Composition of fatty acids in a high-fat diet affects adipose tissue inflammation by inducing calreticulin on adipocytes and activating group 1 innate lymphoid cells

Obesity-induced adipose tissue inflammation plays a critical role in the development of metabolic diseases. For example, NK1.1⁺ group 1 innate lymphoid cells (G1-ILCs) in adipose tissues are activated in the early stages of inflammation in response to a high-fat diet (HFD). In this study, we examined whether the composition of fatty acids affected adipose inflammatory responses induced by an HFD. Mice were fed a stearic acid (C18:0)-rich HFD (HFD-S) or a linoleic acid (C18:2)-rich HFD (HFD-L). HFD-L-fed mice showed significant obesity compared with HFD-S-fed mice. Visceral and subcutaneous fat pads were enlarged and contained more NK1.1⁺KLRG1⁺ cells, indicating that G1-ILCs were activated in HFD-L-fed mice. We examined early changes in adipose tissues during the first week of HFD intake, and found that mice fed HFD-L showed increased levels of NK1.1⁺CD11b⁺KLRG1⁺ cells in adipose tissues. In adipose tissue culture, addition of 4-hydroxynonenal, the most frequent product of lipid peroxidation derived from unsaturated fatty acids, induced NK1.1⁺CD11b⁺CD27⁻ cells. We found that calreticulin, a ligand for the NK activating receptor, was induced on the surface of adipocytes after exposure to 4-hydroxynonenal or a 1-week feeding with HFD-L. Thus, excess fatty acid intake and the activation of G1-ILCs initiate and/or modify adipose inflammation.