The role of Grp 78 in alpha 2-macroglobulin-induced signal transduction. Evidence from RNA interference that the low density lipoprotein receptor-related protein is associated with, but not necessary for, GRP 78-mediated signal transduction.
The low density lipoprotein receptor-related protein (LRP) is a scavenger receptor that binds to many proteins, some of which trigger signal transduction. Receptor-recognized forms of alpha(2)-Macroglobulin (alpha(2)M*) bind to LRP, but the pattern of signal transduction differs significantly from that observed with other LRP ligands. For example, neither Ni(2+) nor the receptor-associated protein, which blocks binding of all known ligands to LRP, block alpha(2)M*-induced signal transduction. In the current study, we employed alpha(2)-macroglobulin (alpha(2)M)-agarose column chromatography to purify cell surface membrane binding proteins from 1-LN human prostate cancer cells and murine macrophages. The predominant binding protein purified from 1-LN prostate cancer cells was Grp 78 with small amounts of LRP, a fact that is consistent with our previous observations that there is little LRP present on the surface of these cells. The ratio of LRP:Grp 78 is much higher in macrophages. Flow cytometry was employed to demonstrate the presence of Grp 78 on the cell surface of 1-LN cells. Purified Grp 78 binds to alpha(2)M* with high affinity (K(d) approximately 150 pm). A monoclonal antibody directed against Grp 78 both abolished alpha(2)M*-induced signal transduction and co-precipitated LRP. Ligand blotting with alpha(2)M* showed binding to both Grp 78 and LRP heavy chains in these preparations. Use of RNA interference to silence LRP expression had no effect on alpha(2)M*-mediated signaling. We conclude that Grp 78 is essential for alpha(2)M*-induced signal transduction and that a "co-receptor" relationship exists with LRP like that seen with several other ligands and receptors such as the uPA/uPAR (urinary type plasminogen activator or urokinase/uPA receptor) system.
Department of Pathology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Uma K
UK
Misra
Mario
M
Gonzalez-Gronow
Govind
G
Gawdi
Justin P
JP
Hart
Carrie E
CE
Johnson
Salvatore V
SV
Pizzo
D000818:Animals; D002352:Carrier Proteins / Q000502:physiology*; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D006801:Humans; D026503:Low Density Lipoprotein Receptor-Related Protein-1 / Q000737:chemistry / Q000502:physiology*; D008297:Male; D051379:Mice; D018832:Molecular Chaperones / Q000502:physiology*; D034622:RNA Interference; D015398:Signal Transduction; D014407:Tumor Cells, Cultured; D000511:alpha-Macroglobulins / Q000502:physiology*
D002352:Carrier Proteins; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D018832:Molecular Chaperones; D000511:alpha-Macroglobulins