Abstract P157: Altered ERK-mediated Control Of AgRP And Metabolic Rate During Obesity

Abstract

Adaptation of resting metabolic rate (RMR) contributes to the maintenance of human obesity, yet the control of RMR - and how this system changes during obesity - remains poorly defined. Leptin normally suppresses Agouti-related peptide (AgRP) expression in the arcuate nucleus of the hypothalamus (ARC) to disinhibit melanocortin-mediated stimulation of RMR. Leptin stimulates ERK signaling in the ARC (a known suppressor of AgRP), and the inhibition of ARC ERK attenuates thermogenic sympathetic nervous activity responses to leptin. Diet-induced obesity (DIO) increases leptin, but paradoxically exaggerates ARC AgRP expression and causes RMR adaptation. Thus, we hypothesize that leptin increases ERK signaling in AgRP cells, to reduce AgRP and thereby ultimately increase RMR, but that this signaling mechanism becomes dysfunctional during DIO. Leptin application to GT1-7 cells (a model of AgRP neurons) increased pERK/ERK ratio (1 uM, 15 min leptin n=3, 2.9-fold of vehicle n=2, p<0.05). In chow-fed adult mice, leptin injection caused an increase in ARC pERK/ERK ratio (2 ug/g ip 30 min leptin n=11, +43.8% vs saline n=9, p<0.05) without affecting total ERK levels (normalized to beta-actin). DIO caused increases in endogenous plasma leptin and an increase in ARC pERK/ERK ratio (HFD n=5, +54.4% vs chow n=4, p<0.05) without affecting total ERK levels - yet, canonical transcriptomic effects of ERK (ie, suppression of AgRP) were lost. Finally, reanalysis of our published single-nucleus RNAsequencing dataset describing the transcriptome of ARC after DIO demonstrates loss of canonical effects of increased ERK signaling specifically within the AgRP cell type. Together, these findings indicate that although DIO is associated with expected increases in ERK activation in AgRP neurons and the ARC, the consequences of ERK signaling are attenuated. We therefore hypothesize that DIO results in RMR adaptation through changes in signaling mechanisms that link ERK to its downstream targets, such as AgRP. Ongoing studies are focused on the development of novel animal models with tamoxifen-dependent conditional deletion of ERK within the adult AgRP neuron, to test whether loss of ERK signaling is sufficient to explain changes in ERK/AgRP/RMR control during DIO.