Insulin-like growth factor receptor signaling in tumorigenesis and drug resistance: a challenge for cancer therapy

Sustained growth is a typical hallmark of cancer. Growth factors, such as epidermal growth factor (EGF), transforming growth factor (TGF), platelet-derived growth factor (PDGF), hepatocyte growth factor (HGF), fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF), and insulin-like growth factor (IGF), stimulate cancer cell and stromal cell proliferation, migration, and invasion thereby promoting tumor growth, angiogenesis, and metastasis. While IGF deficiency may inhibit development and cause metabolic problems, excessive IGF levels disturb health. Increased IGF levels are inversely associated with longevity and positively associated with cancer risk. IGF is mainly produced by the liver, as well as tumor cells and cancer-associated macrophages (Fig. 1). Furthermore, the bioavailability of IGF is regulated by IGF-binding proteins (IGFBPs), which are identified as the serum reservoirs of IGF [1]. Epidemiological studies have demonstrated that circulating IGF and IGFBP levels are associated with some types of cancer. For example, higher levels of circulating IGF are associated with increased risk of breast and prostate cancer [2, 3]. Recent study also reveals that there is a modest positive association between IGF1 and lung cancer risk in current smokers [4]. Although IGF1 was not associated with overall colorectal cancer risk, it appears to be associated with the increased risk for advanced colorectal cancer [5]. In addition, serum levels of IGFBP3 are inversely associated with esophageal and gastrointestinal cancer [6, 7]. However, a recent nested case-control study reveals that circulating concentration of IGF1 is not associated with bladder cancer, lymphoma, and melanoma risk in a European population [8‐10].

IGF binds to its cell surface receptors such as type I insulin-like growth factor receptor (IGF-IR) and then initiates multiple signaling pathways including PI3K/Akt, MAPK, JAK/STAT, Src, and focal adhesion kinase (FAK), which act in concert to stimulate cancer cell proliferation, survival and migration. Activation of IGF signaling pathways promotes the growth, metastasis, and drug resistance in many types of human tumors, including mesenchymal, epithelial, and hematopoietic cancer [11‐13]. Previous study indicates that IGF-IR may be involved in cell-fate determination. Elevated expression of IGF-IR in BCR/ABL⁺ cells may promote the development and self-renewal of chronic myeloid leukemia, while downregulation of IGF-IR in BCR/ABL⁺ cells leads to acute lymphoblastic leukemia (ALL) [14]. However, another study demonstrates that IGF-IR overexpression is essential for leukemia-initiating cell activity in T-ALL [15]. In addition, Bcr-Abl can induce autocrine IGF1 signaling in leukemia cells [16]. Autocrine IGF1 signaling also promotes growth and survival of human acute myeloid leukemia cells [17]. Moreover, IGF-IR is involved in the progression of chronic lymphocytic leukemia [18]. IGF-IR interacts with NPM-ALK and then promotes T cell ALK⁺ anaplastic large cell lymphoma cells survival [19].

Given the important roles of IGF-IR in tumor progression, inhibition of IGF-IR activity has been proposed as a therapeutic strategy for solid tumors and hematologic malignancies [20]. Many anti-IGF-IR monoclonal antibodies (mAbs) and small-molecule inhibitors have been developed. Although anti-IGF-IR mAbs and IGF-IR inhibitors have potent anti-cancer effects in preclinical models, clinical trials of these agents are largely disappointing in unselected cancer patients. Nevertheless, some anti-IGF-IR mAbs, such as ganitumab, show clinical benefits in some types of cancer, and are still under active evaluation. In this review, we update recent progress in exploring the complex IGF signaling, its role in drug resistance, and the mechanisms for resistance to IGF-IR inhibition. We also revisit the strategies to target IGF-IR for cancer therapy.

The physiological and pathological functions of IGF are largely mediated by its receptors. There are two types of IGF receptors, namely type I IGF receptor (IGF-IR) and type II IGF receptor (IGF-IIR). IGF-IR is a receptor tyrosine kinase (RTK) that mediates the stimulatory effects of IGF1 and IGF2 on cell proliferation, migration, and invasion. In contrast, IGF-IIR lacks the kinase activity. Therefore, IGF-IIR can sequester IGF2 and even deliver it to lysosome for degradation, thereby abrogating IGF2 signaling (Fig. 1) [21]. IGF-IR consists of extracellular α subunit (IGF-IRα) and transmembrane β subunit (IGF-IRβ). Both IGF-IRα and IGF-IRβ are encoded by a single gene on chromosome 15q26.3. After the translation of IGF-IR mRNA, the 210 kDa protein product (pro-IGF-IR) is subject to endoproteolytic cleavage by the pro-protein convertases such as furin and pro-protein convertase 5, leading to the generation of α chain (706 amino acids) and β chain (627 amino acids) [22]. The mature IGF-IR is a heterotetramer with two α chains and two β chains [23]. In addition, IGF-IR has high homology with the closely related insulin receptor (InsR), which has two isoforms, InsR-A and InsR-B [24]. The InsR-A is generated by alternative splicing of exon 11 in InsR gene [25]. InsR-A differs from InsR-B in ligand-binding and signaling properties. While InsR-A homodimer binds both insulin and IGF2 with high affinity, InsR-B homodimer and InsR-A/B heterodimer preferentially bind insulin but not IGF. IGF-IR may heterodimerize with InsR-A or InsR-B and then forms a hybrid receptor. IGF1/2 binds to IGF-IRα homodimer and IGF-IR/InsR heterodimer. Especially, IGFBP7 may compete with IGF to bind to the extracellular domain of IGF-IR and then suppress the activation of IGF-IR by IGF [26]. In addition, IGF2 can bind to InsR-A homodimer (Fig. 1) [27]. InsR-B, which contains 12 amino acids encoded by exon 11 of InsR, mediates the metabolic actions of insulin such as the uptake of glucose by muscle and adipose [25, 28]. To prevent metabolic disorders, targeting IGF-IR/InsR for cancer therapy should avoid compromising the function of InsR-B.

Upon binding to IGF, IGF-IR undergoes changes in its conformation, which in turn abolishes the restraints of intrinsic kinase activity by the ectodomain, and triggers transphosphorylation of its tyrosine kinase domains [29]. Phosphorylation of a triple-tyrosine cluster (Y1131/1135/1136) in the kinase domain of β subunit can further amplify the kinase activity of IGF-IR. Except for autophosphorylation, IGF-IR can be phosphorylated and activated by other kinases such as Src and FAK [30, 31]. Of note, the protein kinase mTOR has tyrosine kinase activity [32]. mTORC2 interacts with IGF-IR, and phosphorylates IGF-IR at Y1131/1136 thereby activating IGF-IR [32]. The conformational changes in IGF-IR create docking sites for its substrates, such as Shc and InsR substrates (IRS), which relay the signaling to downstream effectors including PI3K, MAPK, and STAT3. The IRS1/Akt pathway is critical for regulation of metabolism by insulin and InsR. Instead, InsR-A mediates the mitogenic effects of both IGF and insulin, which is dependent on receptor internalization, phosphorylation of SHC and MAPK [33].

IGF-IR has both beneficial and detrimental effects on health. Studies in IGF-IR-deficient mice demonstrate that these mice die within a few days after birth [34], indicating that IGF-IR has critical roles in development and health. However, overexpression of IGF-IR may induce cellular transformation. IGF-IR is frequently overexpressed or activated in a variety of cancer. IGF-IR expression is elevated in about 50% of breast cancers, and more frequently in luminal A-type breast cancer than luminal B and triple-negative breast cancer [35]. The prognostic impact of IGF-IR expression in human breast cancer remains inconclusive. While some studies suggest that overexpression of IGF-IR or phosphorylated IGF-IR is inversely associated with patient prognosis among all subtypes of breast cancer [36‐38], another study reveals that IGF-IR is only inversely associated with prognosis in ErbB2-positive breast cancer [39]. One study even shows that luminal A/B breast cancer patients with high IGF-IRα and negative EGFR expression have better prognosis than the rest [40]. The reasons for discrepancy among these studies are unclear. It remains to know whether the levels of IGF-IRα and IGF-IRβ have different impact on the prognosis. One possibility is that the IGF-IRα/IGF-IRβ ratio can dictate the different outcome.

In addition, IGF-IR is overexpressed in about 30% of human prostate cancer. Overexpression of IGF-IR in prostate cancer is associated with high Gleason grade and increased risk of tumor recurrence and metastasis [41]. Moreover, cell membrane expression of IGF-IR is present in 36% of head and neck squamous carcinoma (HNSCC), while cytoplasmic IGF-IR is detected in 92% of HNSCC [42]. Regardless of the location of IGF-IR, high levels of IGF-IR are associated with high tumor stage, HPV negativity, and short overall survival [42]. Overexpression of IGF-IRβ was correlated with a decreased survival in patients with cervical carcinoma [43]. Except for the levels of IGF-IR, the IGF-activated gene transcription signature is strongly associated with poor prognosis in breast cancer patients [38]. Therefore, both the levels and activity of IGF-IR are positively correlated with tumor progression.

Once activated by the ligands, IGF-IR and InsR may initiate diverse signaling pathways to regulate cell proliferation, differentiation, survival, metabolism, migration, and stemness (Fig. 2). IRS1/2, Shc, and Grb, are typical adaptors to transduce signaling from IGF-IR to downstream effectors. While most of the canonical IGF-IR signaling processes are conducted in the cytosol, IGF-IR may translocate to the nuclei and then regulate gene transcription, which represents a non-canonical function of IGF-IR.

IGF-IR not only promotes cell proliferation, but also prevents stress-induced cell death. Previous studies demonstrate that IGF-IR contributes to resistance to various category of cancer therapeutics including chemotherapy, endocrine therapy, radiotherapy and targeted therapy. Overexpression and activation of IGF-IR during chemotherapy, radiotherapy or targeted therapy predicts poor outcome in cancer patients [91]. Meanwhile, preclinical studies indicate that inhibition of IGF-IR may reverse the resistance to many drugs and improve the efficacy of those anti-cancer agents.

The identification of IGF-IR as an important cancer-promoting protein has stimulated the development of many IGF-IR-targeted agents, including anti-IGF-IR mAbs and small-molecule inhibitors. Given the promising anti-cancer effects of IGF-IR-targeted agents in preclinical models, clinical trials have been initiated to evaluate the efficacy of anti-IGF-IR mAbs (Table 1) [144‐151]. However, anti-IGF-IR mAbs monotherapy is largely ineffective in unselected cancer patients. Some clinical trials suggest that IGF-IR inhibition may be effective only in selective cancer patients [152, 153]. A meta-analysis of 311 Ewing sarcoma patients in multiple trials demonstrates that treatment with anti-IGF-IR mAb figitumumab, R1507, or ganitumab results in complete response in 0.7% cases, partial response in 11% cases, and stable diseases in 21% cases [154]. Clinical evaluation of ganitumab remains active in patients with Ewing sarcoma, rhabdomyosarcoma and breast cancer. In fact, ganitumab has been approved as orphan drug in Ewing sarcoma by the US Food and Drug Agency. It is being tested to combine ganitumab with Src inhibitor dasatinib in patients with embryonal and alveolar rhabdomyosarcoma, and with chemotherapy, radiotherapy or CDK4/6 inhibitor in Ewing sarcoma (Table 1). In addition, combination of ganitumab with metformin is being tested in an ongoing I-SPY2 trial, which aims to identify  most effective agents to treat various types of breast cancer and learn about the predictors of treatment response [155]. Moreover, a randomized clinical trial in patients with advanced pancreatic carcinoma demonstrates that combination of the anti-IGF-IR mAb MK-0646 with gemcitabine is tolerable and associated with improvement in overall survival but not progression-free survival as compared with gemcitabine plus erlotinib [151]. As an alternative strategy to target IGF signaling, results of phase 1 clinical trials of the dual IGF1/2-neutralizing antibodies have been reported, while no results of phase 2/3 trials in cancer therapy have been reported [156, 157]. Currently, the dual IGF1/2-neutralizing antibody xentuzumab in combination with everolimus and exemestane is subject to clinical trial in ER⁺/ErbB2^- breast cancer. Combination of everolimus with exemestrane is an effective treatment for ER⁺/ErbB2^- breast cancer. It remains to know whether xentuzumab may further improve the efficacy of everolimus and exemestrane.

Except for anti-IGF-IR or anti-IGF1/2 mAbs, many small-molecule IGF-IR inhibitors have been developed [158‐163]. The most extensively evaluated IGF-IR inhibitor is linsitinib (OSI-906), a dual IGF-IR/InsR inhibitor (Table 2). A phase 2 trial in recurrent small cell lung cancer patients shows that linsitinib has no clinical activity in these patients, although it is a safe and tolerable treatment [159]. The response to linsitinib is inferior to the topoisomerase inhibitor topotecan, with median progression-free survival of 3.0 (95% CI 1.5–3.6) and 1.2 (95% CI 1.1–1.4) months for topotecan and linsitinib, respectively [159]. Besides, linsitinib fails to show activity in patients with metastatic prostate cancer, locally advanced or metastatic adrenocortical carcinoma [158, 161]. Combination of linsitinib with everolimus shows no objective response in patients with refractory metastatic colorectal cancer [160]. In addition, combination of linsitinib with erlotinib did not improve the progression-free survival and overall survival in patients with advanced non-small cell lung cancer, compared to erlotinib alone (Table 2). According to the recently reported results of a phase 2 trial in patients with gastrointestinal stromal tumors, linsitinib achieved no objective responses in these patients, although the rate of 9-month clinical benefit, progression-free survival, and overall survival at 9 months was 40%, 52% and 80%, respectively [162]. Despite these disappointing results, an early phase study suggests that AXL1717 may produce prolonged stable disease and survival of patients with relapsed malignant astrocytomas [163].

The high failure rate of anti-IGF-IR mAb/inhibitor monotherapy or combinational therapy in clinical trials promotes the investigators to inspect the mechanisms underlying resistance to IGF-IR inhibition in cancer and revisit the strategies to target IGF-IR pathway. While mutations in the drug target are involved in the resistance to many molecularly targeted agents [164], mutations in IGF-IR are barely detected in cancer cells resistant to anti-IGF-IR agents. There are multiple alternative signaling pathways to relay IGF/insulin signaling after IGF-IR is inhibited (Fig. 3).

Mounting evidence demonstrates that receptor tyrosine kinases play important roles in carcinogenesis and tumor progression. Even with the remarkably good response to EGFR/ErbB2 inhibitors in some types of human cancer, the majority of patients eventually develop drug resistance, in which IGF-IR is also involved. Although it is clear that IGF-IR confers resistance to molecularly targeted therapy, chemotherapy and radiotherapy, the complex IGF biology is a great challenge for targeting this pathway in cancer. Either intrinsic or adaptive resistance to anti-IGF-IR agents is a troublesome roadblock. The high degree of tumor heterogeneity and adaptive cellular signaling plasticity, selection pressure, and clonal evolution substantially contribute to the resistance to molecularly targeted therapy including anti-IGF/IGF-IR agents [191].

The efficacy of molecularly targeted therapeutics may be highly selective in cancer patients. However, most of the clinical trials of anti-IGF-IR agents are conducted in unselected cancer patients. There are responders and non-responders in multiple clinical trials of IGF-IR inhibitors. While identification of the different molecular signatures and genotypes among responders and non-responders may help develop predictive biomarkers to enable patient stratification in clinical setting, the molecular and genetic heterogeneity in many tumors is still a big challenge for successful cancer therapy [182]. Nevertheless, identification of potential predictive biomarkers is an urgent task to advance the anti-IGF-IR therapeutics. The use of integrative proteomic-genomic techniques to analyze the samples of patients may be a reliable strategy. Preclinical studies have suggested potential biomarkers such as circulating free IGF levels, mutations of p53, BRCA1, KRAS and BRAF, IRS2 copy number gain, InsR/IGF-IR overexpression, InsR/IGF-IR pathway activation, PTEN loss, CD24, mesenchymal markers, β-catenin/TCF activity, FUS-DDIT3 fusion, and ERG [192‐201]. Another study indicates that exclusive nuclear localization of IGF-IR is associated with better response to anti-IGF-IR antibody in patients with sarcomas [79]. These biomarkers should be evaluated in the clinic. Stratification of anti-IGF-IR agent responders according to molecular subtype remains to be a future direction.

Learning from the lessons of clinical trials, we need to adjust the strategy to target the complex IGF/IGF-IR signaling. Given the collaboration and compensation among different signaling pathways, IGF-IR and InsR should not be the sole targets to be blocked. IGF-IR-cotargeted multikinase inhibitors are preferable to treat cancer. The multikinase (FAK/IGF-IR) inhibitor CT-707 has anti-cancer effect in preclinical models and is subject to phase I clinical trial (clinicaltrials.​gov, NCT02695550) [49]. In addition, the 4-aminopyrazolo[3,4-d]pyrimidine-based dual IGF-IR/Src inhibitor LL28 inhibits tumor growth in preclinical models [202]. We look forward to evaluating the efficacy of these multikinase inhibitors in clinical trials. Recently, the FDA-approved ALK inhibitor ceritinib has been identified as a multikinase inhibitor targeting IGF-IR/InsR, FAK, and RSK1/2 as well [203]. A recent study also demonstrates that ceritinib inhibits InsR phosphorylation and induces tumor regression in a pediatric patient with an unclassified brain tumor [204]. Another study suggests that IGF-IR is as an attractive target for treating patients with BCOR-alternated high-grade neuroepithelial tumor of the central nervous system [205]. It is worthwhile to ensure whether ceritinib can be repurposed to treat IGF-IR-activated cancer. In addition, the combined treatment with IGF-IR/InsR and CDK inhibitors warrants clinical evaluation.

Rather than targeting IGF-IR/InsR themselves, an alternative strategy would be inhibition of the master regulators of RTKs or downstream signaling hubs such as IRS1/2 [205‐208]. Moreover, IGF-IR has kinase-independent functions, such as nuclear translocation and regulation of gene expression, and interaction with SGLT1 to maintain intracellular glucose levels [209]. These kinase-independent functions of IGF-IR are another challenge to treat cancer with anti-IGF-IR agents. Inhibition of the kinase activity of IGF-IR may be not enough to eliminate its pro-tumor effects. Targeting InsR/IGF-IR expression or degradation should be considered as another strategy awaiting further exploration [210, 211]. Given that inhibition of IGF-IR may paradoxically lead to upregulation of IGF1/2 expression, and IGF1/2 have IGF-IR/InsR-independent functions, it warrants to investigate whether co-targeting IGF and IGF-IR/InsR-A may achieve objective responses.