VEGFR-2 expression in carcinoid cancer cells and its role in tumor growth and metastasis

Animal models and cell lines of pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (PNETs), also known as islet cell tumors, are mostly indolent neoplasms that probably arise from a network of endocrine cells that includes islet cells and pluripotent precursors in the pancreatic ductal epithelium. The incidence and prevalence of PNETs continue to rise in recent years because of more sensitive detection. The molecular pathogenesis, early detection, molecular predictors of tumor behavior, and targeted drug therapy of PNETs are not well understood and require additional basic and translational research. The rarity and indolent nature of these tumors, difficulty of access to appropriate patient tissue samples, and varying histopathology and secreted hormones pose particular challenges to PNET researchers. Animal models and cell lines are indispensable tools for investigating the pathogenesis, pathophysiology, mechanisms for tumor invasion and metastasis, and therapeutics of PNETs. This review summarizes currently available animal models and cell lines of PNETs, which have provided valuable insights into the pathogenesis and natural history of human PNETs. In the future, animal models and cell lines of PNETs should also be used to study early tumor detection and molecular predictors of tumor behavior and to test the responses to, and mechanisms for, novel targeted drug therapies.

Tumor VEGF:VEGFR2 autocrine feed-forward loop triggers angiogenesis in lung cancer

The molecular mechanisms that control the balance between antiangiogenic and proangiogenic factors and initiate the angiogenic switch in tumors remain poorly defined. By combining chemical genetics with multimodal imaging, we have identified an autocrine feed-forward loop in tumor cells in which tumor-derived VEGF stimulates VEGF production via VEGFR2-dependent activation of mTOR, substantially amplifying the initial proangiogenic signal. Disruption of this feed-forward loop by chemical perturbation or knockdown of VEGFR2 in tumor cells dramatically inhibited production of VEGF in vitro and in vivo. This disruption was sufficient to prevent tumor growth in vivo. In patients with lung cancer, we found that this VEGF:VEGFR2 feed-forward loop was active, as the level of VEGF/VEGFR2 binding in tumor cells was highly correlated to tumor angiogenesis. We further demonstrated that inhibition of tumor cell VEGFR2 induces feedback activation of the IRS/MAPK signaling cascade. Most strikingly, combined pharmacological inhibition of VEGFR2 (ZD6474) and MEK (PD0325901) in tumor cells resulted in dramatic tumor shrinkage, whereas monotherapy only modestly slowed tumor growth. Thus, a tumor cell-autonomous VEGF:VEGFR2 feed-forward loop provides signal amplification required for the establishment of fully angiogenic tumors in lung cancer. Interrupting this feed-forward loop switches tumor cells from an angiogenic to a proliferative phenotype that sensitizes tumor cells to MAPK inhibition.

Phase I study of sorafenib in combination with everolimus (RAD001) in patients with advanced neuroendocrine tumors

PURPOSE

Sorafenib and everolimus are both active against neuroendocrine tumors (NET). Because of potential synergy between VEGF pathway and mTOR inhibitors, we performed a phase I study to evaluate the safety and feasibility of combining sorafenib and everolimus in patients with advanced NET.

METHODS

Patients were treated with everolimus 10 mg daily in combination with sorafenib (dose level 1: 200 mg twice daily; dose level 2: 200 mg per morning, 400 mg per evening) using standard phase I dose escalation design. Dose-limiting toxicity (DLT) was defined within the first cycle (28 days) of therapy. Treatment was continued until tumor progression, unacceptable toxicity, or withdrawal of consent. Twelve additional patients were treated at the maximum tolerated dose (MTD) level to further characterize safety and a preliminary assessment of activity.

RESULTS

One patient in Cohort 1 experienced DLT (grade 3 skin rash); the cohort was expanded to 6 patients with no further DLTs. All 3 patients in Cohort 2 experienced DLT, consisting of thrombocytopenia, hand-foot skin reaction, and rash/allergic reaction. Sorafenib 200 mg twice daily in combination with everolimus 10 mg daily was established as the MTD. Independently reviewed best objective responses revealed that 62 % of patients had some degree of tumor shrinkage. By RECIST, we observed partial response in 1 patient, stable disease in 13 patients, and progressive disease in 3 patients.

CONCLUSION

Sorafenib 200 mg twice daily with everolimus 10 mg daily represents the MTD of this combination in patients with advanced NET. While the combination is active, toxicity concerns may preclude more widespread use.

Development of VEGFR2-specific Nanobody Pseudomonas exotoxin A conjugated to provide efficient inhibition of tumor cell growth

Angiogenesis targeting is an attractive approach for cancer treatment. Vascular endothelial growth factor receptor 2 (VEGFR2) is such an important target that is overexpressed in tumor vasculature compared to the endothelium cells of resting blood vessels and blocking of its signaling inhibits neovascularization and tumor metastasis. Immunotoxins represent a promising group of targeted therapeutics to combat tumors. They consist of an antibody linked to a toxin and are designed to kill specifically the tumor cells. In this study, we fused a VEGFR2-specific Nanobody, the antigen-binding single-domain fragment derived from functional Heavy-chain antibody of Camelidae, to the truncated form of Pseudomonas exotoxin A and evaluated its ability to bind the VEGFR2 molecule on the cell surface. We demonstrate that this immunotoxin inhibits the proliferation of VEGFR2-expressing cells in vitro. This finding is considered to be a significant achievement in tumor therapy and it forms a basis for further studies in animal models.

VEGF inhibits tumor cell invasion and mesenchymal transition through a MET/VEGFR2 complex

Inhibition of VEGF signaling leads to a proinvasive phenotype in mouse models of glioblastoma multiforme (GBM) and in a subset of GBM patients treated with bevacizumab. Here, we demonstrate that vascular endothelial growth factor (VEGF) directly and negatively regulates tumor cell invasion through enhanced recruitment of the protein tyrosine phosphatase 1B (PTP1B) to a MET/VEGFR2 heterocomplex, thereby suppressing HGF-dependent MET phosphorylation and tumor cell migration. Consequently, VEGF blockade restores and increases MET activity in GBM cells in a hypoxia-independent manner, while inducing a program reminiscent of epithelial-to-mesenchymal transition highlighted by a T-cadherin to N-cadherin switch and enhanced mesenchymal features. Inhibition of MET in GBM mouse models blocks mesenchymal transition and invasion provoked by VEGF ablation, resulting in substantial survival benefit.

Expression of tumor-derived vascular endothelial growth factor and its receptors is associated with outcome in early squamous cell carcinoma of the lung

PURPOSE

Antiangiogenic therapies targeting the vascular endothelial growth factor (VEGF) pathway have yielded more modest clinical benefit to patients with non-small-cell lung cancer (NSCLC) than initially expected. Clinical data suggest a distinct biologic role of the VEGF pathway in the different histologic subtypes of lung cancer. To clarify the influence of histologic differentiation in the prognostic relevance of VEGF-mediated signaling in NSCLC, we performed a concomitant analysis of the expression of three key elements of the VEGF pathway in the earliest stages of the following two principal histologic subtypes: squamous cell carcinoma (SCC) and adenocarcinoma (ADC).

PATIENTS AND METHODS

We evaluated tumor cell expression of VEGF, VEGF receptor (VEGFR) 1, and VEGFR2 using automatic immunostaining in a series of 298 patients with early-stage NSCLC recruited as part of the multicenter European Early Lung Cancer Detection Group project. A score measuring the VEGF signaling pathway was calculated by adding the tumor cell expression value of VEGF and its two receptors. The results were validated in two additional independent cohorts of patients with NSCLC.

RESULTS

The combination of high VEGF, VEGFR1, and VEGFR2 protein expression was associated with lower risk of disease progression in early SCC (univariate analysis, P = .008; multivariate analysis, hazard ratio, 0.62; 95% CI, 0.42 to 0.92; P = .02). The results were validated in two independent patient cohorts, confirming the favorable prognostic value of high VEGF signaling score in early lung SCC.

CONCLUSION

Our results clearly indicate that the combination of high expression of the three key elements in the VEGF pathway is associated with a good prognosis in patients with early SCC but not in patients with ADC.

Association between genetic variations of vascular endothelial growth factor receptor 2 and glioma in the Chinese Han population

Tumor angiogenesis, which is an important step in the development of cancer, is directly regulated by vascular endothelial growth factor receptor 2 (VEGFR-2). In this study, we examined the association of five potentially functional VEGFR-2 polymorphisms with glioma risk in a Chinese Han population. Three SNPs, rs2071559, rs7667298 and rs2305948, showed a statistically significant increased association with the risk of glioma (P = 0.006, 0.005, and 0.012, respectively). Both haplotype and diplotype analyses consistently revealed that subjects carrying two copies of the haplotype "CGT" had a 42% reduced glioma risk compared with their respective noncarriers. Our findings suggested that VEGFR-2 gene variants might contribute to glioma susceptibility.

The effect of PTEN on serotonin synthesis and secretion from the carcinoid cell line BON

BACKGROUND

Carcinoid tumors are associated with the carcinoid syndrome, a set of symptoms resulting from the peptide and amine products, including serotonin, secreted from the cancer cells. The purpose of this study was to investigate the relationship between the phosphatidylinositol-3-kinase/protein kinase B (PI3K/Akt) inhibitor PTEN (phosphatase and tensin homolog deleted on chromosome ten) and serotonin synthesis and secretion in the carcinoid cancer cell line BON.

MATERIALS AND METHODS

PTEN was inhibited by pharmacological and molecular approaches, and the resultant secretion of serotonin and expression of tryptophan hydroxylase 1 (TPH1), the rate-limiting enzyme in serotonin synthesis, was assessed.

RESULTS

Inhibition of PTEN in vitro, with concomitant increased Akt signaling, resulted in decreased secretion of serotonin, as well as decreased serotonin synthesis, as confirmed by reduced expression of TPH1. Inhibition of PTEN in BON cells in an animal model resulted in decreased serum serotonin.

CONCLUSION

By inhibiting signaling through Akt, PTEN indirectly promotes serotonin synthesis and secretion.

The Phosphatidylinositol 3-kinase/Akt Signaling Pathway in Neuroendocrine Tumors

The phosphatidylinositol 3-kinase (PI3K)-Akt pathway is often aberrantly activated in neuroendocrine-derived cancers. Therefore, selectively targeting this pathway using small-molecule inhibitors may reduce neuroendocrine tumor burden, potentiate adjunct therapies, and achieve symptomatic control for patients with hormonally active and inoperable disease. Here, we discuss the role of the PI3K-Akt pathway in the malignant transformation of neuroendocrine tumors, specifically carcinoids and small cell lung cancers. The collective findings presented in this review propose that selective targeting of the PI3K-Akt pathway may mitigate neuroendocrine tumor progression, thus offering a viable therapeutic approach for managing systemic disease.

Molecular pathogenesis of pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (PNETs) are rare primary neoplasms of the pancreas and arise sporadically or in the context of genetically determined syndromes. Depending on hormone production and sensing, PNETs clinically manifest due to a hormone-related syndrome (functional PNET) or by symptoms related to tumor bulk effects (non-functional PNET). So far, radical surgical excision is the only therapy to cure the disease. Development of tailored non-surgical approaches has been impeded by the lack of experimental laboratory models and there is, therefore, a limited understanding of the complex cellular and molecular biology of this heterogeneous group of neoplasm. This review aims to summarize current knowledge of tumorigenesis of familial and sporadic PNETs on a cellular and molecular level. Open questions in the field of PNET research are discussed with specific emphasis on the relevance of disease management.