C-Reactive Protein Protects Myeloma Cells from Apoptosis Via Activating ITAM-Containing Fcγ RII.

Human C-reactive protein (CRP) is an acute-phase protein, and elevated levels of CRP are present in patients with infections, inflammatory diseases, necrosis such as myocardial infarction, or malignancies including multiple myeloma (MM), lymphoma, and carcinoma. CRP is able to bind a variety of ligands and receptors, activating the classical complement pathway. Accumulating evidence strongly suggests that in cardiovascular disease CRP is not only a marker of inflammation, but also contributes to pathogenesis of the disease. These findings led to our hypothesis that CRP may have a functional effect on tumor cells. The present study was undertaken in a myeloma setting to determine whether CRP might affect tumor cell growth and survival. When added to the culture of primary myeloma cells isolated from patients, CRP promoted cell proliferation and reduced cell apoptosis in a dose-dependent manner. To confirm this result, we examined the effects of CRP on myeloma cell lines under stressed conditions and showed that CRP protected myeloma cells from apoptosis induced by serum starvation or IL-6 deprivation. More importantly, CRP also protected myeloma cells from apoptosis induced by dexamethasone or melphalan, two common chemotherapy drugs for MM. The protection was significant since CRP reduced cell death by 50 to 60%, and may be clinically relevant because the results were reproduced in myeloma-SCID mouse models. Injection of CRP prior to treatment of myeloma-bearing mice with dexamethasone or melphalan significantly undermined the therapeutic effects of these chemotherapy drugs. Mice receiving CRP with dexamethasone or melphalan had significantly larger tumor burdens compared with mice treated with dexamethasone or melphalan alone, whereas treatment with CRP alone had no effects on the tumor cells. CRP protected tumor cells from apoptosis by downregulating Bax expression, inhibiting phosphorylation of Bcl-2, and upregulating phosphorylation of Bad, which led to inhibited caspase-9, caspase-3 and PARP activation induced by dexamethasone. We next examined cell surface receptors for CRP and found that CRP bound Fcγ RII CD32A and/or CD32C, but not CD32B. Specific siRNAs that inhibited CD32A or CD32C but not CD32B expression, and antibodies against CD32A/C that blocked receptor-ligand interaction abrogated CRP-mediated protection of cell apoptosis. These results indicate that CRP mediated its effects via activating immunoreceptor tyrosine-based activation motif (ITAM)-containing Fcγ RII. By binding to these receptors, CRP increased the level of phosphorylated Akt, ERK1/2, and IkBa; relocalized NFkB p65 to the nucleus; and inhibited p38 kinase activity. Inhibitors against these signaling molecules blocked the activity of these pathways and abrogated CRP-mediated protection of myeloma cell apoptosis induced by dexamethasone. Therefore, our results provide strong evidence for a novel effect of CRP on myeloma cells. This study also implicates CRP as a potential target for MM therapy.