Enhancement of Cytokine-Mediated Beta-Cell Apoptosis by Free Fatty Acids

Type 1 diabetes (T1D) results from dysfunction and loss of the insulin-producing pancreatic β cells. The body’s lipid metabolism is strongly regulated during this process but there is a need to understand how this regulation contributes to the β-cell death. Here, we investigated the role of free fatty acids (FFAs) in T1D development. Lipidomics data from the case-control study of The Determinants of Diabetes in the Young (TEDDY) consortium were re-analyzed to determine temporal changes in lipid profiles during T1D development. Fatty acid distribution across the pancreas was measured by mass spectrometry imaging. To model islet inflammation, FFAs were measured in human islets treated with the pro-inflammatory cytokines IL-1β + IFNγ by gas chromatography-mass spectrometry. Further, the effects of FFAs on MIN6 insulin-producing cells were measured by proteomics analysis along with biochemical and cell biology assays. We investigated if similar effects occur in vivo during T1D on the islets’ single-cell RNA sequencing data from Human Pancreas Analysis Program (HPAP). During islet autoimmunity, prior to T1D onset, plasma phosphatidylcholine and triacylglycerols are reduced with a simultaneous increase of FFAs. Similarly in human islets, IL-1β + IFNγ induce the increase of palmitate. Moreover, FFAs are abundantly detected across islets and surrounding exocrine tissue. FFAs synergistically enhanced cytokine-mediated apoptosis in MIN6 cells by downregulating the production of nicotinamide adenosine dinucleotide (NAD) via downregulating nicotinamide phosphoribosyltransferase (NAMPT), a rate-limiting enzyme of the NAD salvage pathway. The enhancement of cytokine-mediated apoptosis was reverted by supplementing cells with nicotinic acid and nicotinamide mononucleotide, metabolites that bypass NAMPT in NAD biosynthesis. NAMPT downregulation was further observed during T1D development, supporting that NAD production might be compromised in vivo. Our findings show that fatty acids are released during islet autoimmunity. These fatty acids enhance pro-inflammatory cytokine-mediated apoptosis through impaired NAD metabolism.