Hepatocyte‐specific and extra‐hepatocyte actions of perilipin‐2 during fatty liver disease: benefits of being extra

Background

Non‐alcoholic fatty liver disease (NAFLD) is characterized by hepatic steatosis, typically induced by consumption of a Western diet (WD), and hepatocytes are the main target of injury. Severity of hepatic steatosis correlates with increased development of non‐alcoholic steatohepatitis (NASH) and liver fibrosis, identifying the importance of hepatocyte injury during NAFLD progression (James & Day, 1998).

Typically, lipids are stored as droplets and it is here that cell metabolism occurs; however, droplet composition varies based on pathology and influences cell signalling and metabolism. Perilipin 2 (Plin2) modulates lipid storage, and loss of Plin2 protects against obesity, NASH and insulin resistance in models of NAFLD (McManaman et al. 2013). However, the impact of dysregulated Plin2 signalling on NAFLD progression is poorly understood.

In an article by Orlicky and colleagues in this issue of The Journal of Physiology, the authors evaluated the role of Plin2 in obesity, progression to NASH and liver fibrosis in the chronic WD feeding model of NAFLD (Orlicky et al. 2019).

Novel findings

The authors used an innovative approach to understand the role of hepatocyte Plin2 by generating a hepatocyte‐specific knockout mouse model (Plin2‐HepKO), as well as total Plin2 knockout (Plin2‐Null) to understand extra‐hepatocyte Plin2 actions. Furthermore, the authors utilized a WD feeding model, which is an ideal approach since it (i) has a composition similar to a human diet and (ii) closely mimics the pathophysiological progression of human NAFLD.

When evaluating overall body changes, the authors noted that extra‐hepatocyte Plin2 loss ameliorates WD‐induced obesity, insulin resistance and epidydimal white adipose tissue (eWAT) inflammation. Interestingly, Plin2‐HepKO mice fed WD had similar increases in these characteristics to wild‐type (WT) mice fed WD. The findings suggest that hepatocyte‐derived Plin2 does not regulate WD‐induced eWAT damage or metabolic functions.

Obesity and metabolic dysfunction have been hypothesized to be drivers of fatty liver during NAFLD. As expected, Plin2‐Null mice were resistant to WD‐induced hepatomegaly and hepatosteatosis compared to WT mice fed WD. Interestingly, Plin2‐HepKO mice had a decrease in WD‐induced hepatomegaly and hepatosteatosis, with decreases in micro‐ but not macro‐vesicular steatosis, suggesting a local role for hepatocyte‐derived Plin2 on steatosis development.

Steatosis development is dependent on de novo lipid synthesis and hepatic free fatty acid uptake. The authors found that WD‐fed Plin2‐HepKO mice downregulate hepatic de novo lipogenesis, while Plin2‐Null mice show reduced hepatic fatty acid uptake. Hepatocyte versus extra‐hepatocyte Plin2 activation has differential signalling mechanisms during NAFLD development.

NAFLD to NASH progression is characterized by hepatic inflammation and immune cell infiltration, with increased hepatic stellate cell (HSC) activation and hepatic fibrosis at later stages. The authors noted that both Plin2‐Null and Plin2‐HepKO mice fed a WD had a decrease in these phenotypes when compared to WT mice fed WD. Specifically, it was found that the number of CD3⁺ T cells and CD45‐positive lymphocytes were significantly increased in WD‐fed WT mice, but reduced in WD‐fed Plin2‐Null and Plin2‐HepKO mice. It is important to note that CD3⁺ T cells and CD45‐positive lymphocytes are implicated in the progression of hepatic fibrosis during NAFLD (Bertola et al. 2010).

Hepatic inflammation and fibrosis are regulated by a variety of signalling mechanisms. WT mice fed WD had an increase in transforming growth factor (TGF)‐β1 and tumour necrosis factor (TNF)‐α levels alongside increased transcription of toll‐like receptor (TLR)‐2 and ‐4, interleukin (IL)‐33, chemokine ligand (CCL)‐2 and IL‐10. Expression of all of these factors was significantly reduced in Plin2‐Null mice fed WD; however, Plin2‐HepKO mice fed WD only had decreases in TNF‐α, CCL‐2 and TLR‐4. Interestingly, TGF‐β1, TLR‐2 and IL‐10 levels were unchanged between WT and Plin2‐HepKO mice fed WD. These findings underline the differential signalling mechanisms mediated by hepatocyte versus extra‐hepatocyte Plin2.

Lastly, the authors evaluated macrophage recruitment and polarization and found that Plin2‐Null and Plin2‐HepKO mice fed WD had a reduced macrophage recruitment and M1 (pro‐inflammatory) polarization compared to WT mice fed WD. Therefore, the recruitment of pro‐inflammatory macrophages may be mediated by hepatocyte‐derived Plin2.

Future directions

In their article, Orlicky et al. identified the role of hepatocyte versus extra‐hepatocyte Plin2 signalling in WD‐induced obesity, insulin resistance, and NAFLD to NASH progression. The authors identified that extra‐hepatocyte sources of Plin2 promote eWAT damage and obesity, which is not mediated by hepatocyte‐derived Plin2; however, these findings are based on total knockout of Plin2. Future work should identify the specific, non‐hepatocyte sources of Plin2 that modulate these phenotypes.

The authors found that both Plin2‐Null and Plin2‐HepKO mice were resistant to WD‐induced hepatic inflammation and fibrosis; however, both of these models are lacking hepatocyte‐derived Plin2. Future work should tease out the specific cell types mediating these changes. Understanding the role of Plin2 in other hepatic sources, such as HSCs and Kupffer cells, may be important for understanding NAFLD progression. Additionally, it will be important to recognize if non‐hepatic sources of Plin2, such as eWAT, mediate WD‐induced liver injury.

One noteworthy finding of the article was the differential signalling mechanisms mediated by hepatocyte versus extra‐hepatocyte Plin2. Complete loss of Plin2 mediates hepatic free fatty acid uptake, while hepatocyte‐derived Plin2 regulates de novo lipogenesis. This was accompanied by complete loss of steatosis in Plin2‐Null mice, but only a decrease in micro‐vesicular steatosis in Plin2‐HepKO mice. Different stages of fatty liver may be mediated by distinct cell types and signalling mechanisms, and understanding these differences may be beneficial for personalized treatment.

Similar to the development of fatty liver, different signalling pathways were affected by total versus hepatocyte‐specific Plin2 knockout. Differential signalling pathways in Plin2‐Null and Plin2‐HepKO mice suggest that hepatocyte‐derived Plin2 mediates hepatic inflammation, as well as hepatic fibrogenesis via alterations of immune cell properties, during NAFLD progression to NASH. This finding may generate new approaches for the treatment of NAFLD versus NASH.

Overall, this article is the first to identify that Plin2 has varying signalling mechanisms in hepatocytes versus extra‐hepatocyte sources. This work enhances our understanding of the role of hepatocyte lipid deposition in the promotion of NAFLD‐associated injury, and may lead to the development of new therapeutic targets.

Additional information

Competing interests

None declared.

Author contributions

All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

Funding

None declared.