GRP78 and Cripto form a complex at the cell surface and collaborate to inhibit transforming growth factor beta signaling and enhance cell growth.
Cripto is a multifunctional cell surface protein with important roles in vertebrate embryogenesis and the progression of human tumors. While Cripto has been shown to modulate multiple signaling pathways, its binding partners do not appear to fully explain its molecular actions. Therefore, we conducted a screen aimed at identifying novel Cripto-interacting proteins. This screen led to our identification of glucose-regulated protein 78 (GRP78), an endoplasmic reticulum (ER) chaperone that is also expressed at the surfaces of tumor cells. Here we demonstrate that Cripto and GRP78 interact at the cell surfaces of multiple cell lines and that their interaction is independent of prior association within the ER. Interestingly, short hairpin RNA knockdown of endogenous GRP78 resulted in enhanced transforming growth factor beta (TGF-beta) signaling, indicating that like Cripto, GRP78 inhibits this pathway. We further show that when coexpressed, GRP78 and Cripto collaborate to antagonize TGF-beta responses, including Smad phosphorylation and growth inhibition of prostate cancer cells grown under anchorage-dependent or -independent conditions. Finally, we provide evidence that cells coexpressing GRP78 and Cripto grow much more rapidly in soft agar than do cells expressing either protein individually. Together, our results indicate that these proteins bind at the cell surface to enhance tumor growth via the inhibition of TGF-beta signaling.
Shani
Gidi
G
Clayton Foundation Laboratories for Peptide Biology, The Salk Institute, La Jolla, CA, 92037, USA.
Fischer
Wolfgang H
WH
Justice
Nicholas J
NJ
Kelber
Jonathan A
JA
Vale
Wylie
W
Gray
Peter C
PC
D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism*; D049109:Cell Proliferation / Q000187:drug effects; D000091342:Endoplasmic Reticulum Chaperone BiP; D004815:Epidermal Growth Factor / Q000235:genetics / Q000378:metabolism*; D006360:Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018832:Molecular Chaperones / Q000235:genetics / Q000378:metabolism*; D009363:Neoplasm Proteins / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D011485:Protein Binding; D034622:RNA Interference; D018125:Receptors, Transforming Growth Factor beta / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects*; D051899:Smad2 Protein / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000494:pharmacology*
D000091342:Endoplasmic Reticulum Chaperone BiP; C000717247:HSPA5 protein, human; D006360:Heat-Shock Proteins; C000717248:Hspa5 protein, mouse; D008562:Membrane Glycoproteins; D018832:Molecular Chaperones; D009363:Neoplasm Proteins; D018125:Receptors, Transforming Growth Factor beta; D051899:Smad2 Protein; C489450:Tdgf1 protein, mouse; D016212:Transforming Growth Factor beta; D004815:Epidermal Growth Factor