Stable complexes formed by Grp94 with human IgG promoting angiogenic differentiation of HUVECs by a cytokine-like mechanism

To explore the molecular mechanisms by which complexes of Grp94 with IgG, purified from the plasma of diabetic subjects, could drive an inflammatory risk in vascular cells, native Grp94 was co-incubated with human, non-immune IgG to obtain the formation of complexes that were then tested on human umbilical vein endothelial cells (HUVECs). Co-incubation of Grp94 with IgG led to the formation of stable, SDS-resistant complexes that displayed effects partly similar and partly significantly different from those of Grp94 alone. Both Grp94 alone and with IgG stimulated the cell growth and promoted angiogenesis by a mechanism of autocrine/paracrine activation of the expression of heat shock protein (HSP)90 and HSP70. However, the most striking alterations in the cell cytoskeleton, characterized by dramatic rearrangement of actin and increased formation of podosomes, were induced by Grp94 with IgG, and were mediated by the enhanced expression of HSP90. At variance with Grp94 alone, Grp94 with IgG promoted the angiogenic differentiation by activating a signaling pathway apparently independent of the intense stimulation of the ERK1/2 pathway that was instead more directly involved in mediating the proliferative effects on HUVECs. Results show unprecedented cytokine-like effects of Grp94 and a so far undisclosed capacity to bind irreversibly IgG, forming complexes that, with respect to Grp94 alone, display a more intense angiogenic transforming capacity that may predict an increased inflammatory risk in vascular cells in vivo.