Hepatic steatosis is defined by the accumulation of lipid droplets (LDs). Once thought to be only inert energy storage depots, LDs are increasingly recognized as organelles that have important functions in hepatocytes beyond lipid storage. The lipid and protein composition of LDs is highly dynamic and influences their intrinsic metabolism and signaling properties, which ultimately links them to the changes in hepatic function. This concise review will highlight recent discoveries in LD biology and highlight unique aspects of hepatic LDs and their role in liver disease.

During normal proliferation, hepatocytes accumulate triglycerides (TGs) in lipid droplets (LDs), but the underlying mechanisms and functional significance of this steatosis are unknown. In the current study, we examined the coordinated regulation of cell cycle progression and LD accumulation. As previously shown, hepatocytes develop increased LD content after mitogen stimulation. Cyclin D1, in addition to regulating proliferation, was both necessary and sufficient to promote LD accumulation in response to mitogens. Interestingly, cyclin D1 promotes LD accumulation by inhibiting the breakdown of TGs by lipolysis through a mechanism involving decreased lipophagy, the autophagic degradation of LDs. To examine whether inhibition of lipolysis is important for cell cycle progression, we overexpressed adipose TG lipase (ATGL), a key enzyme involved in TG breakdown. As expected, ATGL reduced LD content but also markedly inhibited hepatocyte proliferation, suggesting that lipolysis regulates a previously uncharacterized cell cycle checkpoint. Consistent with this, in mitogen‐stimulated cells with small interfering RNA‐mediated depletion of cyclin D1 (which inhibits proliferation and stimulates lipolysis), concurrent ATGL knockdown restored progression into S phase. Following partial hepatectomy, a model of robust hepatocyte proliferation in vivo, ATGL overexpression led to decreased LD content, cell cycle inhibition, and marked liver injury, further indicating that down‐regulation of lipolysis is important for normal hepatocyte proliferation. Conclusion: We suggest a new relationship between steatosis and proliferation in hepatocytes: cyclin D1 inhibits lipolysis, resulting in LD accumulation, and suppression of lipolysis is necessary for cell cycle progression.

Caloric restriction (CR) and endurance exercise elicit wide-ranging health benefits including reduced risk of select cancers. In addition, diet composition influences oncogenesis, although its interactions with exercise and CR are not well understood. Therefore, to investigate the potential interactions between diet and lifestyle interventions on liver tumorigenesis, the carcinogen diethylnitrosamine was administered to 72 male C57Bl/6 mice that were subsequently fed diets enriched with lard (CTL) or olive oil and were further stratified to voluntary wheel running (Ex) or 30% CR for 49 weeks. Although Ex and diet composition did not influence liver oncogenesis, CR prevented hepatic tumor formation. In addition, CR reduced steatosis, hepatocyte ballooning, inflammation, and immune cell infiltration, all of which are hallmarks in the progression of nonalcoholic fatty liver disease to liver tumorigenesis. RNA sequencing of nontransformed liver tissues from CR mice revealed changes in metabolic pathways and reduced inflammation, cytokine production, stellate cell activation and migration, and genes associated with liver injury and oncogenesis. These data demonstrate that CR protects against steatosis, liver inflammation, and liver injury and is a robust deterrent of carcinogen-induced hepatic oncogenesis..

<p>Fig. S1. Phenotypic and metabolic parameters are influenced by Ex and CR. Fig. S2. CR modulates additional markers of hepatic inflammation and immune cell infiltration. Fig. S3. CR causes divergence of genes in comparison to CTL mice. Fig. S4. CR affects gene signature pathways and networks associated with hepatic inflammation and metabolism. Table S1. Description of components (g/Kg) and macronutrient content (% kcal) of diets.</p>

<div>Abstract<p>Caloric restriction (CR) and endurance exercise elicit wide-ranging health benefits including reduced risk of select cancers. In addition, diet composition influences oncogenesis, although its interactions with exercise and CR are not well understood. Therefore, to investigate the potential interactions between diet and lifestyle interventions on liver tumorigenesis, the carcinogen diethylnitrosamine was administered to 72 male C57Bl/6 mice that were subsequently fed diets enriched with lard (CTL) or olive oil and were further stratified to voluntary wheel running (Ex) or 30% CR for 49 weeks. Although Ex and diet composition did not influence liver oncogenesis, CR prevented hepatic tumor formation. In addition, CR reduced steatosis, hepatocyte ballooning, inflammation, and immune cell infiltration, all of which are hallmarks in the progression of nonalcoholic fatty liver disease to liver tumorigenesis. RNA sequencing of nontransformed liver tissues from CR mice revealed changes in metabolic pathways and reduced inflammation, cytokine production, stellate cell activation and migration, and genes associated with liver injury and oncogenesis. These data demonstrate that CR protects against steatosis, liver inflammation, and liver injury and is a robust deterrent of carcinogen-induced hepatic oncogenesis. <i>Cancer Prev Res; 10(11); 660–70. ©2017 AACR</i>.</p></div>