Acute Alcohol Intake Induces SOCS1 and SOCS3 and Inhibits Cytokine-Induced STAT1 and STAT3 Signaling in Human Monocytes

Alcohol affects multiple functions of the immune system; however the most detrimental effects are seen in innate immunity related to antimicrobial defense and inflammation. Associations between alcohol consumption and increased risk of infections due to immunosuppression were found in numerous studies (Boe et al., 2003; Happel and Nelson, 2005; Mandrekar et al., 2006; Szabo et al., 1999). We previously reported that moderate acute alcohol consumption induces IL-10, a potent anti-inflammatory cytokine, via activation of the Src-STAT3 pathway (Norkina et al., 2007). Signal transducers and activators of transcription (STAT) family proteins play an essential role in signal transduction of cytokines (Takeda and Akira, 2000). STATs are present in the cytoplasm in a latent form and become activated through phosphorylation of tyrosine residues via cytokine receptor associated tyrosine kinases and nonreceptor tyrosine kinases, JAKs and Src kinases, respectively (Murray, 2007; Takeda and Akira, 2000). We had recently reported that STAT proteins play a critical role in alcohol-initiated production of IL-10 (Norkina et al., 2007). Further, we identified implications of Src kinases in alcohol-induced STAT3 activation in human monocytes (Norkina et al., 2007), however the mechanisms of STAT regulation during acute alcohol treatment are currently unknown. Cytokine-initiated signaling in immune cells must be under stringent control to ensure an appropriate cellular response. Suppressors of cytokine signaling (SOCS) are one of 3 major families of regulatory proteins aimed at attenuation of signal transduction induced by cytokines (Alexander and Hilton, 2004; Rakesh and Agrawal, 2005). SOCS proteins provide a negative feedback loop aimed to attenuate signal transduction through the JAK, TYK/STAT pathway and the mechanisms of SOCS proteins activity have been widely investigated in recent years. SOCS1 interacts with tyrosine in the activation loop of JAKs through its SH2 domain (Murray, 2007; Takeda and Akira, 2000). Phospho-tyrosine residues in the cytoplasmic domains of cytokine receptors serve as a site for interaction with SOCS3 (Alexander and Hilton, 2004; Murray, 2007; Takeda and Akira, 2000). Both SOCS1 and SOCS3 carry an N-terminal domain, known as the kinase inhibitory region (KIR), that acts as a pseudo-substrate for JAKs (Rakesh and Agrawal, 2005). SOCS3 is a negative regulator of signaling induced by cytokines such as Interferon (IFN) γ and Interleukin (IL)-6, while SOCS1 attenuates signaling induced by IFNγ, IL-2, IL-6, IL-7, IL-12, and IL-15 (Murray, 2007). Both SOCS1 and SOCS3 inhibit IFN-α-induced expression of antiviral proteins (Vlotides et al., 2004). Besides the limiting effects on STAT-dependent signaling, SOCS proteins themselves are regulated through activation of STAT-dependent elements (Gatto et al., 2004; He et al., 2003; Ramana et al., 2005; Venkataraman et al., 1999). More recently, it has been reported that SOCS gene expression can be induced by chemokines, but also by some Toll-like receptor (TLR) agonists, such as LPS (via TLR4) and CpG-DNA (via TLR9) (Hu et al., 2007;Mellor et al., 2005). Previous reports from us, and others, demonstrated that alcohol alters TLR4- and TLR9-induced signaling (Goral and Kovacs, 2005; Goral et al., 2004; Mandrekar et al., 2006; Pruett et al., 2004). Further, in vivo alcohol consumption leads to STAT3 activation (Norkina et al., 2007). In this study, we investigated the effect of alcohol on STAT protein activation in order to evaluate the hypothesis that moderate acute alcohol intake may result not only in activation of STATs, but also in STAT-dependent induction of suppressors of cytokine signaling (SOCS). Finally, we addressed the question, whether moderate acute alcohol exposure could influence subsequent cytokine signaling induced by inflammatory cytokines IL-6, IFNα, and IFNγ, which are targets of SOCS proteins in humanmonocytes.