Acquired resistance to EGFR tyrosine kinase inhibitors in cancer cells is mediated by loss of IGF-binding proteins

Perspectives in mammalian IGFBP-3 biology: local vs. systemic action

Insulin-like growth factor (IGF) binding protein (IGFBP)-3 has traditionally been defined by its role as a binding protein and its association with IGF delivery and availability. Development of non-IGF binding IGFBP-3 analogs and the use of cell lines devoid of type 1 IGF receptors (IGF-R) have led to critical advances in the field of IGFBP-3 biology. These studies show that IGFBP-3 has IGF-independent roles in inhibiting cell proliferation in cancer cell lines. Nuclear transcription factor, retinoid X receptor (RXR)-alpha, and IGFBP-3 functionally interact to reduce prostate tumor growth and prostate-specific antigen in vivo. Moreover, IGFBP-3 inhibits insulin-stimulated glucose uptake into adipocytes independent of IGF. The purpose of this review is to highlight IGFBP-3 as a novel effector molecule and not just another "binding protein" by discussing its IGF-independent actions on metabolism and cell growth. Although this review presents studies that assume the role of IGFBP-3 as either an endocrine or autocrine/paracrine molecule, these systems may not exist as distinct entities, justifying the examination of IGFBP-3 in an integrated model. Also, we provide an overview of factors that regulate IGFBP-3 availability, including its production, methylation, and ubiquitination. We conclude with the role of IGFBP-3 in whole body systems and possible future applications of IGFBP-3 in physiology.

Mechanisms of tumor resistance to EGFR-targeted therapies

BACKGROUND

Much effort has been devoted to development of cancer therapies targeting EGFR, based on its role in regulating cell growth. Small-molecule and antibody EGFR inhibitors have clinical roles based on their efficacy in a subset of cancers, generally as components of combination therapies. Many cancers are either initially resistant to EGFR inhibitors or become resistant during treatment, limiting the efficacy of these reagents.

OBJECTIVE/METHODS

To review cellular resistance mechanisms to EGFR-targeted therapies.

RESULTS/CONCLUSIONS

The best validated of these mechanisms include activation of classic ATP-binding casette (ABC) multidrug transporters; activation or mutation of EGFR; and overexpression or activation of signaling proteins operating in relation to EGFR. We discuss current efforts and potential strategies to override these sources of resistance. We describe emerging systems-biology-based concepts of alternative resistance to EGFR-targeted therapies, and discuss their implications for use of EGFR-targeted and other targeted therapies.

Establishment and characterization of a model of acquired resistance to epidermal growth factor receptor targeting agents in human cancer cells

PURPOSE

The epidermal growth factor receptor (EGFR) is recognized as a key mediator of proliferation and progression in many human tumors. A series of EGFR-specific inhibitors have recently gained Food and Drug Administration approval in oncology. These strategies of EGFR inhibition have shown major tumor regressions in approximately 10% to 20% of advanced cancer patients. Many tumors, however, eventually manifest resistance to treatment. Efforts to better understand the underlying mechanisms of acquired resistance to EGFR inhibitors, and potential strategies to overcome resistance, are greatly needed.

EXPERIMENTAL DESIGN

To develop cell lines with acquired resistance to EGFR inhibitors we utilized the human head and neck squamous cell carcinoma tumor cell line SCC-1. Cells were treated with increasing concentrations of cetuximab, gefitinib, or erlotinib, and characterized for the molecular changes in the EGFR inhibitor-resistant lines relative to the EGFR inhibitor-sensitive lines.

RESULTS

EGFR inhibitor-resistant lines were able to maintain their resistant phenotype in both drug-free medium and in athymic nude mouse xenografts. In addition, EGFR inhibitor-resistant lines showed a markedly increased proliferation rate. EGFR inhibitor-resistant lines had elevated levels of phosphorylated EGFR, mitogen-activated protein kinase, AKT, and signal transducer and activator of transcription 3, which were associated with reduced apoptotic capacity. Subsequent in vivo experiments indicated enhanced angiogenic potential in EGFR inhibitor-resistant lines. Finally, EGFR inhibitor-resistant lines showed cross-resistance to ionizing radiation.

CONCLUSIONS

We have developed EGFR inhibitor-resistant human head and neck squamous cell carcinoma cell lines. This model provides a valuable preclinical tool to investigate molecular mechanisms of acquired resistance to EGFR blockade.

Overcoming resistance to tyrosine kinase inhibitors: lessons learned from cancer cells treated with EGFR antagonists

Tyrosine kinase inhibitors (TKIs) are effective anti-cancer therapies but resistance to these agents eventually develops. Several models of resistance to TKIs have been studied including resistance to EGFR inhibitors. Recent studies in EGFR-dependent A431 cells found upregulation of the IGF1R pathway as a mechanism to overcome blockade of EGFR. This was associated with amplification of IGF1R signaling and recovery of downstream PI3K-AKT activation. This work adds to a growing body of cell lines in culture that have provided insights into mechanisms of resistance that can be interrogated in primary tumors in patients. In this review, these model systems and their applicability to human cancers, as well as strategies to identify and overcome resistance to TKIs, are discussed.

Phosphorylated insulin-like growth factor-i/insulin receptor is present in all breast cancer subtypes and is related to poor survival

Drugs that target the insulin-like growth factor-I receptor (IGF-IR) and/or insulin receptor (IR) are currently under investigation for a variety of malignancies including breast cancer. Although we have previously reported that IGF-IR expression in primary breast tumors is common, the activation status of this receptor has not been examined in relation to survival. Phosphorylated IGF-IR/IR (P-IGF-IR/IR) and its downstream signaling partner phospho-S6 (P-S6) were evaluated immunohistochemically in tumor tissue microarrays representing 438 cases of invasive breast cancer. P-IGF-IR/IR (n = 114; P = 0.046) and total levels of IR (n = 122; P = 0.009) were indicative of poor survival, whereas total IGF-IR (n = 112; P = 0.304) was not. P-IGF-IR/IR and P-S6 were coordinately expressed in primary breast tumors (likelihood ratio, 11.57; P = 6.70 x 10(-4)). Importantly, P-IGF-IR/IR was detected in all breast cancer subtypes (luminal, 48.1%; triple negative, 41.9%; and HER2, 64.3%). In vitro, the IGF-IR/IR inhibitor BMS-536924 decreased phospho-RSK and P-S6, and significantly suppressed the growth of breast cancer cell lines MCF-7, SUM149, and AU565 representing the luminal, triple negative, and HER2 subtypes, respectively, in monolayer and soft agar. BMS-536924 also inhibited growth in tamoxifen resistant MCF-7 Tam-R cells while having little effect on immortalized normal breast epithelial cells. Thus, we can determine which patients have the activated receptor and provide evidence that P-IGF-IR/IR is a prognostic factor for breast cancer. Beyond this, P-IGF-IR/IR could be a predictive marker for response to IGF-IR and/or IR-targeted therapies, as these inhibitors may be of benefit in all breast cancer subtypes including those with acquired resistance to tamoxifen.

Effects of erlotinib in EGFR mutated non-small cell lung cancers with resistance to gefitinib

PURPOSE

Most lung cancers with activating epidermal growth factor receptor (EGFR) mutations respond to gefitinib; however, resistance to this tyrosine kinase inhibitor (TKI) invariably ensues. The T790M mutation occurs in 50% and MET amplification in 20% of TKI-resistant tumors. Other secondary mutations (D761Y and L747S) are rare. Our goal was to determine the effects of erlotinib 150 mg/d in EGFR mutated patients resistant to gefitinib 250 mg/d, because the EGFR TKI erlotinib is given at a higher biologically active dose than gefitinib.

EXPERIMENTAL DESIGN

Retrospective review of 18 EGFR mutated (exon 19 deletions, L858R, and L861Q) patients that were given gefitinib and subsequently erlotinib. Seven patients had tumor resampling after TKI therapy and were analyzed for secondary EGFR mutations and MET amplification.

RESULTS

Most patients (14 of 18) responded to gefitinib with median progression-free survival of 11 months (95% confidence interval, 4-16). After gefitinib resistance (de novo or acquired), 78% (14 of 18) of these patients displayed progressive disease while on erlotinib with progression-free survival of 2 months (95% confidence interval, 2-3). Six of 7 resampled patients acquired the T790M mutation, and 0 of 3 had MET amplification. Only 1 gefitinib-resistant patient with the acquired L858R-L747S EGFR, which in vitro is sensitive to achievable serum concentrations of erlotinib 150 mg/d, achieved a partial response to erlotinib.

CONCLUSIONS

In EGFR mutated tumors resistant to gefitinib 250 mg/d, a switch to erlotinib 150 mg/d does not lead to responses in most patients. These findings are consistent with preclinical models, because the common mechanisms of TKI resistance (T790M and MET amplification) in vitro are not inhibited by clinically achievable doses of gefitinib or erlotinib. Alternative strategies to overcome TKI resistance must be evaluated.

IGF binding proteins (IGFBPs) and regulation of breast cancer biology

The IGFBP family comprises six proteins with high affinity for the IGFs. Changes in the balance of the components of the IGF system may contribute to the progression of breast cancer. In tumours the abundance of IGFBPs relates to the estrogen receptor status and their production in the breast is controlled by hormones, principally estrogen and progesterone. Important interactions occur between IGFBPs and key growth regulators such as TGF-beta, PTEN and EGF which are reviewed. The conflicting observations between the effects of IGFBPs on the risk of breast cancer, in particular IGFBP-3, obtained from epidemiology studies in comparison to in vivo observations are highlighted and potential explanations provided. The functional activity of IGFBPs can also be affected by proteolysis, phosphorylation and glycosylation and the implications of these are described. The IGFs are generally present at levels far in excess of that required for maximal receptor stimulation, and the IGFBPs are critical regulators of their cellular actions. IGFBPs can affect cell function in an IGF-dependent or independent manner. The key mechanisms underlying the intrinsic actions of the IGFBPs are still in debate. IGF bioactivity locally in the breast is influenced not only by local tissue expression and regulation of IGFs, IGFBPs and IGFBP proteases, but also by these factors delivered from the circulation. Finally, the therapeutic potential of IGFBPs-2 and -3 are considered together with key questions that still need to be addressed.

Cross-talk between the ErbB/HER family and the type I insulin-like growth factor receptor signaling pathway in breast cancer

Understanding the molecular mechanisms involved in tumorigenesis and their influence on clinical outcome is providing specific molecular markers for targeted therapy. Activation of tyrosine kinase receptors from the human epidermal growth factor receptor family (EGFR, HER2, HER3, HER4) and the insulin-like growth factor receptor I (IGF-IR) plays a key role in the initiation and progression of breast cancer. HER2 overexpression is a validated therapeutic target, as shown by the clinical efficacy of trastuzumab and lapatinib. However, only 25-30% of patients with HER2-overexpressing tumors respond to single-agent trastuzumab or lapatinib, and resistance develops even in responding patients. Therefore, to optimize therapeutic efficacy, it is urgent to elucidate the complex network of signaling pathways that develop in breast cancer cells. Signaling interactions have been reported between ErbB/HER family members and IGF-IR. As increased IGF-IR signaling has been implicated in trastuzumab resistance, agents targeting HER2, and IGF-IR could be potential therapeutic tools in breast cancers that develop resistance to HER2-directed therapy.

Mechanisms of resistance to ErbB-targeted cancer therapeutics

The ErbB receptors, such as EGFR, have been intensely pursued as targets for cancer therapeutics. However, a large percentage of patients who are initially responsive to ErbB-targeted therapies experience tumor recurrence and become refractory to therapy. In this issue of the JCI, Guix et al. demonstrate that downregulation of IGF-binding protein 3 (IGFBP-3) and -4, the negative regulators of IGF-I receptor signaling, contributes to the resistance of human squamous cell carcinomas to the EGFR inhibitor gefitinib (see the related article beginning on page 2609). Understanding the mechanisms involved in the resistance of some tumors to ErbB-targeted molecules may provide guidelines for developing more efficient therapeutic approaches.