Intracellular heat shock protein-70 negatively regulates TLR4 signaling in the newborn intestinal epithelium.

Necrotizing enterocolitis (NEC) is the leading cause of gastrointestinal-related mortality in premature infants, and it develops under conditions of exaggerated TLR4 signaling in the newborn intestinal epithelium. Because NEC does not develop spontaneously, despite the presence of seemingly tonic stimulation of intestinal TLR4, we hypothesized that mechanisms must exist to constrain TLR4 signaling that become diminished during NEC pathogenesis and focused on the intracellular stress response protein and chaperone heat shock protein-70 (Hsp70). We demonstrate that the induction of intracellular Hsp70 in enterocytes dramatically reduced TLR4 signaling, as assessed by LPS-induced NF-κB translocation, cytokine expression, and apoptosis. These findings were confirmed in vivo, using mice that either globally lacked Hsp70 or overexpressed Hsp70 within the intestinal epithelium. TLR4 activation itself significantly increased Hsp70 expression in enterocytes, which provided a mechanism of autoinhibition of TLR4 signaling in enterocytes. In seeking to define the mechanisms involved, intracellular Hsp70-mediated inhibition of TLR4 signaling required both its substrate-binding EEVD domain and association with the cochaperone CHIP, resulting in ubiquitination and proteasomal degradation of TLR4. The expression of Hsp70 in the intestinal epithelium was significantly decreased in murine and human NEC compared with healthy controls, suggesting that loss of Hsp70 protection from TLR4 could lead to NEC. In support of this, intestinal Hsp70 overexpression in mice and pharmacologic upregulation of Hsp70 reversed TLR4-induced cytokines and enterocyte apoptosis, as well as prevented and treated experimental NEC. Thus, a novel TLR4 regulatory pathway exists within the newborn gut involving Hsp70 that may be pharmacologically activated to limit NEC severity.

Toll-like receptor-4 inhibits enterocyte proliferation via impaired beta-catenin signaling in necrotizing enterocolitis.

BACKGROUND & AIMS: Necrotizing enterocolitis (NEC), the leading cause of gastrointestinal death from gastrointestinal disease in preterm infants, is characterized by exaggerated TLR4 signaling and decreased enterocyte proliferation through unknown mechanisms. Given the importance of beta-catenin in regulating proliferation of many cell types, we hypothesize that TLR4 impairs enterocyte proliferation in NEC via impaired beta-catenin signaling. METHODS: Enterocyte proliferation was detected in IEC-6 cells or in ileum or colon from wild-type, TLR4-mutant, or TLR4(-/-) mice after induction of NEC or endotoxemia. beta-Catenin signaling was assessed by cell fractionation or immunoconfocal microscopy to detect its nuclear translocation. Activation and inhibition of beta-catenin were achieved via cDNA or small interfering RNA, respectively. TLR4 in the intestinal mucosa was inhibited with adenoviruses expressing dominant-negative TLR4. RESULTS: TLR4 activation significantly impaired enterocyte proliferation in the ileum but not colon in newborn but not adult mice and in IEC-6 enterocytes. beta-Catenin activation reversed these effects in vitro. To determine the mechanisms involved, TLR4 activation phosphorylated the upstream inhibitory kinase GSK3beta, causing beta-catenin degradation. NEC in both mouse and humans was associated with decreased beta-catenin and increased mucosal GSK3beta expression. Strikingly, the inhibition of enterocyte beta-catenin signaling in NEC could be reversed, and enterocyte proliferation restored, through adenoviral-mediated inhibition of TLR4 signaling in the small intestinal mucosa. CONCLUSION: We now report a novel pathway linking TLR4 with inhibition of beta-catenin signaling via GSK3beta activation, leading to reduced enterocyte proliferation in vitro and in vivo. These data provide additional insights into the pathogenesis of diseases of intestinal inflammation such as NEC.