Reversible exposure of human platelet fibrinogen receptors by antiplatelet tetraspanin monoclonal antibodies via induction of a conformational change in membrane glycoprotein IIb/IIIa complex

The tetraspanin CD9 modulates epidermal growth factor receptor signaling in cancer cells

CD9 is a member of the tetraspanins, and has been shown to be involved in a variety of cellular activities such as migration, proliferation, and adhesion. In addition, it has been known that CD9 can associate with other proteins. Here we demonstrated the physical and functional association of CD9 with epidermal growth factor receptor (EGFR) on MKN-28 cells. Double-immunofluorescent staining and immunoprecipitation demonstrated the complex formation of CD9-EGFR and CD9-beta(1) integrin, and that both complexes are colocalized on the cell surface especially at the cell-cell contact site. Anti-CD9 monoclonal antibody ALB6 induced a dotted or patch-like aggregation pattern of both CD9-EGFR and CD9-beta(1) integrin. The internalization of EGFR after EGF-stimulation was significantly enhanced by the treatment with ALB6. CD9 can associate with EGFR in hepatocellular carcinoma cells (HepG2/CD9) and Chinese hamster ovary cancer cells (CHO-HER/CD9), which were transfected with pTJ/human EGFR/CD9. Furthermore expression of CD9 specifically attenuated EGFR signaling in CHO-HER/CD9 cells through the down regulation of surface expression of EGFR. These results suggest that CD9 might have an important role that attenuates EGFR signaling. Therefore, CD9 not only associates EGFR but also a new regulator, which may affect EGF-induced signaling in cancer cells.

CD36 associates with CD9 and integrins on human blood platelets

The membrane glycoprotein CD36 is involved in platelet aggregation, inhibition of angiogenesis, atherosclerosis, and sequestration of malaria-parasitized erythrocytes. In this study, immunoprecipitations with anti-CD36 antibodies were performed to identify proteins that associate with CD36 in the platelet membrane. Platelets were solubilized in 1% Triton X-100, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), Brij 96, or Brij 99, and the proteins that coprecipitated with CD36 were identified by peptide mass spectrometry and Western blotting. The tetraspanin protein CD9 and the integrins alphaII(b)beta3 and alpha6beta1 specifically coprecipitated with CD36 from platelets that were solubilized in CHAPS and Brij 99 but not from platelets that were solubilized in Triton X-100. Only CD9 is coprecipitated with CD36 from platelets that were solubilized in Brij 96. Reciprocal immunoprecipitations with antibodies to CD9, alpha6, alphaIIb, or beta3 from Brij 99-solubilized platelets coprecipitated CD36. Coprecipitation of CD36, CD9, and alpha6beta1 was also observed on platelets from a patient with Glanzmann thrombasthenia, indicating that alphaII(b)beta3 is not required for the other proteins to associate. Colocalization of alpha6 and CD36, of CD9 and CD36, and of alpha6 and CD9 was observed on intact platelets prior to solubilization, using double immunofluorescence microscopy. These data indicate that CD36 associates with CD9 and integrins on human blood platelets. These associated proteins may mediate or participate in some of the diverse biological functions of CD36.