Abstract
Curcumin (CUR) has been proven to be clinically effective in rheumatoid arthritis (RA) therapy, but its low oral bioavailability eclipses existent evidence that attempts to explain the underlying mechanism. Small intestine, the only organ exposed to a relatively high concentration of CUR, is the main site that generates gut hormones which are involved in the pathogenesis of RA. This study aims at addressing the hypothesis that one or more gut hormones serve as an intermediary agent for the anti-arthritic action of CUR. The protein and mRNA levels of gut hormones in CUR-treated rats were analyzed by ELISA and RT-PCR. Somatostatin (SOM) depletor and receptor antagonist were used to verify the key role of SOM in CUR-mediated anti-arthritic effect. The mechanisms underlying CUR-induced upregulation of SOM levels were explored by cellular experiments and immunohistochemical staining. The data showed that oral administration of CUR (100 mg/kg) for consecutive two weeks in adjuvant-induced arthritis rats still exhibited an extremely low plasma exposure despite of a dramatic amelioration of arthritis symptoms. When injected intraperitoneally, CUR lost anti-arthritic effect in rats, suggesting that it functions in an intestine-dependent manner. CUR elevated SOM levels in intestines and sera, and SOM depletor and non-selective SOM receptor antagonist could abolish the inhibitory effect of CUR on arthritis. Immunohistochemical assay demonstrated that CUR markedly increased the number of SOM-positive cells in both duodenum and jejunum. In vitro experiments demonstrated that CUR could augment SOM secretion from intestinal endocrine cells, and this effect could be hampered by either MEK1/2 or Ca(2+)/calmodulin-dependent kinase II (CAMKII) inhibitor. In summary, oral administration of CUR exhibits anti-arthritic effect through augmenting SOM secretion from the endocrine cells in small intestines via cAMP/PKA and Ca(2+)/CaMKII signaling pathways.
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