Lipids link immune suppression to effective immunotherapy in steatotic hepatocellular carcinoma

Despite advances in immunotherapy using immune checkpoint inhibitors (ICIs), most cancer patients do not respond to this treatment. In this issue of Hepatology, Murai et al. found that intratumoral lipid accumulation linked immune exhaustion to effective ICI therapy in nonviral hepatocellular carcinoma (HCC).

Nonalcoholic fatty liver disease (NAFLD) affects up to 30% of Americans (1). NAFLD is a precursor of cirrhosis and HCC. While viral and alcohol related cirrhosis and HCC are in decline, NAFLD is increasing in incidence. The diagnosis is based on biopsy or imaging evidence of fat exceeding 5% of the liver examined from an individual lacking a significant history of alcohol use. Histologically, NAFLD spans a spectrum from simple fatty infiltration of the liver (steatosis) to nonalcoholic steatohepatitis (NASH), with the latter demonstrating inflammation and hepatocyte ballooning. NASH can progress to cirrhosis and HCC (1). Whereas NASH is present in 2–5% of the general population, the median incidence increases to 33% among individuals who are obese. The manuscript by Murai et al. evaluates drivers of the increasingly common fraction of HCCs that are of nonviral origin.

The investigators (2) highlight recent evidence that immunotherapy with ICIs [programmed death 1 (PD-1) inhibitors, programmed death ligand-1 (PD-L1) inhibitors, and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) inhibitors] are effective in a subset of HCCs. To identify nonviral HCCs that might respond to immunotherapy, the investigators performed RNA-seq on 113 tumors and genome sequencing on 55 tumors, analyzing 69 genes associated with HCC disease recurrence. The resulting data clustered the tumors into three prognosis stratified classes. Tumors from patients with the worst prognosis (Class 1) had clinical characteristics associated with a poor prognosis (larger tumor size and portal vein invasion), proliferation genes and TP53 mutations. Class 2 tumors, with an intermediate prognosis, had an interferon profile and a high degree of intratumoral immune cell infiltration including cytotoxic T lymphocytes, while Class 3 tumors, with the best prognosis, were associated with cadherin-associated protein beta (CTNNB)1 mutations and low levels of intratumoral immune cell infiltration. The mutation findings for Class I and Class III tumors were confirmed through an external validation sample of 180 nonviral HCCs. These observations are consistent with earlier reports of HCC (3) using the Chiang classification (4) of HCCs into proliferation, CTNNB1, interferon, and polysomy 7. The current authors’ analysis also found that steatotic HCCs (which they defined as tumors in which fat comprised at least 10% of the liver evaluated, a higher bar than the 5% used by many others), have an immune enriched but immune exhausted tumor immune microenvironment (TIME). As the authors note, ICIs are thought to breathe new life into exhausted T cells, and have been shown to be more effective in tumors with a high concentration of cytotoxic T lymphocytes and PD-L1 expression (5).

Relative to the TIME and treatment with immunotherapy, the authors observed that lipid accumulation in HCC cells induced immunosuppression in macrophages and fibroblasts through PD-L1 upregulation. Two PD1 inhibitors, nivolumab and atezolizumab, and ipilimumab, a CTLA-4 inhibitor, have received FDA approval for individuals with unresectable advanced HCC. Nivolumab alone demonstrated an objective response rate (ORR) of 14%, the combination of nivolumab plus ipilumumab had an ORR of 32% (6), while atezolizumab plus bevacizumab, an anti-vascular endothelial growth factor antibody, had an ORR of 33% (7). Unfortunately, as can be seen by the ORRs, many patients with HCC treated with ICIs fail to respond (8). The authors conducted a retrospective analysis of 30 patients with advanced HCC who were treated with atezolizumab and bevacizumab. Zero of seven patients with steatotic HCC progressed compared to all those with other HCC variants during 5.4 months of follow-up, a significant advantage in progression free survival for the former group. The authors posit that intratumor steatosis may be a useful imaging biomarker for predicting the efficacy of immunotherapy in HCC.

To better understand the association of steatotic HCC with immune suppression and effective ICI therapy, the investigators conducted fatty acid (FA) profiling. They observed that the long chain FA palmitic acid (PA) was significantly higher in steatotic than in nonsteatotic HCC tissues. Based on this observation, they added PA to HCC cells in vitro, observing lipid accumulation with increased expression of PD-L1 and other chemokines (e.g., CXCL8, CSF1) and cytokines (e.g., TGF, IL-10), associated with immune cell infiltration and exhaustion (Figure 1). These findings were more noted in steatotic than other HCC samples.

Figure 1 Lipid accumulation induces immune exhaustion and effective ICI therapy in steatotic HCC. HCC cells containing long chain FAs (e.g., PA) have immune cell infiltration and exhaustion, which contributes to effective ICI therapy. The mechanism(s) by which FAs induce PD-L1 upregulation and immunosuppression in HCC remain unclear. FABPs facilitate FA uptake, trafficking and response. It has been shown that FABP1, FABP4, FABP5 are highly expressed in hepatocytes, macrophages and CD8⁺ cytotoxicity T cells, respectively, which could contribute to FA-mediated PD-1/PD-L1 expression in the tumor microenvironment of HCC. HCC, hepatocellular cell carcinoma; ICI, immune checkpoint inhibitor; FABP, FA binding protein; FA, fatty acid; PD-1, programmed death 1; PD-L1, programmed death ligand-1; CTL, cytotoxic T-lymphocyte; PA, palmitic acid.

An interesting area of future research is to determine the underlying molecular mechanisms by which lipid accumulation in HCC leads to increased expression of PD-1/PD-L1 in the TIME. PA and other long chain FAs are insoluble, and the family of FA binding proteins (FABPs) has demonstrated a central role in FA trafficking and response (9). Hepatocytes mainly express FABP1 (also known as liver FABP, or L-FABP), which is involved in the uptake, transport and metabolism of FAs and contributes to steatosis and other obesity associated diseases of the liver (10). Serum levels of FABP1 have been linked to poor survival in chronic liver diseases ranging from chronic hepatitis to HCC (11). FABP1 polymorphisms have been associated with steatotic HCC.

FABP4 has been identified as a functional marker for pro-tumor macrophages (12), and FABP4 upregulation contributes to the increased risk of breast cancer and nonviral and nonalcoholic HCC (13,14). Moreover, FABP5 expressed in CD8⁺ CTLs has been shown to promote the uptake of another long chain FA, linoleic acid, and ROS-mediated T cell death, suggesting that FABP5 plays a role in mediating FA-mediated immune exhaustion in the TIME (15). It is likely that FABPs function to mediate PD-1/PD-L1 upregulation in the TIME (Figure 1), thus linking lipid accumulation to effective ICI therapy in HCC.

Acknowledgments

This material should not be interpreted as representing the viewpoint of the U.S. Department of Health and Human Services, the National Institutes of Health, or the National Cancer Institute.

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Annals of Translational Medicine. The article did not undergo external peer review.

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://atm.amegroups.com/article/view/10.21037/atm-22-6548/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are 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.

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