Complement Factor D protects mice from ethanol-induced inflammation and liver injury

Abstract

Complement plays a crucial role in microbial defense and clearance of apoptotic cells. Emerging evidence suggests complement is an important contributor to alcoholic liver disease. While complement component 1, Q subcomponent (C1q)-dependent complement activation contributes to ethanol-induced liver injury, the role of the alternative pathway in ethanol-induced injury is unknown. Activation of complement via the classical and alternative pathways was detected in alcoholic hepatitis patients. Female C57BL/6J [wild type (WT)], C1q-deficient ( C1qa, lacking classical pathway activation), complement protein 4-deficient ( C4, lacking classical and lectin pathway activation), complement factor D-deficient ( FD, lacking alternative pathway activation), and C1qa/FD (lacking classical and alternative pathway activation) mice were fed an ethanol-containing liquid diet or pair-fed control diet for 4 or 25 days. Following chronic ethanol exposure, liver injury, steatosis, and proinflammatory cytokine expression were increased in WT but not C1qa, C4, or C1qa/FD mice. In contrast, liver injury, steatosis, and proinflammatory mediators were robustly increased in ethanol-fed FD mice compared with WT mice. Complement activation, assessed by hepatic accumulation of C1q and complement protein 3 (C3) cleavage products (C3b/iC3b/C3c), was evident in livers of WT mice in response to both short-term and chronic ethanol. While C1q accumulated in ethanol-fed FD mice (short term and chronic), C3 cleavage products were detected after short-term but not chronic ethanol. Consistent with impaired complement activation, chronic ethanol induced the accumulation of apoptotic cells and fibrogenic responses in the liver of FD mice. These data highlight the protective role of complement factor D (FD) and suggest that FD-dependent amplification of complement is an adaptive response that promotes hepatic healing and recovery in response to chronic ethanol. NEW & NOTEWORTHY Complement, a component of the innate immune system, is an important pathophysiological contributor to ethanol-induced liver injury. We have identified a novel role for factor D, a component of the alternative pathway, in protecting the liver from ethanol-induced inflammation, accumulation of apoptotic hepatocytes, and profibrotic responses. These data indicate a dual role of complement with regard to inflammatory and protective responses and suggest that accumulation of apoptotic cells impairs hepatic healing/recovery during alcoholic liver disease.

Inflammatory pathways in alcoholic steatohepatitis

“…For example, complement activation via the alternative pathway is critical to the removal of injured and dying hepatocytes in models of both fibrosis and early ASH. 82,83 MicroRNA, EVs MicroRNAs (miRNAs), small non-coding RNA molecules of 19-25 nucleotides, can induce RNA silencing and regulate gene expression at posttranscriptional levels, playing important roles in a variety of cellular functions. Recent studies have demonstrated that many miRNAs are involved in directly or indirectly regulating inflammatory pathways in ASH.…”

Anaphylatoxin Receptors C3aR and C5aR1 Are Important Factors That Influence the Impact of Ethanol on the Adipose Secretome

“…The overall cellularity of adipose consists of adipocytes and the stromal vascular fraction, including perivascular cells, vascular endothelium, stem cells, pre-adipocytes and immune cells ( 1 , 47 ). Because ethanol activates complement to generate anaphylatoxins ( 8 , 18 ), known activators of the innate immune system, we hypothesized that adipose SVCs, including macrophages, would be the major contributor to ethanol-induced adipose inflammation in mALD. Despite high C5aR1 expression on SVCs compared to adipocytes, data from bone marrow chimeras suggest that C5aR1 expression on cells of non-myeloid origin, likely adipocytes, are the major contributor to the induction of inflammatory responses caused from ethanol feeding in adipose (Figure 3 ).…”

“…This is of particular interest for alcohol-mediated pathologies, as gut-derived LPS can stimulate multiple components of the innate immune system and perpetuate systemic inflammation. Indeed, peripheral C5a ( 8 ) as well as circulating endotoxin is increased in ALD patients ( 48 , 49 ). Here we find that ethanol feeding, directly via the activation of complement, stimulates spontaneous secretion of multiple mediators, including chemokines (MCP-1 and chemerin), adipokines (LCN2) and pro-inflammatory cytokines (TNFα) from isolated adipocytes in a C5aR1-dependent manner (Figures 4 , 5 ).…”

“…In addition to acting as an immune surveillance mechanism, a growing body of evidence suggests complement is a key modulator of metabolic diseases, including non-alcoholic and alcoholic liver disease. Complement is activated via the classical, lectin and alternative pathways, converging at complement component 3 (C3) ( 8 ). Initiation of the terminal pathway leads to C3 and complement component 5 (C5) cleavage, generating the potent anaphylatoxins C3a and C5a, respectively.…”

“…Complement activation products are increased in ALD patients and in murine ALD (mALD) ( 8 , 17 , 18 ). While the anaphylatoxin receptors have well-defined roles in modulating inflammation, less is known about their role in moderating adipose inflammation in mALD.…”