Hepatic lipolysis by β‐adrenergic stimulation is inhibited by ethanol exposure
Authors: Schott Micah , Rasineni Karuna , Weller Shaun , Schulze Ryan , Casey Carol , McNiven Mark 
Published: November 2023

Steatotic hepatocytes accumulate profound numbers of large neutral lipid‐storage organelles called lipid droplets (LDs). The process by which the formation or utilization of these organelles are perturbed during NAFLD and NASH is poorly defined. EtOH consumption can also exacerbate these conditions markedly by yet unknown mechanisms. Thus, there is a critical need to understand lipolytic mechanisms for LD catabolism in hepatocytes. In adipocytes, catecholamines induce lipolysis through a β‐adrenergic/cAMP pathway that activates cytosolic lipases ATGL and HSL to catabolize LD triglycerides. Importantly, β‐adrenergic induction of LD catabolism has not been demonstrated in hepatocytes. Thus, the GOAL of this study was to determine whether hepatocytes, like adipocytes, also undergo cAMP‐mediated lipolysis in response to β‐adrenergic (β‐AR) stimulation. First, treatment of primary rat hepatocytes or Hep3B human hepatoma cells with the β‐AR agonist isoproterenol caused a 20–40% reduction in total cellular LD content within 24 hours. β‐AR induced LD loss was mimicked by the cAMP agonist forskolin/IBMX and was abrogated by the cAMP‐dependent protein kinase (PKA) inhibitor H89. In correlation, PKA activation following β‐AR stimulation was confirmed by FRET biosensors and biochemical analysis. β‐AR stimulation directly activated the lipolytic pathway, as evidenced by cAMP/PKA‐mediated phosphorylation of hormone sensitive lipase (HSL) and adipose triglyceide lipase (ATGL). In addition, pharmacological inhibitors against ATGL or HSL each blocked β‐AR induced LD loss, suggesting that lipase activation is critical for this process in stimulated cells. To examine whether β‐AR/cAMP stimulation triggered lipase recruitment directly to the LD, live‐cell imaging of Hep3B cells expressing either ATGL‐EGFP or HSL‐sfGFP showed that within 60 min of stimulation, each lipase was significantly displaced from the cytosol and recruited to the LD surface. This recruitment was blocked by the PKA inhibitor H89. Finally, to test if EtOH exposure attenuates the activation of lipolysis in hepatocytes, we isolated primary hepatocytes from rats fed a 6‐week ethanol‐containing Lieber‐DeCarli diet. Compared with controls, ethanol‐exposed hepatocytes showed a drastic inhibition in lipase activation and LD breakdown following β‐AR/cAMP stimulation. Remarkably, in ethanol‐metabolizing VA13 hepatocytes, ethanol insult completely prevented isoproterenol‐induced translocation of ATGL‐EGFP to the LD. In summary, these results show that β‐adrenergic stimulation mobilizes cytosolic lipases for LD breakdown in hepatocytes, and perturbation of this pathway could be a major consequence of chronic ethanol insult leading to fatty liver. Manipulation of this regulatory cascade could be useful in developing new therapies to attenuate fatty liver disease.Support or Funding InformationThis study was generously supported by funding from NIAAA 5R01AA020735 (MAM and CAC) and NIDDK 5R37DK044650 (MAM), T32DK007352‐36 (MBS), and P30DK084567 (Mayo Clinic Center for Cell Signaling in Gastroenterology).