NAMPT and BMAL1 Are Independently Involved in the Palmitate-Mediated Induction of Neuroinflammation in Hypothalamic Neurons

Obesity is a prominent metabolic disease that predisposes individuals to multiple comorbidities, including type 2 diabetes mellitus, cardiovascular diseases, and cancer. Elevated circulating levels of fatty acids contribute to the development of obesity, in part, by targeting the hypothalamus. Palmitate, the most abundant circulating saturated fatty acid, has been demonstrated to dysregulate NAMPT and circadian clock proteins, as well as induce neuroinflammation. These effects ultimately result in hypothalamic dysregulation of feeding behavior and energy homeostasis. NAMPT is the rate-limiting enzyme of the NAD+ salvage pathway and its expression is under the control of the circadian clock. NAD+ produced from NAMPT can modulate the circadian clock, demonstrating bidirectional interactions between circadian and metabolic pathways. Using NPY/AgRP-expressing mHypoE-46 neurons as well as the novel mHypoA-BMAL1-WT/F and mHypoA-BMAL1-KO/F cell lines, we studied whether there were any interactions between NAMPT and the core circadian clock protein BMAL1 in the palmitate-mediated induction of neuroinflammation. We report that palmitate alteredNampt, Bmal1, Per2and the inflammatory genesNf-kappa b, I kappa B alpha, Il-6, andTlr4. Contrary to studies performed with peripheral tissues, the palmitate-mediated induction inNamptwas independent of BMAL1, and basalNamptlevels did not appear to exhibit rhythmic expression. Palmitate-induced downregulation ofBmal1andPer2was independent of NAMPT. However, NAMPT and BMAL1 were both involved in the regulation ofNf-kappa b, I kappa B alpha, Il-6, andTlr4, as NAMPT inhibition resulted in the repression of basalNf-kappa bandI kappa B alpha and normalized palmitate-mediated increases inIl-6, andTlr4. On the other hand, BMAL1 deletion repressed basalNf-kappa b, but increased basalIl-6. We conclude that NAMPT and BMAL1 do not interact at the transcriptional level in hypothalamic neurons, but are independently involved in the expression of inflammatory genes.