Immunomodulatory roles of CTRP3 in endotoxemia and metabolic stress

Abstract

C1q/TNF-related protein 3 (CTRP3) is a secreted hormone that modulates hepatic glucose and lipid metabolism. Its circulating levels are reduced in human and rodent models of obesity, a metabolic state accompanied by chronic low-grade inflammation. Recent studies have demonstrated an anti-inflammatory role for recombinant CTRP3 in attenuating LPS-induced systemic inflammation, and its deficiency markedly exacerbates inflammation in a mouse model of rheumatoid arthritis. We used genetic mouse models to explore the immunomodulatory function of CTRP3 in response to acute (LPS challenge) and chronic (high-fat diet) inflammatory stimuli. In a sublethal dose of LPS challenge, neither CTRP3 deficiency nor its overexpression in transgenic mice had an impact on IL-1β, IL-6, TNF-α, or MIP-2 induction at the serum protein or mRNA levels, contrary to previous findings based on recombinant CTRP3 administration. In a metabolic context, we measured 71 serum cytokine levels in wild-type and CTRP3 transgenic mice fed a high-fat diet or a matched control low-fat diet. On a low-fat diet, CTRP3 transgenic mice had elevated circulating levels of multiple chemokines (CCL11, CXCL9, CXCL10, CCL17, CX3CL1, CCL22 and sCD30). However, when obesity was induced with a high-fat diet, CTRP3 transgenic mice had lower circulating levels of IL-5, TNF-α, sVEGF2, and sVEGFR3, and a higher level of soluble gp130. Contingent upon the metabolic state, CTRP3 overexpression altered chemokine levels in lean mice, and attenuated systemic inflammation in the setting of obesity and insulin resistance. These results highlight a context-dependent immunomodulatory role for CTRP3.

Keywords: CTRP3; LPS; cytokines; inflammation; obesity.

Figure 1

Increased expression of inflammatory cytokines in the epididymal fat depot of WT, CTRP3 Tg, and KO mice challenged with LPS. Quantitative real‐time PCR analysis of Il1β (A), Il6 (B), Tnfα (C), and Mip2/Cxcl2 (D) in the visceral (epididymal) fat depot of WT (n = 7), CTRP3 Tg (n = 8), and CTRP3 KO (n = 6) male mice injected with saline control or LPS (1 μg). (E) Expression of Ctrp3 in the visceral adipose tissue of LPS‐injected WT male mice. Expression levels were normalized to cyclophilin A (CypA). All data are expressed as mean ± SEM. *P < 0.05; **P < 0.01; ***P < 0.005 (saline vs. LPS injected mice)

Figure 2

Increased serum levels of inflammatory cytokines in WT, CTRP3 Tg, and KO mice challenged with LPS. Serum IL‐1β (A), IL‐6 (B), and TNF‐α (C) levels in WT (n = 7), CTRP3 Tg (n = 8), and CTRP3 KO (n = 6) male mice injected with saline control or LPS (1 μg). All data are expressed as mean ± SEM.

Figure 3

Altered serum cytokine levels in CTRP3 Tg compared to WT mice fed a control low‐fat diet. Serum levels of CCL11 (A), CXCL9 (B), CXCL10 (C), secreted CD30 (D), CCL17 (E), CX3CL1 (F), CCL22 (G) and CXCL5 (H) in WT and CTRP3 Tg male mice fed a high‐fat diet. All data are expressed as mean ± SEM (N = 8–9 per group). *P < 0.05.

Figure 4

Altered serum cytokine levels in CTRP3 Tg compared to WT mice fed calorie‐dense high‐fat diet. Serum levels of IL‐5 (A), TNF‐α (B), sgp130 (C), secreted VEGF2 (D), and secreted VEGFR3 (E) in WT and CTRP3 Tg fed a low‐fat diet. All data are expressed as mean ± SEM (N = 8–9 per group). *P < 0.05.

Figure 5

Altered serum cytokine levels in CTRP3 Tg fed a control low‐fat diet or a calorie‐dense high‐fat diet. Serum levels of CCL11 (A), CXCL5 (B), CXCL10 (C), CCL21 (D), CCL22 (E), CXCL1 (F), sCD30 (G), sgp130 (H), sRAGE (I), Pentraxin‐3 (J), and AGP (K) in CTRP3 Tg fed a low‐fat diet (LFD) or high‐fat diet (HFD). All data are expressed as mean ± SEM (N = 8–9 per group). *P < 0.05.