Paroxetine is a selective serotonin reuptake inhibitor (SSRI) that is clinically used for the treatment of depression in human patients. Because of recent reports on the role of serotonin in modulating inflammation and the link between inflammation and depression, we sought to test the effect of paroxetine directly on macrophage response to an inflammatory stimulus. Lipopolysaccharide (LPS) treatment of mouse macrophages significantly enhanced TNFα and IL-6 production. Paroxetine treatment of macrophages however, significantly inhibited LPS-induced IL-6 production. In contrast, paroxetine enhanced LPS-induced TNFα production in macrophages. These effects of paroxetine were mimicked by fluoxetine, another SSRI. To determine if the effects of paroxetine are mediated via modulation of the 5-HT system, we treated macrophages with 5-HT or 5-HT receptor antagonist (LY215840) in the presence of LPS and/or paroxetine. 5-HT treatment by itself did not affect LPS-induced cytokine production. LY215840 however, reversed paroxetine's effect on LPS-induced TNFα production but not IL-6. To understand the signaling mechanisms, we examined paroxetine's effect on MAPK and NFκB pathways. While paroxetine inhibited LPS-induced IκBα phosphorylation, MAPK pathways were mostly unaffected. Together these data demonstrate that paroxetine has critical but differential effects on IL-6 and TNFα production in macrophages and that it likely regulates these cytokines via distinct mechanisms.

G-protein coupled receptor kinases (GRKs) are serine/threonine kinases that regulate a large and diverse class of G-protein coupled receptors (GPCRs). Through GRK phosphorylation and β-arrestin recruitment GPCRs are desensitized and their signal terminated. Recent work on these kinases has expanded their role from canonical GPCR regulation to include non-canonical regulation of non-GPCR and non-receptor substrates through phosphorylation as well as via scaffolding functions. Due to these and other regulatory roles, GRKs have been shown to play a critical role in the outcome of a variety of physiological and pathophysiological processes including chemotaxis, signaling, migration, inflammatory gene expression, etc. This diverse set of functions for these proteins makes them popular targets for therapeutics. Role for these kinases in inflammation and inflammatory disease is an evolving area of research currently pursued in many laboratories. In this review, we describe the current state of knowledge on various GRKs pertaining to their role in inflammation and inflammatory diseases.

The intestinal epithelial barrier plays an essential role in maintaining host homeostasis. The barrier regulates nutrient absorption as well as prevents the invasion of pathogenic bacteria in the host. It is composed of epithelial cells, tight junctions, and a mucus layer. Several factors, such as cytokines, diet, and diseases can affect this barrier. These factors have been shown to increase intestinal permeability, inflammation, and translocation of pathogenic bacteria. In addition, dysregulation of the epithelial barrier can result in inflammatory diseases such as inflammatory bowel disease. Our lab and others have also shown that barrier disruption can have systemic effects including bone loss. In this chapter, we will discuss the current literature to understand the link between intestinal barrier and bone. We will discuss how inflammation, aging, dysbiosis and metabolic diseases can affect intestinal barrier-bone link. In addition, we will highlight the current suggested mechanism between intestinal barrier and bone.

β-arrestin2 (β-arr2) identified as a scaffolding protein in GPCR desensitization, is a negative regulator of inflammation in polymicrobial sepsis. In this study we wanted to investigate the role of β-arr2 in intestinal inflammation, a site of persistent microbial stimulation. In the absence of β-arr2, mice exhibited greater extent of mucosal inflammation determined by cellular infiltration and expression of inflammatory mediators even under homeostatic conditions. Further, β-arr2 deficient mice were more susceptible to DSS induced colitis as demonstrated by greater body weight loss, higher disease activity index and shortened colon as compared to wild type (WT) mice. We also show that T cells from β-arr2 KO mice exhibit altered activation status under both basal and colitic conditions, implicating their involvement in disease induction. Further assessment of the role of β-arr2 in intrinsic T cell differentiation confirmed its importance in T cell polarization. Utilizing the T cell transfer model of colitis we demonstrate that T-cell specific-β-arr2 is important in limiting colitic inflammation; however it plays a paradoxical role in concurrent systemic wasting disease. Together, our study highlights a critical negative regulatory role of β-arr2 in intestinal inflammation and demonstrates a distinct role of T-cell-specific β-arr2 in systemic wasting disease.

In recent years a link between the gastro-intestinal tract and bone health has started to gain significant attention. Dysbiosis of the intestinal microbiota has been linked to the pathology of a number of diseases which are associated with bone loss. In addition modulation of the intestinal microbiota with probiotic bacteria has revealed to have both beneficial local and systemic effects. In the present chapter we discuss the intestinal and bone immune systems, explore how intestinal disease affects the immune system and examine how these pathologic changes could adversely impact bone health.