Cracking HCC: Targeting Diacylglycerol Kinase Eta to Tackle Therapy Resistance and Cancer Stemness

Introduction

Hepatocellular carcinoma (HCC) is the most prevalent form of primary liver cancer and ranks as the third leading cause of cancer-related deaths worldwide.(1) Despite advances in diagnostics and treatments, HCC remains highly lethal and is often diagnosed at an advanced stage, where therapeutic options are very limited.(2) The high mortality in HCC is driven by multiple factors such as resistance to existing anticancer agents, a lack of reliable biomarkers for detecting early-stage disease amenable to surgical resection, and underlying liver conditions that limit the use of chemotherapeutic drugs.(3) Since the early 2010s, HCC management has made significant progress, particularly in cases where early-stage tumors can be resected surgically.(1, 2) For small or intermediate-stage tumors, treatments like radiofrequency ablation, microwave ablation, and transarterial chemoembolization show efficacy, although recurrence remains an challenge.(4) Patients with advance HCC rely on systemic therapies such as tyrosine kinase inhibitors (TKIs) like sorafenib, or immune checkpoint inhibitors (ICIs) like nivolumab and pembrolizumab.(2, 5) However, these treatments vary in response and efficacy, carry significant side effects, and are costly, leaving many patients with advanced HCC with limited therapeutic options.(6)

Drug resistance and stemness in HCC are closely linked to metabolic dysregulation. Given the limitations of current therapies for HCC, there is growing interest in targeting the unique metabolic profile of HCC cells, characterized by altered pathways to meet the high energy demands of malignancy. Identifying new metabolic regulators that influence therapy resistance and self-renewal in HCC could open novel therapeutic avenues. A recent study by Loh et al. identifies diacylglycerol kinase eta (DGKH) as a critical regulator of HCC metabolism,(7) demonstrating that DGKH promotes aggressive HCC features, including therapy resistance, cancer stemness, and metastatic potential.(7)

DGKH as A Driver of Cancer Stemness and Therapy Resistance in HCC

Loh et al. performed a comprehensive analysis of HCC data from The Cancer Genome Atlas (TCGA) using weighted gene co-expression network analysis (WGCNA). Their analysis identified a gene module strongly associated with advanced HCC and cancer stem cell characteristics, with DGKH emerging as the most enriched gene in poorly differentiated HCC tumors, suggesting its potential as a biomarker for aggressive HCC. Moreover, DGKH expression was elevated in tumor tissue compared to non-tumor tissue, and patients with high DGKH levels showed poorer differentiation and a worse response to TKI treatment. Functional assays with DGKH-manipulated HCC cell lines demonstrated that DGKH plays a critical role in promoting stemness, therapy resistance, and metastatic behavior in HCC cells, which are the major challenges in treating HCC. Mechanistically, DGKH supports mTOR signaling, a key driver of cellular growth and proliferation, by promoting the production of phosphatidic acid (PA). This positions DGKH as a central player in a unique metabolic pathway that enhances HCC cell survival and resistance. By facilitating mTOR signaling, DGKH could provide cells with the energy and biosynthetic machinery required for maintaining stemness and aggressive phenotypes under therapeutic stress. However, the authors overlooked the broader role of PA, the production of DGKH. In addition to modulating cell signaling, PA plays a critical role in membrane dynamics and protein structure.(8) Activation of DGKH likely promotes PA production, providing essential building blocks for the membranes of growing malignant hepatocytes. This role further underscores the potential of DGKH as a therapeutic target for HCC. Further highlighting the importance of this study, Loh et al. identified the transcriptional co-activator p300 (EP300) as an upstream regulator of DGKH. EP300 directly binds to the DGKH promoter, enhancing its transcription. The EP300-DGKH-mTOR axis provides new insights into the mechanisms driving therapy resistance and stemness in HCC, highlighting promising avenues for targeted interventions.

Potential Role of DGKH in the Tumor Microenvironment

The TME is the internal environment where tumor cells survive. It comprises not only the tumor cells but also cancer-associated fibroblasts (CAFs), endothelial cells, immune cells, and non-cellular components such as exosomes and cytokines. Advances in single-cell RNA sequencing (scRNA-Seq) have revealed that certain genes may act as tumor suppressors in one context while functioning as oncogenes in another.(9) Database mining revealed that DGKH is significantly upregulated in malignant hepatocytes, but reduced in different subpopulations of tumor-associated macrophages (TAM) compared to M1 macrophages (Fig. 1). As previously reported, DGKH plays a role in immune cell activation, specifically in macrophages.(10) By regulating PA levels, DGKH positively regulates Toll-like receptor-induced proinflammatory cytokine production.(10) In vitro, DGKH deficiency results in reduced production of TNF-α, IL-6, and IL-1β and in limited M1 macrophage polarization.(11) All these findings underscore the contribution of DGKH to immune activation and enhancement of antitumor immunity. In addition to regulating macrophage polarization, DGKH inhibits Treg cell generation by limiting the activation of ERK through TCR stimulation.(12) This suggests that DGKH may help prevent immunosuppression and thereby restore antitumor immunity. In summary, the potential dual role of DGKH in modulating malignant hepatocytes and the immune microenvironment makes DGKH an interesting but complex target for the development of therapies aimed at both malignant hepatocytes and immune system. Although antagonizing DGKH can directly reduce the growth of malignant hepatocytes, it may also impair antitumor immunity. Thus, inhibiting DGKH may serve as a double-edged sword, especially within the immune system context. Hepatocyte-specific DGKH knockdown combined with DGKH overexpression in immune cells might offer a powerful and unique therapeutic strategy for treating HCC.

Figure 1. mRNA levels of DGKH in various cell types within human HCC.

(A) DGKH mRNA expression levels in normal versus malignant hepatocytes, based on single-cell RNA-Seq data from human HCC tumor samples. (B) DGKH mRNA expression levels in different subpopulations of TAMs compared to M1-polarized macrophages, derived from single-cell RNA-Seq of human HCC tumors. This analysis uses a comprehensive single-cell RNA-Seq dataset including 409,775 cells from 347 human HCC samples.

Further Investigation of DGKH in HCC

The study offers a comprehensive analysis of DGKH’s role in cancer stemness, metastasis, and drug resistance. However, the complexity of hepatocarcinogenesis and DGKH’s multifaceted function highlight the need for further research to address existing gaps and improve the clinical applicability of DGKH-targeted therapies. The first gap lies in the diversity and generalizability of patient samples. The human data in this study primarily derive from TCGA, which, while valuable, may not fully capture the heterogeneity seen in HCC patients. HCC is notably heterogeneous across global populations, with distinct genetic and epigenetic profiles depending on geographic and ethnic factors. In particular, differences in HCC risk factors, such as hepatitis B or C infection, alcohol-related liver disease, and metabolic syndrome, may influence metabolic pathways like DGKH expression and activity. Expanding the study to incorporate data from diverse patient cohorts would offer a more complete understanding of DGKH’s role across different patient populations. Second, further investigation is warranted into how DGKH modulates the immune microenvironment. This study focuses primarily on DGKH’s role within cancer cells offering limited insight into its interaction with the TME. HCC often thrives in an immunosuppressive environment that inhibits antitumor immunity. Given that malignant hepatocytes can function as “immune cells” by secreting cytokines,(17) further investigation is warranted to determine whether elevated DGKH in malignant hepatocytes has downstream effects on immune cells, including TAMs, T cells, Kupffer cells (KCs), and natural killer (NK) cells. Exploring the effects of DGKH manipulation in immune cells like KCs, TAMs, regulatory T cells (Tregs), and cytotoxic T lymphocytes (CTLs) could provide valuable insights into DGKH’s impact on antitumor immunity and offer a more comprehensive understanding of its role in HCC. Lastly, potential off-target effects of DGKH inhibition should be thoroughly investigated. Targeting metabolic pathways like DGKH and mTOR could have unintended effects due to the widespread role of these pathways in normal cellular processes. For instance, DGKH is known to be expressed in various tissues and could potentially affect non-cancerous cells if broadly inhibited. The study does not extensively address the specificity of DGKH inhibition for HCC cells versus healthy liver or non-liver tissues. Toxicology studies and an assessment of DGKH expression in other tissues would help clarify the safety profile of DGKH-targeted therapies.

Maximizing the Clinical Potential of DGKH-Targeted Therapies

Despite the limitations described above, identifying DGKH as a metabolic driver in HCC represents a significant step forward in understanding the metabolic landscape of this malignancy. In an immunocompetent mouse model, the authors combined sorafenib treatment with liver-directed AAV8-mediated DGKH depletion, observing significant reductions in tumor burden, cancer stemness, PA production, and mTOR signaling. This dual treatment approach suggests that DGKH inhibition may not only slow HCC progression but also enhance tumor sensitivity to existing therapies like sorafenib. The DGKH-mTOR axis offers a valuable target for therapeutic intervention, particularly in HCC patients who are refractory to TKIs. Moving forward, several strategies could enhance the clinical impact of these findings. First, combination therapies could help amplify the effects of DGKH inhibition. Given that DGKH inhibition has shown promise in sensitizing tumors to TKIs, exploring other potential drug combinations could help amplify this effect. For example, combining DGKH inhibitors with immunotherapies or other metabolic modulators may enhance anti-tumor effects. The second alternative strategy is to manipulate DGKH across various cell types, including KCs, hepatocytes, T cells, and other liver immune cells. This approach could uniquely inhibit malignant hepatocyte growth while simultaneously boosting antitumor immunity. In summary, this research on DGKH represents a forward leap in our understanding of HCC metabolism and therapy resistance. As single-cell atlas research in HCC advances, selectively targeting DGKH in malignant hepatocytes and immune cells holds great promise for improving patient outcomes and could help redefine the standard of care for this challenging cancer.