Natural killer T (NKT) cells in adipose tissue (AT) contribute to whole body energy homeostasis.
RESULTS: Unraveling adipocyte-iNKT cell communication may help to fight
obesity-induced AT dysfunction. Overproduction and/or accumulation of
ceramide and
ceramide metabolites, including
glucosylceramides, can lead to
insulin resistance. However,
glucosylceramides also fulfill important physiological functions. They are presented by antigen presenting cells (APC) as endogenous
lipid antigens via CD1d to activate a unique lymphocyte subspecies, the CD1d-restricted invariant (i) natural killer T (NKT) cells. Recently, adipocytes have emerged as
lipid APC that can activate adipose tissue-resident iNKT cells and thereby contribute to whole body energy homeostasis. Here we investigate the role of the
glucosylceramide biosynthesis pathway in the activation of iNKT cells by adipocytes.
UDP-glucose ceramide glucosyltransferase (Ugcg), the first rate limiting step in the
glucosylceramide biosynthesis pathway, was inhibited via chemical compounds and
shRNA knockdown in vivo and in vitro. β-1,4-Galactosyltransferase (B4Galt) 5 and 6,
enzymes that convert
glucosylceramides into potentially inactive
lactosylceramides, were subjected to
shRNA knock down. Subsequently, (pre)adipocyte cell lines were tested in co-culture experiments with iNKT cells (IFNγ and
IL4 secretion). Inhibition of Ugcg activity shows that it regulates presentation of a considerable fraction of
lipid self-antigens in adipocytes. Furthermore, reduced expression levels of either B4Galt5 or -6, indicate that B4Galt5 is dominant in the production of cellular
lactosylceramides, but that inhibition of either
enzyme results in increased iNKT cell activation. Additionally, in vivo inhibition of Ugcg by the aminosugar
AMP-DNM results in decreased iNKT cell effector function in adipose tissue. Inhibition of endogenous
glucosylceramide production results in decreased iNKT cells activity and
cytokine production, underscoring the role of this biosynthetic pathway in
lipid self-antigen presentation by adipocytes.