SOCS1-Negative Feedback of STAT1 Activation Is a Key Pathway in the dsRNA-Induced Innate Immune Response of Human Keratinocytes

Abstract

Toll-like receptor (TLR)3 is a receptor for virus-associated double-stranded RNA, and triggers antiviral immune responses during viral infection. Epidermal keratinocytes express TLR3 and provide an innate immune defense against viral infection. Since the intracellular regulatory mechanism is unknown, we hypothesized that the signal transducers and activators of transcription (STAT)-suppressors of cytokine signaling (SOCS) system regulates the innate immune response of keratinocytes. Treatment with polyinosinic-polycytidylic acid (poly(I:C)) resulted in the rapid translocation of IFN regulatory factor (IRF)-3 into the nucleus, followed by phosphorylation of STAT1 and STAT3. The activation of STATs by poly(I:C) probably occurs in an indirect fashion, through poly(I:C)-induced IFN. We infected cells with the dominant-negative forms of STAT1 (STAT1F), STAT3 (STAT3F), and SOCS1 using adenovirus vectors. Infection with STAT1F suppressed the induction of macrophage inflammatory protein (MIP)-1alpha by poly(I:C), whereas STAT3F had a minimal effect, which indicates that STAT1 mediates MIP-1alpha induction. SOCS1, which is a negative feedback regulator of STAT1 signaling, was induced by treatment with poly(I:C). SOCS1 infection inhibited the phosphorylation of STAT1 and significantly reduced poly(I:C)-induced MIP-1alpha production. Furthermore, STAT1-SOCS1 regulated poly(I:C)-induced TLR3 and IRF-7 expression. However, SOCS1 did not affect NF-kappaB signaling. Thus, the STAT1-SOCS1 pathway regulates the innate immune response via TLR3 signaling in epidermal keratinocytes.

Double-Stranded RNA Induces an Antiviral Defense Status in Epidermal Keratinocytes through TLR3-, PKR-, and MDA5/RIG-I-Mediated Differential Signaling

“…The parallel presence of all dsRNA recognition molecules in human keratinocytes in a functional and differentially regulated way is a novel and unexpected finding (45). Although NF-B activation following stimulation with poly(I:C) has also been observed in other cells of the epithelial lineage, such as in human respiratory (46 -49), reproductive (50), uterine (51), and intestinal (52,53) epithelium, this has been generally attributed to the expression and function of TLR3.…”

Molecular analysis of human cancer cells infected by an oncolytic HSV-1 reveals multiple upregulated cellular genes and a role for SOCS1 in virus replication

“…Inhibition of STAT1 signaling by SOCS1 regulates innate immune recognition via double-stranded RNA-activated pathways in human keratinocytes. 42 SOCS1, via its SH2-binding domain, has been reported to possess tumor suppressor activity and inhibit JAK/STAT, c-Kit and IL-3 receptor signaling. 43,44 Intriguingly, a role for SOCS1 in antitumor dendritic cell vaccines has also been identified.…”

TLR3 Sensing of Viral Infection~!2009-10-01~!2010-03-30~!2010-04-28~!

“…The human promoter for TLR3 contains interferon response elements (IREs) and a STAT element; the cognate transcription factors are suggested to be IRF1 and STAT1 [34]. Indeed dominant-negative expression of STAT1 can abolish poly I:C induced TLR3 expression in murine cells [42]. LPS also upregulates TLR3 expression via the autocrine action of LPS-induced IFN secretion.…”

“…Expression of SOCS1 can inhibit poly I:C induced STAT1 phosphorylation activity. This may represent an autocrine regulatory mechanism to control TLR3 signalling [42]. West Nile virus non structural protein 1 (NS1) inhibited activation and nuclear translocation of both NF-B and IRF3 in response to poly I:C stimulation of HeLa cells [118].…”