and as Matrigel colonies, (b) cell invasion and migration in Boyden chambers, and (c) vascular endothelial growth factor (VEGF) expression and secretion by cells. We found, in addition, that β2M mediates its action through increased phosphorylation of cyclic AMP - responsive element-binding protein (CREB) via the protein kinase A-CREB axis, resulting in increased VEGF expression and secretion. In convergence with this signal axis, β2M overexpression also activated both phosphatidylinositol 3-kinase/Akt and mitogen-activated protein kinase pathways. β2M overexpression induced accelerated growth of SN12C in mouse subcutis and bone. Interrupting the β2M signaling pathway using small interfering RNA led to apoptosis with increased activation of caspase-3 and caspase-9 and cleaved poly(ADP-ribose) polymerase. Conclusions: Our results showed for the first time that the β2M-protein kinase A-CREB-VEGF signaling axis plays a crucial role in support of renal cell carcinoma growth and progression and reveals a novel therapeutic target.