Alpha-fetoprotein (AFP) is a glycoprotein synthesized primarily by the yolk sac and fetal liver during embryogenesis. Its primary function is to promote cell growth and development during fetal development. In adulthood, AFP concentrations are very low, sometimes even zero. However, when liver cells undergo malignant transformation, AFP concentrations increase significantly, potentially signaling a tumor. AFP normally produced by the fetus is called normal AFP (nAFP); the fetal glycoprotein produced by most human hepatocellular carcinoma tumors is called tumor-derived AFP (tAFP). AFP is an important tumor marker for the diagnosis and treatment of liver cancer and a key indicator for the clinical diagnosis of liver cancer metastasis.
Structure and function of AFP
AFP is a fetal-specific alpha-globulin synthesized during fetal development. It is present in fetal blood and tissues and can also be detected in adult liver and some malignant tumors. AFP is composed of 609 amino acids with a molecular weight of approximately 69 kDa. AFP consists of an N-terminal domain, a C-terminal domain, and a central domain. These three domains of AFP are linked by disulfide bonds to form a V-shaped structure, and each domain exhibits distinct biological activities. Domain I binds to the phosphatase domain of PTEN, thereby affecting PTEN activity; Domain II is highly flexible and easily digested by proteases; and Domain III is the most conserved domain. This domain consists of several consecutive hydrophobic amino acids forming a leucine zipper-like structure. Domain III is responsible for binding to signaling proteins and receptors, thereby regulating their biological activities.
(Data source: Lu Y, et al. Front Oncol. 2024)
AFP difference
AFP variability depends on species, tissue, isoform, binding ligand, binding partner, and type of post-translational modification (such as N-glycosylation). Different AFP variants have distinct biological functions; for example, studies have found elevated levels of fucosylated AFP variants in the serum of HCC patients but undetectable in the serum of normal patients. Polyunsaturated fatty acid-bound AFP variants play an important role in the TME and influence the activation of dendritic cells (DCs) and natural killer cells (NK cells).
(Data source: Munson PV. Trends Immunol. 2022)
AFP signaling pathway and regulation in hepatocellular carcinoma (HCC):
AFP binds to the AFP receptor (AFPR), activating the PKA pathway and inducing Ca2 + influx, which increases intracellular cAMP and PKA, enhances DNA synthesis, promotes the expression of oncogenes c-Fos, c-Jun, and Ras, and stimulates the growth of liver cancer cells. Binding of AFP to AFPR triggers growth-promoting signals, promoting AFP internalization. Internalized AFP interacts with PTEN, activating the PI3K/AKT/mTOR pathway and promoting the malignant behavior of hepatocellular carcinoma ( HCC ) cells by upregulating mTOR protein expression. AFP not only plays a key role in regulating tumor proliferation but also enhances cancer cells' resistance to apoptosis. Cytoplasmic AFP enhances resistance to apoptotic factors by affecting the TGF-β and p53/Bax/caspase-3 signaling pathways. AFP also inhibits death receptor 3 (DR3) expression and binds to caspase-3, inhibiting tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in tumor cells. AFP can also promote HCC cell invasion and metastasis by upregulating the expression of MMP2/9, epithelial cell adhesion molecules EpCAM and CXC chemokine receptor 4 (CXCR4), and AFP knockout can significantly inhibit the migration and invasion ability of HCC cells.
(Data source: Li W, et al. Curr Med Chem. 2021)
Immunosuppressive function of AFP
AFP can inhibit the activity of immune cells such as dendritic cells (DCs), NK cells, and T cells. AFP reduces the antigen-presenting capacity of DCs, thereby preventing DC-mediated T cell activation. AFP can also attenuate NK cell cytotoxicity against liver cancer cells by regulating NK cell signaling pathways. AFP generally does not directly impair NK cell function, but rather indirectly inhibits NK cell function by hindering DC maturation and reducing DC IL-12 secretion. AFP also inhibits T cell proliferation and cytotoxicity, thereby weakening T cell responses against liver cancer cells. In the absence of AFP, DCs enhance the function of CD4+ T cells, CD8+ T cells, and NK lymphocytes, thereby hindering tumor growth. In the presence of AFP, DCs promote Treg differentiation but inhibit the function of CD8+ T cells and NK cells, thereby creating an immunosuppressive environment and promoting tumor growth.
(Data source: Lu Y, et al. Front Oncol. 2024)
AFP-based HCC treatment
Development of AFP vaccines: Vaccines can be designed to target the D1 region of AFP, which does not have an immunosuppressive effect. Vaccination can promote the activity of dendritic cells and stimulate anti-tumor responses in hepatocellular carcinoma. If AFP is presented by more AFP-engineered dendritic cells, beneficial CD8+ and CD4+ T cell immune responses can be generated. Studies have found that AFP-pulsed dendritic cells can divert specific cytotoxic T lymphocytes to AFP-producing hepatocellular carcinoma cells.
CAR-T cell therapy: AFP-specific chimeric antigen receptor engineered T cells (CAR-T cells) are currently being developed against MHC-restricted AFP peptides. These T cells can target and kill AFP-expressing tumor cells and can overcome the limitations of vaccine-induced T cells in the tumor microenvironment, such as low frequency and exhaustion phenotype.
(Data source: Mandlik DS, et al. World J Gastroenterol. 2023)
