Short-term high fat diet feeding of mice suppresses catecholamine-stimulated Ca 2+ signalling in hepatocytes and intact liver.

Excess consumption of carbohydrates, fat, and calories leads to non-alcoholic fatty liver disease (NAFLD) and hepatic insulin resistance; major factors in the pathogenesis of type II diabetes. Hormones and catecholamines acting through G-protein coupled receptors (GPCRs) linked to phospholipase C (PLC) and increases in cytosolic Ca 2+ ([Ca 2+ ] c ) regulate many metabolic functions of the liver. In the intact liver, catabolic hormones such as glucagon, catecholamines and vasopressin integrate and synergize to regulate the frequency and extent to which [Ca 2+ ] c waves propagate across hepatic lobules to control metabolism. Dysregulation of hepatic Ca 2+ homeostasis has been implicated in the development of metabolic disease, but changes in hepatic GPCR-dependent Ca 2+ signalling have been largely unexplored in this context. We show that short-term, 1-week, high fat diet (HFD) feeding of mice attenuates norepinephrine-stimulated Ca 2+ signalling, reducing the number of cells responding and suppressing the frequency of [Ca 2+ ] c oscillations in both isolated hepatocytes and intact liver. The 1-week HFD feeding paradigm did not change basal Ca 2+ homeostasis; endoplasmic reticulum Ca 2+ load, store-operated Ca 2+ entry and plasma membrane Ca 2+ pump activity were unchanged compared to low fat diet (LFD) fed controls. However, norepinephrine-induced IP 3 production was significantly reduced after HFD feeding, demonstrating an effect of HFD on receptor-stimulated PLC activity. Thus, we have identified a lesion in the PLC signalling pathway induced by short-term HFD feeding, which interferes with hormonal Ca 2+ signalling in isolated hepatocytes and the intact liver. These early events may drive adaptive changes in signalling, which lead to pathological consequences in fatty liver disease. KEY POINTS: Non-alcoholic fatty liver disease (NAFLD) is a growing epidemic. In healthy liver, the counteracting effects of catabolic and anabolic hormones regulate metabolism and energy storage as fat. Hormones and catecholamines promote catabolic metabolism via increases in cytosolic Ca 2+ ([Ca 2+ ] c ). We show that 1 week high fat diet (HFD) feeding of mice attenuated the Ca 2+ signals induced by physiological concentrations of norepinephrine. Specifically, HFD suppressed the normal pattern of periodic [Ca 2+ ] c oscillations in isolated hepatocytes and disrupted the propagation of intralobular [Ca 2+ ] c waves in the intact perfused liver. Short-term HFD inhibited norepinephrine-induced inositol 1,4,5-trisphosphate (IP 3 ) generation, but did not change basal endoplasmic reticulum Ca 2+ load or plasma membrane Ca 2+ fluxes. We propose that impaired Ca 2+ signalling plays a key role in the earliest phases of the etiology of NAFLD, and is responsible for many of the ensuing metabolic and related dysfunctional outcomes at the cellular and whole tissue level. Abstract figure legend Ca2 -dependent hormones such as norepinephrine activate the PLC/IP3 signaling pathway to elicit frequency-modulated Ca2 oscillations in hepatocytes, which are propagated as periodic Ca2 waves across entire liver lobules. These Ca2 signals stimulate oxidative metabolism and inhibit lipogenesis. Short-term high fat diet feeding in mice inhibits norepinephrine-stimulated IP3 formation, reducing Ca2 oscillations frequency and disrupting Ca2 wave propagation into the pericentral zone of the hepatic lobule. Thus, high fat diet suppresses the Ca2 signaling that would otherwise oppose triglyceride accumulation, particularly in the pericentral zone, and may drive adaptive changes in Ca2 homeostasis that contribute to fatty liver disease. This article is protected by copyright. All rights reserved.