Phospholipase C-related catalytically inactive protein regulates lipopolysaccharide-induced hypothalamic inflammation-mediated anorexia in mice.

Peripheral lipopolysaccharide (LPS) injection induces systemic inflammation through the activation of the inhibitor of nuclear factor kappa B (NF-κB) kinase (IKK)/NF-κB signaling pathway, which promotes brain dysfunction resulting in conditions including anorexia. LPS-mediated reduction of food intake is associated with activation of NF-κB signaling and phosphorylation of the transcription factor signal transducer and activator of transcription 3 (STAT3) in the hypothalamus. We recently reported phospholipase C-related catalytically inactive protein (PRIP) as a new negative regulator of phosphatidylinositol 3-kinase/AKT signaling. AKT regulates the IKK/NF-κB signaling pathway; therefore, this study aimed to investigate the role of PRIP/AKT signaling in LPS-mediated neuroinflammation-induced anorexia. PRIP gene (Prip1 and Prip2) knockout (Prip-KO) mice intraperitoneally (ip) administered with LPS exhibited increased anorexia responses compared with wild-type (WT) controls. Although few differences were observed between WT and Prip-KO mice in LPS-elicited plasma pro-inflammatory cytokine elevation, hypothalamic pro-inflammatory cytokines were significantly upregulated in Prip-KO rather than WT mice. Hypothalamic AKT and IKK phosphorylation and IκB degradation were significantly increased in Prip-KO rather than WT mice, indicating further promotion of AKT-mediated NF-κB signaling. Consistently, hypothalamic STAT3 was further phosphorylated in Prip-KO rather than WT mice. Furthermore, suppressor of cytokine signaling 3 (Socs3), a negative feedback regulator for STAT3 signaling, and cyclooxogenase-2 (Cox2), a candidate molecule in LPS-induced anorexigenic responses, were upregulated in the hypothalamus in Prip-KO rather than WT mice. Pro-inflammatory cytokines were upregulated in hypothalamic microglia isolated from Prip-KO rather than WT mice. Together, these findings indicate that PRIP negatively regulates LPS-induced anorexia caused by pro-inflammatory cytokine expression in the hypothalamus, which is mediated by AKT-activated NF-κB signaling. Importantly, hypothalamic microglia participate in this PRIP-mediated process. Elucidation of PRIP-mediated neuroinflammatory responses may provide novel insights into the pathophysiology of many brain dysfunctions.

Immobilization stress-induced anorexia is mediated independent of MyD88.

MyD88 is an adaptor protein for the toll-like receptor, which is involved in regulating innate immune function. Lipopolysaccharide-induced activation of toll-like receptor 4 signaling induces hypothalamic signal transducer and activator of transcription 3 (STAT3) phosphorylation and anorexia through MyD88. In the present study, we investigated the possible role of MyD88 in psychological stress-induced anorexia. We found that immobilization stress inhibited food intake in both wild-type mice and MyD88-deficient mice. Immobilization stress slightly increased STAT3 phosphorylation in the hypothalamus, but it was weaker than the lipopolysaccharide-induced increase in STAT3 phosphorylation. These observations suggest that the mechanisms involved in psychological stress-induced anorexia may be regulated differently from those involved in anorexia that is induced by infection.

MyD88 plays a key role in LPS-induced Stat3 activation in the hypothalamus.

Infection causes the production of proinflammatory cytokines, which act on the central nervous system (CNS) and can result in fever, sleep disorders, depression-like behavior, and even anorexia, although precisely how cytokines regulate the functions of the CNS remain unclear. In the present study, we investigated the regulatory-molecular mechanisms by which cytokines affect hypothalamic function in a state of infection. The intraperitoneal administration of lipopolysaccharide (LPS), a ligand of Toll-like receptor 4 (TLR4), time-dependently (2-24 h) increased signal transducer and activator of transcription 3 (STAT3) phosphorylation in the hypothalamus and liver, which corresponded with anorexia observed within 24 h. Interestingly, the pattern of phosphorylation in response to LPS differed between the hypothalamus and liver. In the hypothalamus, LPS increased STAT3 phosphorylation from 2 h, with a peak at 4 h and a decline thereafter, whereas, in the liver, the peak activation of STAT3 persisted from 2 to 8 h. The time course of the LPS-induced expression of suppressor of cytokine signaling 3 (SOCS3), a STAT3-induced negative regulator of the Janus kinase-STAT pathway, was similar to that of STAT3 phosphorylation. Using mice deficient in myeloid differentiation primary-response protein 88 (MyD88), an adapter protein of TLR4, we found that LPS-induced STAT3 phosphorylation and SOCS3 expression in the hypothalamus and liver were predominantly mediated through MyD88. Moreover, we observed that MyD88-deficient mice were resistant to LPS-induced anorexia. Taken together, our findings reveal a novel mechanism, i.e., MyD88 plays a key role in mediating STAT3 phosphorylation and anorexia in the CNS in a state of infection and inflammation.