Therapeutic targets of SphK1 and SphK2 in hepatocellular carcinoma

SphK1 and SphK2 are critical therapeutic targets in hepatocellular carcinoma (HCC) due to their involvement in tumor progression, survival, and resistance mechanisms. Below is a detailed overview of their therapeutic implications:

### **1. SphK1 as a Therapeutic Target**

SphK1 plays a significant oncogenic role in HCC by promoting tumor proliferation, migration, angiogenesis, and chemoresistance. Its therapeutic targeting focuses on inhibiting its activity and disrupting its signaling pathways:

• **Mechanisms of Action:**
• SphK1 activates PI3K/AKT/mTOR and MAPK/ERK signaling pathways, driving tumor growth and survival.
• It enhances NF-κB signaling, epithelial–mesenchymal transition (EMT), and chemoresistance, contributing to tumor aggressiveness.
• SphK1 upregulation increases VEGF secretion, promoting angiogenesis and linking lipid metabolism to tumor vascularization.

• **Therapeutic Strategies:**
• **Selective SphK1 Inhibitors:**
• **PF-543**: Potently inhibits SphK1 activity, suppressing sphingosine-1-phosphate (S1P) production and reducing tumor invasion.
• **SKI-178**: Induces apoptosis and inhibits tumor proliferation in preclinical models.
• **Dual Inhibition Approaches:**
• Combined blockade of SphK1 and SphK2 (e.g., SKI-II, SKI-349) enhances apoptosis, suppresses FAK/IGF-1R and AKT/mTOR signaling, and improves chemosensitivity.
• **Immunotherapy Potential:**
• Inhibiting SphK1/S1P signaling may reactivate tumor necrosis factor (TNF)-mediated apoptosis and enhance immune checkpoint inhibitor responses.

### **2. SphK2 as a Therapeutic Target**

SphK2 has a unique role in HCC by regulating nuclear gene transcription, mitochondrial function, and metabolic reprogramming. Its inhibition has shown promise in overcoming resistance and reducing tumor progression:

• **Mechanisms of Action:**
• Nuclear SphK2-generated S1P enhances histone acetyltransferase activity, promoting oncogene transcription (e.g., c-Myc) and metabolic reprogramming.
• Mitochondrial SphK2 maintains telomere activity by stabilizing telomerase (hTERT) structure, supporting tumor cell survival and resistance to regorafenib.
• SphK2 modulates mTORC2 signaling and very-low-density lipoprotein (VLDL) secretion, contributing to HCC cell survival and lipid metabolism.

• **Therapeutic Strategies:**
• **Selective SphK2 Inhibitors:**
• **ABC294640 (Opaganib)**: Selectively inhibits SphK2, reducing MAPK/ERK pathway activation and showing synergy with sorafenib in HCC models.
• **Dual Inhibition Approaches:**
• Combined targeting of SphK1 and SphK2 enhances apoptosis and suppresses key survival pathways, such as AKT/mTOR.
• **Regorafenib Resistance Overcoming:**
• SphK2 inhibition can restore drug efficacy by blocking NF-κB/STAT3 activation, which contributes to regorafenib resistance.
• **Metabolic Reprogramming:**
• SphK2 inhibition induces lipotoxic stress and cell death by disrupting mTORC2 signaling and lipid secretion.

### **3. Diagnostic and Biomarker Potential**

• Elevated serum levels of S1P and C16-ceramide are being investigated as potential early biomarkers for malignant transformation in liver diseases, including HCC. These biomarkers could guide therapeutic targeting of SphK1 and SphK2.

### **4. Future Directions in Therapy**

• **Isoform-Specific Inhibitors:** Developing highly specific inhibitors for SphK1 and SphK2 to minimize off-target effects and maximize therapeutic efficacy.
• **Combination Therapies:** Integrating SphK inhibitors with immune checkpoint inhibitors, chemotherapy, or targeted therapies (e.g., sorafenib or regorafenib) for enhanced outcomes.
• **Multi-Omics Analysis:** Utilizing genomics, proteomics, and metabolomics to tailor precision therapies targeting SphK isoforms in HCC.
• **Metabolism–Immune Targeting:** Combining SphK inhibition with therapies targeting immune modulation and metabolic reprogramming.

### **Summary**

Targeting SphK1 and SphK2 in HCC offers promising therapeutic opportunities by disrupting key oncogenic pathways, overcoming drug resistance, and inducing tumor cell apoptosis. Selective inhibitors like PF-543, SKI-178, and ABC294640 have demonstrated preclinical efficacy, while dual inhibition strategies and combination therapies hold potential for clinical application. Additionally, diagnostic biomarkers and precision medicine approaches are advancing the field of SphK-targeted therapies in HCC.