TLR4 and metabolic syndrome: Mounting evidence indicates that air pollutions are associated with a high incidence of NAFLD and metabolic syndrome [38,39,40,41,42] through the activation of Kupffer cells [43], c-Jun N-terminal Kinases-activator protein 1, nuclear factor-κB, Toll-like receptor-4, and peroxisome proliferator-activated receptors [44].