Significance of Alternative Splicing in Cancer Cells
Alternative splicing is a highly regulated process in human cells that allows a single gene to produce multiple protein isoforms. Discovered by Walter in 1978, this mechanism has been shown to occur in over 90% of human genes, as revealed by next-generation sequencing technologies. The spliceosome, composed of five small nuclear ribonucleoprotein (snRNP) particles (U1, U2, U4, U5, and U6), orchestrates this process by recognizing consensus sequences at exon-intron junctions. Additionally, enhancers and silencers within exons or introns regulate the binding of splicing factors, which influence exon recognition. Among these factors, the heterogeneous nuclear ribonucleoprotein (hnRNP) family and the serine/arginine-rich protein (SR) family play antagonistic roles in splicing regulation.
Alternative splicing can occur in several modes, including exon skipping, intron retention, alternative donor/acceptor sites, mutually exclusive exons, alternative promoters, and alternative polyadenylation. These variations can lead to the production of different transcripts, which may alter gene expression levels and protein functionality. While alternative splicing is beneficial for generating protein diversity, it can also contribute to diseases, including cancer.
Alternative Splicing and the Hallmarks of Cancer Cells
Cancer is characterized by several hallmarks, including sustained proliferative signaling, evasion of growth suppressors, resistance to cell death, replicative immortality, induction of angiogenesis, activation of invasion and metastasis, reprogramming of energy metabolism, and evasion of immune destruction. Alternative splicing plays a significant role in these processes, contributing to tumorigenesis, cancer progression, and therapy resistance.
Sustaining Proliferative Signaling
Cancer cells exhibit sustained proliferation, often independent of growth factor stimulation. The RAS/RAF/ERK pathway, involving KRAS, is crucial in epithelial cell-derived tumors. CD44 variant 6 (CD44v6), an alternative splicing variant of CD44, promotes cell proliferation through a positive feedback loop with RAS/MAPK activation. Overexpression of CD44v6 is linked to tumorigenesis and progression in various cancers, including colon, breast, ovarian, and pancreatic cancers. The Wnt/β-catenin pathway, which promotes alternative splicing in colorectal cancers, also regulates the splicing factor PTBP1, further enhancing proliferation.
Evading Growth Suppressors
Cancer cells evade growth suppression by inactivating tumor suppressor genes. In hepatocellular tumors, RAS signaling induces AKT activation and SRSF1-dependent splicing of the SV1 isoform of Krüppel-like factor 6, a cytoplasmic inactive variant of this tumor suppressor. SRSF3 overexpression counteracts p53β-mediated cell senescence by regulating alternative splicing, further promoting cancer progression.
Resisting Cell Death
Apoptosis is a natural barrier to cancer development. In lung cancer cells, hnRNPL phosphorylation by AKT leads to the generation of the anti-apoptotic Casp-9b isoform, contributing to tumorigenesis. In hepatocellular carcinomas, the SVHB splicing variant of SVH mediates apoptosis by interacting with p53. Suppressing SVHB expression accelerates apoptosis in hepatoma cells, suggesting a potential therapeutic strategy.
Enabling Replicative Immortality
Cancer cells achieve unlimited replicative potential through telomere maintenance. The Wnt pathway regulates hTERT, a component of telomerase, which catalyzes telomere production. Alternative splicing of hTERT produces hTERTα, an inhibitor of telomerase, and hTERTβ, which triggers mRNA degradation. Dysregulation of these isoforms is observed in myelodysplastic syndromes and melanoma.
Inducing Angiogenesis
Tumor-associated angiogenesis is essential for supplying nutrients and oxygen. VEGF, a key angiogenesis inducer, is regulated by alternative splicing. VEGF165b, an anti-angiogenic isoform, is downregulated in colorectal tumors, correlating with poor prognosis. Conversely, VEGF165, a pro-angiogenic isoform, is upregulated in prostate cancer and can be inhibited by SRPK1.
Activating Invasion and Metastasis
Cancer cells activate invasion and metastasis through the epithelial-mesenchymal transition (EMT). CD44 splice variants, such as CD44v and CD44s, regulate EMT. In breast cancer, CD44v promotes EMT, while CD44s enhances Akt signaling and cancer cell viability. ESRP1 and ESRP2, splicing regulators, also modulate EMT by influencing CD44 splicing.
Reprogramming Energy Metabolism
Cancer cells reprogram energy metabolism to support growth and division, often through aerobic glycolysis. Pyruvate kinase (PKM) has two splicing variants, PKM1 and PKM2, which influence oxidative phosphorylation and glycolysis, respectively. PKM2 overexpression is observed in glioblastoma, lung cancer, and hepatocellular carcinoma, promoting tumor proliferation.
Evading Immune Destruction
Cancer cells evade immune destruction by modulating immune responses. Alternative splicing of interferon regulatory factor-1 (IRF-1) and CD45, a T cell regulator, influences immune functions. hnRNP proteins and splicing factors like SF2/ASF also play roles in immune modulation.
Alternative Splicing and the Tumor Microenvironment
The tumor microenvironment, including hypoxia, reactive oxygen species (ROS), hyperosmosis, growth factors, and the extracellular matrix (ECM), influences alternative splicing. Hypoxia induces global changes in splicing through HIF activation, while ROS and hyperosmosis affect splicing factor activity. Growth factors like EGF, HGF, TGF-β, and IGF regulate splicing events, and ECM remodeling can induce splicing changes that promote tumor progression.
Alternative Splicing and Therapy in Cancer
Targeting alternative splicing offers a promising strategy for cancer therapy. Conventional therapeutics focus on protein isoforms and splicing factors, but oligonucleotide and RNA-based therapies, such as antisense oligonucleotides (ASOs), provide more precise targeting. ASOs can block splice sites or prevent the binding of splicing factors, redirecting splicing to desired isoforms. Spliceosome inhibitors like Spliceostatin A and Sudemycins have shown cytotoxic effects in leukemia by altering splicing patterns.
Conclusions
Alternative splicing is a critical mechanism in cancer, contributing to tumorigenesis, progression, and therapy resistance. Understanding its role in the hallmarks of cancer and the tumor microenvironment provides opportunities for developing novel therapies and biomarkers. Targeting alternative splicing through conventional and emerging therapies holds promise for improving cancer treatment outcomes.
doi.org/10.1097/CM9.0000000000000542
Was this helpful?
0 / 0