IQ motif-containing GTPase-activating proteins (IQGAPs) are a family of evolutionarily conserved proteins found in eukaryotic cells, ranging from yeast to humans, and consist of three closely related homologs; IQGAP1, IQGAP2, and IQGAP3. These homologs comprise multiple domains involved in regulating a broad spectrum of biological processes such as cytoskeleton remodeling, cytokinesis, protein trafficking, cell adhesion, proliferation, migration, and tumorigenesis (1–6). Aberrant IQGAP expression has been extensively reported in a range of malignancies, including liver, gastric, lung, breast, and colon cancers, and is highly related to poor clinical characteristics and prognosis (7–10).

Globally, liver cancer is a significant public health threat with increasing prevalence. By 2025, liver cancer is anticipated to affect more than one million individuals annually (11). Hepatocellular carcinoma (HCC), the most common subtype of primary liver cancer, which is the sixth most frequently diagnosed type of cancer and the third most common cause of cancer-related mortality worldwide in 2020 (12). Several imperative risk factors such as chronic Hepatitis B and C virus (HBV and HCV) infections, alcoholic and non-alcoholic liver diseases, e.g., non-alcoholic fatty liver disease (NAFLD) and the advanced state of NAFLD termed as non-alcoholic steatohepatitis (NASH), and hereditary hemochromatosis have been described for HCC (13). Apart from these risk factors, any other causes leading to liver cirrhosis, including HBV and HCV, are also identified as crucial risk factors for HCC (11, 14). Over the last decade, the discovery of novel biomarkers and innovation in imaging technologies have led to tremendous progress in the management and therapy of HCC. Hepatectomy and liver transplantation have been the mainstays of treatment for patients with HCC (15). Additionally, alternative adjuvant therapies such as radiofrequency ablation, transarterial chemoembolization, and immunotherapy have dramatically improved patient prognosis (11).

IQGAPs are important molecular factors that play a comprehensive and distinctive role in HCC initiation and progression. IQGAP1 is overexpressed in HCC and contributes to cancer development and advancement (16, 17). IQGAPs have complex interactions with several different factors at the molecular level and thus play a vital role in cancer progression. Likewise, IQGAP1 can bind to the cell cycle regulators and might boost cell division and enhance the invasive and migratory abilities of HCC cells, as shown for the Huh-7 cell line (18). Moreover, IQGAP1 also interacts with certain transcription factors and co-activators and affects the signaling pathways involved in apoptosis and survival processes (19). Additionally, IQGAP1 has been shown to render anoikis resistance and metastasis in HCC (19, 20). Similar to IQGAP1, IQGAP3 is considered an oncogene in HCC, where it supports intrahepatic and distant metastasis and epithelial-to-mesenchymal transition (EMT) (21, 22). In contrast to isoform 1 and 3 of IQGAP, IQGAP2 is considered a tumor suppressor in HCC since its expression coincides with patient prognosis and is decreased in HCC tissues (23). Furthermore, IQGAP2-deficient mice were more prone to spontaneously growing HCC (24). Therefore, all these findings indicate that IQGAPs are potential therapeutic targets in HCC. This review details the three IQGAP isoforms and their distinct structures. In addition, we compare the characteristics of IQGAPs and summarize their expression and functions in different liver diseases and mainly in HCC, as well as their associated mechanisms.

Although IQGAPs share a similar domain composition, they differ in tissue expression, subcellular localization, and function. IQGAP1, the most extensively investigated isoform among IQGAPs, has gained considerable attention since its discovery in 1994 (25). IQGAP1 is a ubiquitously expressed scaffold protein involved in various vital cellular functions (4, 5). IQGAP1 has been postulated to maintain matrix homeostasis via collagen phagocytosis in physiological tissue remodeling (26). Furthermore, IQGAP1 appears to be an exceptionally attractive therapeutic target because it acts as a hub for signaling pathways involved in cancer progression (20, 27–29).

IQGAP2 was initially identified in 1996 as a large cytoplasmic scaffold protein that is predominantly expressed in the liver but also in the prostate, kidney, stomach, testis, and platelets (30). IQGAP2 is a tumor suppressor, as a decreased expression has been observed in human breast cancer and HCC (23, 31). Moreover, IQGAP2-mutant mice are susceptible to developing HCC. Additionally, IQGAP2 inhibits EMT and angiogenesis and promotes apoptosis in breast cancer. Thereby, low expression levels of IQGAP2 results in a poor prognosis for patients (7, 31).

IQGAP3 has received scant attention despite its discovery in 2007 (32) and its detection in various organs, including the liver, stomach, ovaries, prostate, breast, pancreas, and lung (8, 33–38). IQGAP3 is mainly expressed in brain (39) and is critical for regulation of neurite outgrowth of the neurons (32). In addition to regulating mitosis progression, genome integrity, and stability, IQGAP3 is necessary and sufficient for proper cell proliferation and migration (35, 40). Accordingly, IQGAP3 is classified as an oncogene owing to its intimate association with tumorigenesis and metastasis. Its expression is inversely linked with clinical characteristics and survival in most cancers (8, 38, 41, 42).

IQGAPs have a high degree of amino acid sequence homology and a comparable domain structure ( Figure 1 ), with IQGAP1 having 62% and 59% amino acid sequence identity to IQGAP2 and IQGAP3, respectively (43, 44). IQGAPs contain six distinct functional domains depending on the amino acid sequence. These diverse domains allow them to bind to a variety of partners and modulate the spatiotemporal distribution of various signal transduction complexes (9, 45).

Starting from the N-terminus, IQGAPs comprise a calmodulin homology domain (CHD) that mediates actin-binding (46). For instance, the interaction between the CHD of IQGAP1 and actin filaments governs the cytoskeleton modulation to facilitate actin binding and polymerization, thus further regulating cell division, cell migration, and the stability of cell-cell contacts (47). Moreover, it has been shown previously that IQGAP1 dynamically contributes to the formation of the spine head in the cultured rat hippocampal neurons through CHD (48).

Following the CHD is the coiled-coil (CC), also known as heptad domains, which are characterized by the presence of repetitive hydrophobic and charged amino acids (49). The CC repeat sequence enables IQGAP1 and IQGAP2 to bind to the ERM protein family (ezrin, radixin, and moesin) that crosslinks actin-based cytoskeletons to plasma membranes and participates in multiple intracellular signaling pathways, reinforcing the role of IQGAPs as cytoskeletal regulators and signal transduction hubs (50, 51). Additionally, the CC repeat region of all IQGAPs isoforms stimulates neural development and spine morphogenesis via interaction with valosin containing protein (VCP) which also suggests a crucial role of IQGAPs in the pathophysiology of neurodegenerative diseases (52).

The polyproline protein-protein (WW) domain contains two functionally conserved tryptophans, W, which interacts with other proteins, including proline-rich regions. WW domain of IQGAP1, through a polyproline motif, binds to classical MAP kinases (MAPK) and activates downstream signaling pathways and advances tumor development, growth, and invasion (53, 54). Likewise, interruption of IQGAP1-kinase- connections by ectopic expression of WW peptide in mice selectively suppressed Ras-MAPK -mediated oncogenesis and tumor invasion. Moreover, this effect also remarkably improves the life span of tumor-bearing mice (54). However, a later study reported contradictory findings that the WW domain of IQGAP1 was neither sufficient nor essential for binding to MAPK, whereas the IQ domain that immediately follows WW domain was both adequate and necessary for high-affinity IQGAP1-kinase interaction (55).

IQ domain comprises of four tandem isoleucine-glutamine (IQ) motifs. This domain interacts with calmodulin, a calcium-sensing protein capable of binding to and regulating a diverse array of target proteins (56). The IQ domain also interacts with protein kinases and cell surface receptors such as epidermal growth factor receptor (EGFR) (55, 57–60). The IQ domain of IQGAP1 activates MAPK signaling cascade by binding to EGFR, which is associated with epidermal cancer progression and invasion (61). Subsequently, IQGAP1-IQ motif decoy peptide supplementation in vivo inhibited the oncogenic signaling pathway and carcinogenesis without compromising normal epidermal proliferation or differentiation (61).

The IQ domain is followed by a GTPase activation-related structural domain (GRD) that is responsible for the nomenclature of IQGAPs (49). This domain is highly homologous to the functional components of Ras GTPase-activating proteins (GAPs) and interacts with small GTPases (3, 9, 62). The crystal structure of GRD showed that threonine replaces the catalytic “arginine finger” required for GTP hydrolysis; thus, it does not have GAP function, failing to hydrolyze GTP but rather stabilizing the GTP-bound protein in its active state (63). Such interaction between GRD domain of IQGAP2 and small GTPases is mechanistically crucial for the polymerization of actin filaments during cancer progression (64, 65).

Finally, IQGAPs contain a RasGAP C-terminal (RGCT) structural domain that significantly contributes to cell-cell adhesion, cell polarization, and directional migration by binding to a variety of proteins, including E-cadherin and β-catenin (66–69). Furthermore, IQGAP1 enhanced extracellular matrix (ECM) degradation by binding to the exocyst subunits via RGCT domain to prime tumor cell metastasis and invasion, while the elimination of the exocyst binding site abrogated the enhancement of IQGAP1-induced ECM degradation (70, 71). This indicates that IQGAP1-regulated cytoskeletal remodeling plays an aggressive role in tumor progression. Likewise, mutations of the RGCT can severely impair the interaction of IQGAP1 with small GTPases; therefore, complying that the domains adjacent to GRD are also crucial for the binding of IQGAPs with small GTPases of Rho family proteins (72–75).

As delineated above, IQGAP scaffold proteins are crucial for a variety of vital cellular processes, and their molecular interactions with other biological molecules are very important for the development and progression of cancer. Likewise, role of IQGAPs has been quite extensively explored in various cancers including HCC. However, the investigations for the implications of IQGAPs in other liver diseases such as fatty liver disease, fibrosis, and cirrhosis are still too scarce and need more scientific research to explore the role and importance of these scaffold proteins in liver diseases (76–79).

Aberrant activation of multiple signaling pathways in HCC, such as the Wnt/β-catenin, Hippo, transforming growth factor β (TGF-β), receptor tyrosine kinase (RTK)-activated phosphatidylinositol 3-kinase/AKT/mTOR (PI3K/AKT/mTOR), and Ras/Raf/MEK/ERK (also known as the MAPK/ERK) pathways, leads to uncontrolled cell division, differentiation, proliferation, motility, and apoptosis ( Figure 2 ). Mounting studies have shown that dysregulated IQGAP expression promotes HCC progression and plays a crucial role in HBV-induced HCC.

This review described the structures of the three members of the IQGAP family and their expression and diverse roles in different liver diseases and mainly in HCC. The signaling pathways associated with IQGAPs and the functional mechanisms in HCC were also presented. Although IQGAPs are promising therapeutic targets for HCC, some issues need to be addressed. For example, IQGAP1 plays distinct roles in primary HCC and hepatic metastatic carcinoma, as well as the concrete mechanism of inter-regulation between IQGAP families. Furthermore, the oncogenes IQGAP1 and IQGAP3 play critical roles in cytoskeletal remodeling of the liver and bile ducts, and in liver regeneration after liver injury, respectively (136–138). Thus, the inhibition of IQGAP1 and IQGAP3 may disrupt the integrity of the liver and bile ducts and affect liver regeneration after liver injury. Future detailed studies on the mechanistic roles of IQGAPs in the cytoskeleton and HCC could promote more precise targeted therapy for HCC while avoiding disruption of the normal physiological functions of IQGAPs. Likewise, it is equally important to explore the role of these fundamental scaffolding proteins in other liver diseases such as fatty liver disease, fibrosis, and cirrhosis to unravel the functional relationship of these proteins with chronic liver diseases which if left untreated ultimately lead to HCC.

QD and QA summarized and wrote the article. MR, FS, and AZ commented and revised the manuscript. All authors contributed to the article and approved the submitted version.

This study was supported by funding from the Friedrich-Schiller-Universität Jena and Foundation “Else Kröner-Fresenius-Stiftung” within the Else Kröner Graduate School for Medical Students “Jena School for Ageing Medicine (JSAM)”.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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