A novel receptor function for the heat shock protein Grp78:: silencing of Grp78 gene expression attenuates α2M-induced signalling

The activated proteinase inhibitor alpha(2)-macroglobulin (alpha(2)M*) binds to two receptors, the low density lipoprotein receptor-related protein (LRP-1) and the alpha(2)M* signalling receptor (alpha(2)MSR). Silencing LRP-1 gene expression in macrophages by RNA interference does not block alpha(2)M* activation of signalling cascades. We now demonstrate that transfection of macrophages with a double-stranded RNA homologous in sequence to the Grp78 gene markedly decreased induction of inositol 1,4,5-trisphosphate (IP3) and subsequent IP3-dependent elevation of [Ca2+](i) induced by alpha(2)M*. Concomitantly, alpha(2)M*-induced increase in [H-3]thymidine uptake was abolished in these transfected cells. Insulin treatment significantly upregulates alpha(2)MSR and it also caused a marked increase in Grp78 expression which could be blocked by RNA interference. alpha(2)M* treatment of cells activates the Ras- and PI 3-kinase-dependent signalling pathways. Suppressing Grp78 expression leads to the loss of these activation events in transfected macrophages. We thus conclude that Grp78 is the alpha(2)M* signalling receptor. (C) 2004 Elsevier Inc. All rights reserved.