SIGNALING PATHWAYS INVOLVED IN ICC PROGRESSION

Notch, Wnt, and Hedgehog Signaling Pathways

The Notch, Wnt, and Hedgehog signaling pathways are essential regulators of embryonic development, tissue homeostasis, and cellular fate determination. Abnormal activation of these pathways significantly contributes to the development and progression of different malignancies, including iCCA.The Notch signaling system is essential for cell differentiation, proliferation, and death. In iCCA, the overactivation of Notch receptors, especially Notch1 and Notch2, has been associated with enhanced tumor proliferation, survival, and resistance to apoptosis. Notch signaling additionally enhances epithelial-to-mesenchymal transition (EMT), hence promoting tumor invasiveness and metastasis. Preclinical studies indicate that the inhibition of Notch signaling may impede tumor proliferation, positioning it as a viable therapeutic target [42]. The Wnt/β-catenin signaling system is a vital modulator of cellular proliferation and differentiation. In iCCA, deregulation of this system, frequently due to mutations in β-catenin or other pathway constituents, results in the accumulation of β-catenin within the nucleus. This buildup triggers oncogenic transcriptional pathways that facilitate tumor growth, invasion, and chemoresistance [43]. Aberrant Wnt signaling also plays a role in sustaining cancer stem cells, which are associated with tumor recurrence and resistance to standard therapy. The Hedgehog signaling system, crucial for embryonic development, is often dormant in adult tissues but may be triggered in malignancies, such as iCCA [44-48] (Fig. 3). In this environment, Hedgehog signaling facilitates desmoplasia (the creation of fibrotic stroma), hence aiding tumor proliferation, invasion, and metastasis. It also improves the tumor microenvironment's resistance to treatment. Inhibiting Hedgehog signaling with particular agents has demonstrated potential in preclinical investigations for diminishing tumor proliferation and enhancing treatment results. These signaling pathways are essential in the pathogenesis of cholangiocarcinoma, and their blockage offers significant opportunities for innovative, tailored therapeutics to enhance patient outcomes in iCCA [49].

PI3K/AKT/mTOR Pathway and its Role in ICC

The PI3K/AKT/mTOR signaling pathway is a vital modulator of cellular growth, proliferation, metabolism, and survival [50]. Its dysregulation significantly contributes to the development and progression of iCCA.The initiation of this pathway generally commences with the activation of PI3K (phosphoinositide 3-kinase) by growth factors that attach to receptor tyrosine kinases (RTKs), like EGFR or FGFR. The activation of PI3K results in the synthesis of PIP3, which subsequently recruits and activates AKT, a serine/threonine kinase. AKT subsequently phosphorylates other downstream targets, including mTOR (mechanistic target of rapamycin), which is a crucial regulator of protein synthesis and cellular proliferation [51-54]. In iCCA, mutations or amplifications in upstream RTKs, PI3K subunits, or the loss of tumor suppressors, such as PTEN, which negatively control this pathway, lead to the constitutive activation of PI3K/AKT/mTOR signaling. This facilitates unregulated cellular proliferation, persistence, angiogenesis, and evasion of apoptosis, hence advancing tumor development and chemoresistance. Inhibiting this system with agents, like mTOR inhibitors (e.g., everolimus) or PI3K/AKT inhibitors, has demonstrated promise in preclinical research. The intricacy and redundancy of the route frequently result in resistance, underscoring the necessity for combination medicines to enhance treatment outcomes in iCCA [55].

Role of FGFR and Other Growth Factor Receptors in the Tumor Microenvironment

Fibroblast growth factor receptors (FGFRs), along with other growth factor receptors, including EGFR and VEGFR, are crucial in the tumor microenvironment (TME) of iCCA. FGFRs, especially FGFR2, are frequently overexpressed or altered in iCCA, resulting in the constitutive activation of downstream signaling pathways that enhance tumor cell proliferation, survival, and angiogenesis [56]. FGFR2 fusions or amplifications are notably important in a subset of iCCA patients, facilitating tumor growth and establishing FGFR2 as a target for precision therapeutics. Growth factor receptors, such as EGFR and VEGFR, play a role in the tumor microenvironment by facilitating cellular responses to external stimuli. Activation of EGFR accelerates tumor proliferation and metastasis, whereas VEGFR facilitates angiogenesis, supplying vital nutrients and oxygen to the expanding tumor. The intricate interactions among these receptors in the tumor microenvironment foster a pro-tumorigenic milieu, facilitating tumor proliferation, invasion, and therapeutic resistance. Targeting these receptors presents therapeutic prospects in iCCA [57].

Crosstalk between Signaling Pathways Influencing Tumor Growth

The interaction between signaling pathways is essential in modulating tumor development and advancement. In iCCA, pathways, such as PI3K/AKT/mTOR, Notch, Wnt, and Hedgehog, interact to facilitate carcinogenesis [58]. Notch signaling can augment Wnt pathway activation, facilitating epithelial-to-mesenchymal transition (EMT) and metastasis. Likewise, PI3K/AKT/mTOR signaling can influence Hedgehog and FGFR pathways, facilitating cell survival and angiogenesis. This complex interaction promotes a highly adaptable tumor microenvironment, facilitating cancer cell proliferation, evasion of apoptosis, and resistance to therapies. Simultaneously targeting various pathways may enhance therapeutic effects in iCCA [59].