Targeting HCC Stemness Through SLC27A5: A New Therapeutic Avenue

Hepatocellular carcinoma (HCC) remains a formidable challenge in oncology, ranking as one of the most prevalent malignancies globally and the third leading cause of cancer-related deaths. A critical factor underpinning the aggressive nature of HCC is the presence of liver cancer stem cells (LSCs), which fuel tumor initiation, metastasis, and recurrence. These cancer stem cells possess self-renewal and differentiation potential, rendering them resistant to conventional therapies and responsible for tumor relapse. Despite advances in treatment modalities, effective strategies targeting these elusive cellular culprits have been limited, necessitating a deeper exploration into the molecular circuits governing LSC maintenance.

A groundbreaking study from researchers at Chongqing Medical University unravels the intricate molecular interplay involving the fatty acid transport protein, SLC27A5, and its profound influence on liver cancer stemness. As a liver-specific solute carrier primarily involved in hepatic fatty acid metabolism, SLC27A5 deficiency has been linked to hepatic fibrosis and progression of hepatocellular carcinoma. Intriguingly, beyond its metabolic functions, SLC27A5 has been implicated in RNA-related regulatory pathways, particularly alternative polyadenylation (APA), a post-transcriptional mechanism that diversifies mRNA isoforms through differential cleavage and polyadenylation sites, thereby impacting gene expression regulation.

Alternative polyadenylation represents a pivotal control point in mRNA maturation, generating transcripts with varied 3′ untranslated region (3′-UTR) lengths. This diversity influences mRNA stability, localization, and translational efficiency, often modulating gene expression patterns implicated in oncogenesis. Aberrations in APA dynamics have been documented in numerous cancers, including HCC, highlighting the role of RNA processing dysregulation in tumor biology. Building on previous evidence linking SLC27A5 to RNA processes, the current study sought to delineate its role in modulating APA and elucidate mechanisms through which it impacts liver cancer stem cell biology.

Deploying an integrative screening approach combining immunoprecipitation coupled with mass spectrometry (IP-MS), the researchers identified compelling interactions between SLC27A5 and poly(A)-binding protein cytoplasmic 1 (PABPC1). PABPC1 is a multifaceted RNA-binding protein that shuttles between the nucleus and cytoplasm, playing an instrumental role in mediating 3′UTR-APA and mRNA stability. Notably, PABPC1 is overexpressed in various malignancies and is correlated with poor prognostic outcomes in HCC patients. The identification of SLC27A5-PABPC1 interaction reflects a novel regulatory axis in the post-transcriptional control of gene expression, with broad implications for cancer stemness regulation.

Subsequent mechanistic investigations revealed that SLC27A5 promotes the ubiquitination and proteasomal degradation of PABPC1 via the recruitment of RBBP7, an epigenetic regulator and protein degrader. This degradation of PABPC1 culminates in significant downregulation of its cellular levels, effectively reshaping the landscape of APA regulatory machinery in HCC cells. By tempering PABPC1 abundance, SLC27A5 indirectly influences the APA profile of downstream target transcripts critical for stemness and tumor progression.

A key downstream effector identified in this regulatory cascade is METTL14, an RNA methyltransferase involved in N6-methyladenosine (m6A) modification of mRNA, with established roles in modulating RNA metabolism and cancer cell biology. The study found that SLC27A5, through the repression of PABPC1, modulates the usage frequency of METTL14 distal polyadenylation sites (dPAS), resulting in a switch from transcripts harboring longer 3′UTRs (METTL14-UL) to shorter ones (METTL14-US). Remarkably, this alteration in METTL14 isoform expression is independent of its methyltransferase enzymatic activity, prompting reconsideration of METTL14’s functions beyond catalysis.

Bioinformatics analyses underscored a negative correlation between METTL14 expression and liver cancer stemness markers, supporting the hypothesis that METTL14 isoforms exert differential influences on HCC stem cell traits. Both in vitro cellular assays and in vivo mouse models demonstrated that METTL14-US effectively suppresses stemness phenotypes in HCC. Importantly, SLC27A5 upregulates METTL14-US expression, thereby unleashing its tumor-suppressive capacity and further inhibiting cancer stem cell properties. This regulatory axis highlights the pivotal role of APA in fine-tuning isoform-specific gene function within the tumor microenvironment.

Additional mechanistic insights revealed that METTL14-US mRNA evades microRNA-mediated silencing pathways, affording enhanced transcript stability and sustained expression levels. By contrast, METTL14-UL transcripts with longer 3′UTRs are more susceptible to miRNA targeting, thereby reducing their steady-state abundance. This differential vulnerability reinforces the critical impact of APA-generated isoforms in post-transcriptional gene regulation and tumor biology, further substantiating the therapeutic potential of manipulating APA profiles.

Corroborating these molecular findings, analysis of human HCC specimens revealed that SLC27A5 deficiency correlates with elevated PABPC1 levels and a predominance of short 3′UTR METTL14 isoforms, collectively driving diminished METTL14 function and exacerbated tumor progression. These observations validate the clinical relevance of the SLC27A5-PABPC1-METTL14 axis and underscore its potential as a biomarker for HCC prognosis and treatment stratification.

This study’s revelations about the SLC27A5-mediated regulation of liver cancer stemness via alternative polyadenylation not only deepen our comprehension of the molecular networks governing hepatic tumor biology but also open novel avenues for therapeutic intervention. Targeting components of this axis, particularly the restoration of SLC27A5 function or modulation of METTL14 alternative polyadenylation patterns, could yield innovative strategies to curtail cancer stem cell-driven tumor relapse and metastasis in HCC.

In conclusion, the elucidation of the SLC27A5-PABPC1-METTL14 axis represents a paradigm shift in understanding the convergence of metabolic regulation, RNA processing, and cancer stem cell biology. The findings highlight the profound implications of alternative polyadenylation in oncogenesis, transcending conventional gene expression paradigms, and offer promising new therapeutic targets in the relentless fight against hepatocellular carcinoma. As the field advances, clinical translation of these insights could herald a new era of precision medicine tailored to dismantle cancer stem cell reservoirs and improve patient outcomes.

Web References: http://dx.doi.org/10.1016/j.gendis.2024.101488

References: Original publication in Genes & Diseases, doi: 10.1016/j.gendis.2024.101488