Reaction of a living tissue to injury is known as inflammation and is associated with both positive and negative outcomes. Dysregulated inflammation is often implicated in the etiology of a number of medical conditions in humans. Modulation of inflammation using small molecules represents a potential intervention strategy to mitigate further harmful effects. G-protein coupled receptors (GPCR) play a critical role in immunity by regulating the function of immune cells including macrophages. G-protein receptor kinases (GRKs) desensitize agonist-bound GPCR and affect signaling via GPCRs in macrophages. GRKs also exhibit GPCR-independent roles in modulating various intracellular signaling pathways that regulate inflammatory responses. Therefore GRKs are candidate targets for evaluating the use of small molecules to modulate inflammation. Paroxetine, an FDA-approved selective serotonin (5HT) re-uptake inhibitor (SSRI) used as an anti-depressant, selectively inhibits GRK2 in cardiomyocytes. Previously, GRK2 was demonstrated to modulate lipopolysaccharide (LPS)-induced inflammatory responses in macrophages. Based on these observations, we hypothesized that paroxetine inhibits GRK2 activity and affects LPS-induced inflammatory cytokine production in macrophages. Our results revealed that paroxetine decreases LPS-induced IL-6, IL-1β and increases TNFα levels in macrophages. To further evaluate if paroxetine mediated inflammatory cytokine response is through its ability to inhibit GRK2, RNA interference studies were performed with GRK2 siRNA smartpool. These experiments demonstrated that the effects of paroxetine on macrophages are GRK2-independent. Another SSRI, fluoxetine was also found to exert similar LPS-induced differential regulation of cytokine responses in macrophages. Taken together, these results suggest that the effects of SSRIs could be mediated increased 5HT levels achieved by inhibiting 5HT transporter (SERT) in macrophages. However, exogenous 5HT did not affect the inflammatory cytokine response in LPS-induced macrophages. On the contrary, a 5HT2/5HT7 receptor antagonist (LY 215840) decreases LPS-induced proinflammatory cytokines in macrophages. In conjunction, these results suggest that 5HT is involved in SSRI mediated effects on cytokine response in macrophages. Furthermore, paroxetine did not appear to modulate the following inflammatory signaling kinases in macrophages: pIκBα, pp105, pP38, pJNK1/2, and pERK1/2. In summary, using two different types of macrophages and two SSRIs, we demonstrate that SSRIs can be used to modulate inflammatory cytokine secretion in macrophages. SSRIs, in addition to their potent anti-depressant activity, possess anti-inflammatory properties. A logical continuation of this study would be attempts to elucidate the intracellular pathways involved in SSRI-mediated molecular mechanisms that govern inflammatory cytokine response in macrophages.
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