Targeting the IGF-1R using picropodophyllin in the therapeutical 5T2MM mouse model of multiple myeloma: beneficial effects on tumor growth, angiogenesis, bone disease and survival

A phase-1 trial of linsitinib (OSI-906) in combination with bortezomib and dexamethasone for the treatment of relapsed/refractory multiple myeloma

We report results of a phase-1 study evaluating the safety and anti-cancer activity of the small molecule insulin-like growth factor-1 receptor (IGF-1R) inhibitor, linsitinib combined with bortezomib, and dexamethasone in relapsed/refractory multiple myeloma. Nineteen patients were enrolled across four dose-escalation cohorts (75-150 mg bid). The maximum tolerated dose of linsitinib was 125 mg. The most frequent Grade 3/4 AEs occurring in ≥10% of patients were thrombocytopenia (53%), bone pain (26%), neutropenia (21%), diarrhea (14%), anemia (14%), rash (10%), and lung infection (10%). Study discontinuation due to treatment-related AEs was low (16%). Across all cohorts the ORR was 61% (95% CI: 28.9-75.6%). Three partial response or greater and one stable disease were observed in proteasome inhibitor (PI) refractory patients (n = 5). Median PFS was 7.1 months (95% CI: 3.6-NA). Linsitinib plus bortezomib and dexamethasone demonstrate a manageable safety profile while the clinical benefit particularly in PI refractory patients warrants further exploration.

Targeting the tumor immune microenvironment with "nutraceuticals": From bench to clinical trials

The occurrence of immune effector cells in the tissue microenvironment during neoplastic progression is critical in determining tumor growth outcomes. On the other hand, tumors may also avoid immune system-mediated elimination by recruiting immunosuppressive leukocytes and soluble factors, which coordinate a tumor microenvironment that counteracts the efficiency of the antitumor immune response. Checkpoint inhibitor therapy results have indicated a way forward via activation of the immune system against cancer. Widespread evidence has shown that different compounds in foods, when administered as purified substances, can act as immunomodulators in humans and animals. Although there is no universally accepted definition of nutraceuticals, the term identifies a wide category of natural compounds that may impact health and disease statuses and includes purified substances from natural sources, plant extracts, dietary supplements, vitamins, phytonutrients, and various products with combinations of functional ingredients. In this review, we summarize the current knowledge on the immunomodulatory effects of nutraceuticals with a special focus on the cancer microenvironment, highlighting the conceptual benefits or drawbacks and subtle cell-specific effects of nutraceuticals for envisioning future therapies employing nutraceuticals as chemoadjuvants.

Identifying chemopreventive agents for obesity-associated cancers using an efficient, 3D high-throughput transformation assay

Obesity is associated with ~40% of cancer diagnoses but there are currently no effective preventive strategies, illustrating a need for chemoprevention. We previously demonstrated that fibroblast growth factor 2 (FGF2) from adipose tissue stimulates malignant transformation, as measured by growth in soft agar, the gold-standard in vitro transformation assay. Because the soft agar assay is unsuitable for high throughput screens (HTS), we developed a novel method using 3D growth in ultra-low attachment conditions as an alternative to growth in agar to discover compounds that inhibit transformation. Treating non-tumorigenic, skin epithelial JB6 P⁺ cells with FGF2 stimulates growth in ultra-low attachment conditions analogous to growth in the soft agar. This transformation HTS identified picropodophyllin, an insulin growth factor 1 receptor (IGF1R) inhibitor, and fluvastatin, an HMG-CoA reductase inhibitor, as potential chemopreventive agents. These compounds were validated for efficacy using two non-tumorigenic cell lines in soft agar. Another IGF1R inhibitor and other statins were also tested and several were able to inhibit growth in soft agar. This novel 3D HTS platform is fast, robust and has the potential to identify agents for obesity-associated cancer prevention.

Microenvironment drug resistance in multiple myeloma: emerging new players

Multiple myeloma (MM) drug resistance (DR) is a multistep transformation process based on a powerful interplay between bone marrow stromal cells and MM cells that allows the latter to escape anti-myeloma therapies. Here we present an overview of the role of the bone marrow microenvironment in both soluble factors-mediated drug resistance (SFM-DR) and cell adhesion-mediated drug resistance (CAM-DR), focusing on the role of new players, namely miRNAs, exosomes and cancer-associated fibroblasts.

The insulin-like growth factor system in multiple myeloma: diagnostic and therapeutic potential

Multiple myeloma (MM) is a highly heterogeneous plasma cell malignancy. The MM cells reside in the bone marrow (BM), where reciprocal interactions with the BM niche foster MM cell survival, proliferation, and drug resistance. As in most cancers, the insulin-like growth factor (IGF) system has been demonstrated to play a key role in the pathogenesis of MM. The IGF system consists of IGF ligands, IGF receptors, IGF binding proteins (IGFBPs), and IGFBP proteases and contributes not only to the survival, proliferation, and homing of MM cells, but also MM-associated angiogenesis and osteolysis. Furthermore, increased IGF-I receptor (IGF-IR) expression on MM cells correlates with a poor prognosis in MM patients. Despite the prominent role of the IGF system in MM, strategies targeting the IGF-IR using blocking antibodies or small molecule inhibitors have failed to translate into the clinic. However, increasing preclinical evidence indicates that IGF-I is also involved in the development of drug resistance against current standard-of-care agents against MM, including proteasome inhibitors, immunomodulatory agents, and corticoids. IGF-IR targeting has been able to overcome or revert this drug resistance in animal models, enhancing the efficacy of standard-of-care agents. This finding has generated renewed interest in the therapeutic potential of IGF-I targeting in MM. The present review provides an update of the impact of the different IGF system components in MM and discusses the diagnostic and therapeutic potentials.

Molecular mechanisms, current management and next generation therapy in myeloma bone disease

Multiple myeloma (MM) bone disease is a major cause of morbidity and mortality in MM patients and persists even in patients in remission. This bone disease is caused by an uncoupling of bone remodeling, with increased osteoclast and decreased osteoblast activity and formation, culminating in lytic bone destruction. Bisphosphonates are the current standard of care but new therapies are needed. As the molecular mechanisms controlling MM bone disease are increasingly well understood, new therapeutic targets are extensively explored in the preclinical setting and initial clinical trials with novel compounds now show promising results. In this review, we will provide a comprehensive overview of the biology of MM bone disease, summarize its current clinical management and discuss preclinical and clinical data on next generation therapies.

In vivo treatment with epigenetic modulating agents induces transcriptional alterations associated with prognosis and immunomodulation in multiple myeloma

Histone deacetylase inhibitors (HDACi) and DNA methyltransferase inhibitors (DNMTi) are in early clinical development for multiple myeloma (MM) therapy. Despite all encouraging pre-clinical data, clinical activity of HDACi and DNMTi is mostly lacking. To optimize the trials, characterization of the in vivo response towards HDACi and DNMTi will be crucial. Therefore, we investigated the transcriptional response after in vivo treatment with the HDACi quisinostat or DNMTi decitabine using the murine 5T33MM model. We identified 504 and 154 genes deregulated by quisinostat and decitabine, respectively. Of interest, MM patients' gene expression levels of 62 quisinostat- and 25 decitabine-deregulated genes were predictive for overall survival of patients. This prognostic information was implemented in a DNA methylation and histone acetylation score. A high score was related to a high proliferative and immature phenotype of MM cells. Furthermore, highly scored MM patients had an adverse overall survival. Interestingly, bio-informatic prediction tools revealed an association of quisinostat-deregulated genes with lymphocyte activation, proliferation, immune-effector mechanisms and T-helper-1 development. Overall, treatment of 5T33MM mice with epigenetic modulating agents led to the translation of gene signatures to predict overall survival of MM patients. HDACi mainly deregulated tumoral immunomodulatory pathways, supporting the rationale to combine HDACi with immunomodulatory therapies.

Picropodophyllin inhibits the growth of Ewing's sarcoma cells through the insulin‑like growth factor‑1 receptor/Akt signaling pathway

Ewing's sarcoma (ES) is the second most common type of pediatric bone tumor, and is associated with a poor prognosis. Picropodophyllin (PPP), a novel selective inhibitor of insulin‑like growth factor‑1 receptor (IGF‑1R), is able to strongly inhibit various types of cancers. However, the effect of IGF‑1R on ES remains unclear. Following treatment with various concentrations of PPP for various times, cell viability was determined using an MTT assay. In addition, cell proliferation and apoptosis was investigated separately by bromodeoxyuridine staining and flow cytometry, respectively. The PPP‑associated signaling pathway was also investigated. The results of the present study suggested that PPP inhibited cell proliferation and viability of A673 and SK‑ES‑1 human Ewing's sarcoma cells in a dose- and time‑dependent manner. In addition, cell apoptosis rates were increased following treatment with PPP. Further investigation of the underlying mechanism revealed that PPP inhibited Akt phosphorylation. Fumonisin B1, an Akt‑specific activator, reversed the inhibitory effects of PPP on cell growth. Furthermore, the results suggested that PPP decreased the expression levels of IGF‑1R, a common activator of Akt signaling. PPP inhibited the growth of human Ewing's sarcoma cells by targeting the IGF‑1R/Akt signaling pathway. Therefore, PPP may prove useful in the development of an effective strategy for the treatment of Ewing's sarcoma.

A review of current murine models of multiple myeloma used to assess the efficacy of therapeutic agents on tumour growth and bone disease

Pre-clinical in vivo models of multiple myeloma are essential tools for investigating the pathophysiology of multiple myeloma and for testing new therapeutic agents and strategies prior to their potential use in clinical trials. Over the last five decades, several different types of murine models of multiple myeloma have been developed ranging from immunocompetent syngeneic models, e.g. the 5 T series of myeloma cells, to immunocompromised models including the SCID xenograft models, which use human myeloma cell lines or patient-derived cells. Other models include hybrid models featuring the implantation of SCID mice with bone chips (SCID-hu or SCID-rab) or 3-D bone scaffolds (SCID-synth-hu), and mice that have been genetically engineered to develop myeloma. Bearing in mind the differences in these models, it is not surprising that they reflect to varying degrees different aspects of myeloma. Here we review the past and present murine models of myeloma, with particular emphasis on their advantages and limitations, characteristics, and their use in testing therapeutic agents to treat myeloma tumour burden and bone disease.

Picropodophyllin causes mitotic arrest and catastrophe by depolymerizing microtubules via insulin-like growth factor-1 receptor-independent mechanism

Picropodophyllin (PPP) is an anticancer drug undergoing clinical development in NSCLC. PPP has been shown to suppress IGF-1R signaling and to induce a G2/M cell cycle phase arrest but the exact mechanisms remain to be elucidated. The present study identified an IGF-1-independent mechanism of PPP leading to pro-metaphase arrest. The mitotic block was induced in human cancer cell lines and in an A549 xenograft mouse but did not occur in normal hepatocytes/mouse tissues. Cell cycle arrest by PPP occurred in vitro and in vivo accompanied by prominent CDK1 activation, and was IGF-1R-independent since it occurred also in IGF-1R-depleted and null cells. The tumor cells were not arrested in G2/M but in mitosis. Centrosome separation was prevented during mitotic entry, resulting in a monopolar mitotic spindle with subsequent prometaphase-arrest, independent of Plk1/Aurora A or Eg5, and leading to cell features of mitotic catastrophe. PPP also increased soluble tubulin and decreased spindle-associated tubulin within minutes, indicating that it interfered with microtubule dynamics. These results provide a novel IGF-1R-independent mechanism of antitumor effects of PPP.

Insulin-like growth factor-1 receptor is associated with better prognosis in classical Hodgkin's lymphoma: Correlation with MET expression

The purpose of this study was to examine the prognostic significance of insulin-like growth factor-1 receptor (IGF-1R) expression alone and in relation to the expression of the MET- receptor and the MET-homologous receptor RON, in classical Hodgkin's lymphoma (cHL). Tumour samples from patients with cHL (n = 202; median age 37.5 years) were analysed retrospectively for IGF-R1, MET or RON expression by immunohistochemistry using tissue microarrays. The median follow-up time was 3.7 years (range, 0.1-20 years). Twenty-nine patients (14.3%) expressed IGF-1R protein in Hodgkin/Reed-Sternberg (HRS) cells, which was associated with a better overall survival (OS) (P = 0.036). IGF-1R expression was closely associated with MET receptor expression and low level of lactate dehydrogenase. In patients with cHL receiving doxorubicin, bleomycin, vinblastine and dacarbazine, those expressing IGF-1R showed a trend towards better OS and event-free survival than IGF-1R-negative patients (P = 0.129 and P = 0.115 respectively), but statistical significance was not reached. This study suggests that IGF-1R expression could be associated with better clinical outcome in cHL but is significantly associated with the expression of MET receptor.

Insulin-like growth factor system in cancer: novel targeted therapies

Insulin-like growth factors (IGFs) are essential for growth and survival that suppress apoptosis and promote cell cycle progression, angiogenesis, and metastatic activities in various cancers. The IGFs actions are mediated through the IGF-1 receptor that is involved in cell transformation induced by tumour. These effects depend on the bioavailability of IGFs, which is regulated by IGF binding proteins (IGFBPs). We describe here the role of the IGF system in cancer, proposing new strategies targeting this system. We have attempted to expand the general viewpoint on IGF-1R, its inhibitors, potential limitations of IGF-1R, antibodies and tyrosine kinase inhibitors, and IGFBP actions. This review discusses the emerging view that blocking IGF via IGFBP is a better option than blocking IGF receptors. This can lead to the development of novel cancer therapies.

The IGF-1 receptor inhibitor picropodophyllin potentiates the anti-myeloma activity of a BH3-mimetic

The ABT-analogous 737, 263 and 199 are BH3 mimetics showing potent anti-myeloma (MM) activity, but only on defined molecular subgroups of MM patients presenting a Bcl-2high/Mcl-1low profile. IGF-1 is a major survival factor in MM regulating the expression of Bcl-2 proteins and might therefore be a resistance factor to these ABT-analogous. We first show that IGF-1 protected human MM cell lines (HMCLs) against ABT-737. Concurrently, the IGF-1 receptor inhibitor picropodophyllin (PPP) synergistically sensitized HMCL, primary human MM and murine 5T33MM cells to ABT-737 and ABT-199 by further decreasing cell viability and enhancing apoptosis. Knockdown of Bcl-2 by shRNA protected MM cells to ABT-737, while Mcl-1 shRNA sensitized the cells. PPP overcame the Bcl-2 dependency of ABT-737, but failed to completely overcome the protective effect of Mcl-1. In vivo, co-treatment of 5T33MM bearing mice significantly decreased tumor burden and prolonged overall survival both in a prophylactic and therapeutic setting. Interestingly, proteasome inhibitor resistant CD138- 5T33MM cells were more sensitive to ABT-737, whereas PPP alone targeted the CD138+ cells more effectively. After co-treatment, both subpopulations were targeted equally. Together, the combination of an IGF-1R inhibitor and an ABT-analogue displays synergistic anti-myeloma activity providing the rational for further (pre)clinical testing.

The role of FAK in tumor metabolism and therapy

Focal adhesion kinase (FAK) plays a vital role in tumor cell proliferation, survival and migration. Altered metabolic pathways fuel rapid tumor growth by accelerating glucose, lipid and glutamine processing. Besides the mitogenic effects of FAK, evidence is accumulating supporting the association between hyper-activated FAK and aberrant metabolism in tumorigenesis. FAK can promote glucose consumption, lipogenesis, and glutamine dependency to promote cancer cell proliferation, motility, and survival. Clinical studies demonstrate that FAK-related alterations of tumor metabolism are associated with increased risk of developing solid tumors. Since FAK contributes to the malignant phenotype, small molecule inhibition of FAK-stimulated bioenergetic and biosynthetic processes can provide a novel approach for therapeutic intervention in tumor growth and invasion.

Something old, something new and something borrowed: emerging paradigm of insulin-like growth factor type 1 receptor (IGF-1R) signaling regulation

The insulin-like growth factor type 1 receptor (IGF-1R) plays a key role in the development and progression of cancer; however, therapeutics targeting it have had disappointing results in the clinic. As a receptor tyrosine kinase (RTK), IGF-1R is traditionally described as an ON/OFF system, with ligand stabilizing the ON state and exclusive kinase-dependent signaling activation. Newly added to the traditional model, ubiquitin-mediated receptor downregulation and degradation was originally described as a response to ligand/receptor interaction and thus inseparable from kinase signaling activation. Yet, the classical model has proven over-simplified and insufficient to explain experimental evidence accumulated over the last decade, including kinase-independent signaling, unbalanced signaling, or dissociation between signaling and receptor downregulation. Based on the recent findings that IGF-1R “borrows” components of G-protein coupled receptor (GPCR) signaling, including β-arrestins and G-protein-related kinases, we discuss the emerging paradigm for the IGF-1R as a functional RTK/GPCR hybrid, which integrates the kinase signaling with the IGF-1R canonical GPCR characteristics. The contradictions to the classical IGF-1R signaling concept as well as the design of anti-IGF-1R therapeutics treatment are considered in the light of this paradigm shift and we advocate recognition of IGF-1R as a valid target for cancer treatment.

The association of TP53 mutations with the resistance of colorectal carcinoma to the insulin-like growth factor-1 receptor inhibitor picropodophyllin

BACKGROUND

There is growing evidence indicating the insulin-like growth factor 1 receptor (IGF-1R) plays a critical role in the progression of human colorectal carcinomas. IGF-1R is an attractive drug target for the treatment of colon cancer. Picropodophyllin (PPP), of the cyclolignan family, has recently been identified as an IGF-1R inhibitor. The aim of this study is to determine the therapeutic response and mechanism after colorectal carcinoma treatment with PPP.

METHODS

Seven colorectal carcinoma cell lines were treated with PPP. Following treatment, cells were analyzed for growth by a cell viability assay, sub-G1 apoptosis by flow cytometry, caspase cleavage and activation of AKT and extracellular signal-regulated kinase (ERK) by western blot analysis. To examine the in vivo therapeutic efficacy of PPP, mice implanted with human colorectal carcinoma xenografts underwent PPP treatment.

RESULTS

PPP treatment blocked the phosphorylation of IGF-1R, AKT and ERK and inhibited the growth of TP53 wild-type but not mutated colorectal carcinoma cell lines. The treatment of PPP also induced apoptosis in TP53 wild-type cells as evident by the presence of sub-G1 cells and the cleavage of caspase-9, caspase-3, DNA fragmentation factor-45 (DFF45), poly (ADP-ribose) polymerase (PARP), and X-linked inhibitor of apoptosis protein (XIAP). The loss of BAD phosphorylation in the PPP-treated TP53 wild type cells further suggested that the treatment induced apoptosis through the BAD-mediated mitochondrial pathway. In contrast, PPP treatment failed to induce the phosphorylation of AKT and ERK and caspase cleavage in TP53 mutated colorectal carcinoma cell lines. Finally, PPP treatment suppressed the growth of xenografts derived from TP53 wild type but not mutated colorectal carcinoma cells.

CONCLUSIONS

We report the association of TP53 mutations with the resistance of treatment of colorectal carcinoma cells in culture and in a xenograft mouse model with the IGF-1R inhibitor PPP. TP53 mutations often occur in colorectal carcinomas and could be used as a biomarker to predict the resistance of colorectal carcinomas to the treatment by this IGF-1R inhibitor.

Loss of androgen receptor promotes adipogenesis but suppresses osteogenesis in bone marrow stromal cells

Gender differences have been described in osteoporosis with females having a higher risk of osteoporosis than males. The differentiation of bone marrow stromal cells (BMSCs) into bone or fat is a critical step for osteoporosis. Here we demonstrated that loss of the androgen receptor (AR) in BMSCs suppressed osteogenesis but promoted adipogenesis. The mechanism dissection studies revealed that AR deficiency suppressed osteogenesis-related genes to inhibit osteoblast differentiation from BMSCs. Knockout of AR promoted adipogenesis of BMSCs via Akt activation through IGFBP3-mediated IGF signaling, and the 5' promoter assay and chromatin immunoprecipitation assays further proved that AR could modulate IGFBP3 expression at the transcriptional level. Finally, addition of IGF inhibitors successfully masked the AR deficiency-induced Akt activation, and inhibitions of Akt, IGF1, and IGF2 pathways reversed the AR depletion effects on the adipogenesis process. These results suggested that AR-mediated changes in IGFBP3 might modulate the IGF-Akt axis to regulate adipogenesis in BMSCs.

The role of the insulin-like growth factor-I receptor in malignancy: an update

In the past three decades, evidence has been accumulating that the IGF-I receptor/ligand system plays an important role in malignant disease. This has led to a search for specific inhibitors of the IGF receptor for cancer therapy, revealing some predictable, but also unexpected challenges. Here we review recent data that highlight the essential role of the IGF axis in several important steps in cancer cell progression and metastasis and highlight cellular processes that have been the focus of much interest and new insight in recent years. Strategies used to target the IGF axis clinically are summarized and the obstacles encountered are discussed.

Insulin-like growth factor binding protein-3 suppresses vascular endothelial growth factor expression and tumor angiogenesis in head and neck squamous cell carcinoma

Angiogenesis, the process by which new blood vessels are recruited to existing ones, is essential for tumor development. Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3), which modulates bioavailability of IGF, has been studied for its potential role in angiogenesis during tissue regeneration and cancer development. In this study, we assessed the role of IGFBP-3 in tumor angiogenesis in head and neck squamous cell carcinoma (HNSCC) and human umbilical vein endothelial cells (HUVECs) using adenoviral (Ad-BP3) and recombinant (rBP3) IGFBP-3. Using an in vivo orthotopic tongue tumor model, we confirmed that both Ad-BP3 and rBP3 suppress the growth of UMSCC38 HNSCC cells in vivo. Ad-BP3 inhibited vascularization in tongue tumors and chorio-allantoic membrane, and suppressed angiogenesis-stimulating activities in UMSCC38 cells. In HUVECs, Ad-BP3 decreased migration, invasion, and tube formation. rBP3 also suppressed production of vascular endothelial growth factor (VEGF) in HUVECs and UMSCC38 cells. IGFBP-3-GGG, a mutant IGFBP-3 with loss of IGF binding capacity, suppressed VEGF production. In addition, we found that IGFBP-3 suppressed VEGF expression, even in mouse embryonic fibroblasts from an IGF-1R-null mouse. Finally, we demonstrated that IGFBP-3-GGG inhibits tumor angiogenesis and growth to the same degree as wild-type IGFBP-3. Taken together, these results support the hypothesis that IGFBP-3 has anti-angiogenic activity in HNSCC, at least in part due to IGF-independent suppression of VEGF production from vascular endothelial cells and cancer cells.

Targeting the IGF-1R signaling and mechanisms for epigenetic gene silencing in human multiple myeloma

Multiple myeloma (MM) is a B cell malignancy characterized by the expansion of clonal plasmablast/plasma cells within the bone-marrow. It is well established that the bone-marrow microenvironment has a pivotal role in providing critical cytokines and cell-cell interactions to support the growth and survival of the MM tumor clone. The pathogenesis of MM is, however, only fragmentarily understood. Detailed genomic analysis reveals a heterogeneous and complex pattern of structural and numerical chromosomal aberrations. In this review we will discuss some of the recent results on the functional role and potential clinical use of the IGF-1R, one of the major mediators of growth and survival for MM. We will also describe some of our results on epigenetic gene silencing in MM, as it may indeed constitute a novel basis for the understanding of tumor initiation and maintenance in MM and thus may change the current view on treatment strategies for MM.

Effects of epidermal growth factor receptor and insulin-like growth factor 1 receptor inhibition on proliferation and intracellular signaling in cutaneous SCCHN: potential for dual inhibition as a therapeutic modality

BACKGROUND

Combined inhibition of epidermal growth factor receptor (EGFR) and insulin-like growth factor-1 receptor (IGF-1R) has been proposed as a therapy for cutaneous squamous cell carcinoma of the head and neck (SCCHN).

METHODS

Receptor expression and downstream signaling were assessed in cutaneous squamous cell carcinoma (SCC) cell lines and patient samples. EGFR and IGF-1R signaling was inhibited in cutaneous SCC cell lines using erlotinib and/or picropodophyllin.

RESULTS

EGFR and IGF-1R were overexpressed in cutaneous SCCHN specimens relative to normal skin. Dual inhibition of both receptors prevented cell growth and decreased activation of Akt and p42/44 mitogen-activated protein kinase (MAPK) more effectively than either inhibitor alone.

CONCLUSION

Dual inhibition of EGFR and IGF-1R is effective at blocking cell growth, and is correlated with inhibition of Akt and p42/44 MAPK, suggesting that this may be a promising treatment for cutaneous SCCHN.

Receptor tyrosine kinases and targeted cancer therapeutics

The majority of growth factor receptors are composed of extracellular, transmembrane, and cytoplasmic tyrosine kinase (TK) domains. Receptor tyrosine kinase (RTK) activation regulates many key processes including cell growth and survival. However, dysregulation of RTK has been found in a wide range of cancers, and it has been shown to correlate with the development and progression of numerous cancers. Therefore, RTK has become an attractive therapeutic target. One way to effectively block signaling from RTK is inhibition of its catalytic activity with small-molecule inhibitors. Low-molecular-weight TK inhibitors (TKIs), such as imatinib, targeting tumors with mutant c-Kit, and gefitinib, targeting non-small cell lung cancer with mutant epidermal growth factor receptor (EGFR), have received marketing approval in Japan. MET, fibroblast growth factor receptor (FGFR), and insulin-like growth factor-I receptor (IGF-IR) are frequently genetically altered in advanced cancers. TKIs of these receptors have not yet appeared on the market, but many anticancer drug candidates are currently undergoing clinical trials. Most of these TKIs were designed to compete with ATP at the ATP-binding site within the TK domain. This review will focus on small-molecule TKIs targeting MET, FGFR, and IGF-IR and discuss the merits and demerits of two types of agents, i.e., those with only one or a few targets and those directed at multiple targets. Targeting agents specifically inhibiting the target kinase were previously searched for based on the hypothesis that a narrow target window might reduce unexpected side effects, but agents with multiple targets have been recently developed to overcome tumors resistant against a single-targeting agent.

Multiple antitumor effects of picropodophyllin in colon carcinoma cell lines: clinical implications

Although colorectal cancer can be successfully treated by conventional strategies such as chemo/radiotherapy and surgery, a substantial number of cases, in particular those with liver metastases, remain incurable. Therefore, novel treatment approaches are warranted. The IGF-1R and its ligands, mainly IGF-1 and IGF-2, have been suggested to play pivotal roles in proliferation, survival and migration of adenocarcinoma cells of the colon/rectum. Therefore, interference with IGF-1R-mediated signaling may represent a therapeutic option for this malignancy. In this study, semi-quantitative RT-PCR analyses of 48 paired, colorectal cancer patient samples showed significant overexpression of tumor IGF-1R and IGF-2 mRNA. There was also an overexpression of MMP-7, which was significantly correlated with histopathological parameters. Based on these findings, the effect of the IGF-1R-inhibitory cyclolignan picropodophyllin (PPP) was assessed in the four colon carcinoma cell lines HT-29, HCT-116, DLD-1 and CaCO-2. PPP strongly and dose-dependently inhibited proliferation and migration in all cell lines. However, when exposed to 0.5 μM PPP, only HT-29 showed a net decrease of viable cells as compared with the cell number at the beginning of the experiment, a finding that coincided with decreased expression/phosphorylation of IGF-1R, AKT and ERK. This cell line also exhibited PPP-induced downregulation of MMP-7 and MMP-9. Similar to the DLD-1 and HCT-116 cell lines, HT-29 also showed substantial cell detachment in response to PPP. Although a net reduction of cells by PPP seems to require a synchronized downregulation of IGF-1R, AKT and ERK1/2, part of the antitumor effect may be explained by other, possibly IGF-1R-unrelated mechanism(s). Such a multitude of inhibitory effects of PPP in colon cancer cells together with its low toxicity in vivo makes it a promising drug candidate in the treatment of this disease.

Molecular target characterization and antimyeloma activity of the novel, insulin-like growth factor 1 receptor inhibitor, GTx-134

PURPOSE

Therapeutic strategies that target insulin-like growth factor 1 receptor (IGF-1R) hold promise in a wide variety of cancers including multiple myeloma (MM). In this study, we describe GTx-134, a novel small-molecule inhibitor of IGF-1R and insulin receptor (IR) and characterized its antitumor activity in preclinical models of MM.

EXPERIMENTAL DESIGN

The activity of GTx-134 as a single agent and in combination was tested in MM cell lines and primary patient samples. Downstream effector proteins and correlation with apoptosis was evaluated. Cytotoxcity in bone marrow stroma coculture experiments was assessed. Finally, the in vivo efficacy was evaluated in a human myeloma xenograft model.

RESULTS

GTx-134 inhibited the growth of 10 of 14 myeloma cell lines (<5 μmol/L) and induced apoptosis. Sensitivity to GTx-134 correlated with IGF-1R signal inhibition. Expression of MDR-1 and CD45 were associated with resistance to GTx-134. Coculture with insulin-growth factor-1 (IGF-1) or adherence to bone marrow stroma conferred modest resistance, but did not overcome GTx-134-induced cytotoxicity. GTx-134 showed in vitro synergies when combined with dexamethasone or lenalidomide. Further, GTx-134 enhanced the activity of PD173074, a fibroblast growth factor receptor 3 (FGFR3) inhibitor, against t(4;14) myeloma cells. Therapeutic efficacy of GTx-134 was shown against primary cells and xenograft tumors. Although dysregulation of glucose homeostasis was observed in GTx-134-treated mice, impairment of glucose tolerance was modest.

CONCLUSIONS

These studies support the potential therapeutic efficacy of GTx-134 in MM. Further, they provide a rationale for clinical application in combination with established antimyeloma treatments and novel targeted therapies.

Molecular characterization of acquired tolerance of tumor cells to picropodophyllin (PPP)

Background

Picropodophyllin (PPP) is a promising novel anti-neoplastic agent that efficiently kills tumor cells in vitro and causes tumor regression and increased survival in vivo. We have previously reported that PPP treatment induced moderate tolerance in two out of 10 cell lines only, and here report the acquired genomic and expression alterations associated with PPP selection over 1.5 years of treatment.

Methodology/Principal Findings

Copy number alterations monitored using metaphase and array-based comparative genomic hybridization analyses revealed largely overlapping alterations in parental and maximally tolerant cells. Gain/ amplification of the MYC and PVT1 loci in 8q24.21 were verified on the chromosome level. Abnormalities observed in connection to PPP treatment included regular gains and losses, as well as homozygous losses in 10q24.1-q24.2 and 12p12.3-p13.2 in one of the lines and amplification at 5q11.2 in the other. Abnormalities observed in both tolerant derivatives include amplification/gain of 5q11.2, gain of 11q12.1-q14.3 and gain of 13q33.3-qter. Using Nexus software analysis we combined the array-CGH data with data from gene expression profilings and identified genes that were altered in both inputs. A subset of genes identified as downregulated (ALDH1A3, ANXA1, TLR4 and RAB5A) or upregulated (COX6A1, NFIX, ME1, MAPK and TAP2) were validated by siRNA in the tolerant or parental cells to alter sensitivity to PPP and confirmed to alter sensitivity to PPP in further cell lines.

Conclusions

Long-term PPP selection lead to altered gene expression in PPP tolerant cells with increase as well as decrease of genes involved in cell death such as PTEN and BCL2. In addition, acquired genomic copy number alterations were observed that were often reflected by altered mRNA expression levels for genes in the same regions.

Utility of flow cytometry immunophenotyping in multiple myeloma and other clonal plasma cell-related disorders

In recent years, multiparameter flow cytometry (MFC) immunophenotyping has become mandatory in the clinical management of hematological malignancies, both for diagnostic and monitoring purposes. Multiple myeloma (MM) and other clonal plasma cell-related (PC) disorders should be no exception to this paradigm, but incorporation of immunophenotypic studies in the management of patients with PC disorders is still far from being routinely established in many diagnostic flow cytometry laboratories. For clonal PC disorders, MFC is of clear and established clinical relevance in: (1) the differential diagnosis between MM and other PC-related disorders; (2) the identification of high-risk MGUS and smoldering MM; (3) minimal residual disease investigation after therapy; additionally it may also be useful for (4) the definition of prognosis-associated antigenic profiles; and (5) the identification of new therapeutic targets. In this article, we review the clinical value of MFC in the study of PC disorders, with specific emphasis in those areas where consensus exists on the need to incorporate MFC into routine evaluation of MM and other clonal PC-related disorders.

Inhibition of the insulin-like growth factor I receptor by epigallocatechin gallate blocks proliferation and induces the death of Ewing tumor cells

The insulin-like growth factor I receptor (IGFIR) has emerged as a key therapeutic target in many human malignancies, including childhood cancers such as Ewing family tumors (EFT). In this study, we show that IGFIR is constitutively activated in EFTs and that the major catechin derivative found in green tea, (-)-epigallocatechin gallate (EGCG), can inhibit cell proliferation and survival of EFT cells through the inhibition of IGFIR activity. Treatment of EFT cell lines with EGCG blocked the autophosphorylation of IGFIR tyrosine residues and inhibited its downstream pathways including phosphoinositide 3-kinase-Akt, Ras-Erk, and Jak-Stat cascades. EGCG treatment was associated with dose- and time-dependent inhibition of cellular proliferation, viability, and anchorage-independent growth, as well as with the induction of cell cycle arrest and apoptosis. Apoptosis in EFT cells by EGCG correlated with altered expression of Bcl-2 family proteins, including increased expression of proapoptotic Bax and decreased expression of prosurvival Bcl2, Bcl-XL, and Mcl-1 proteins. Our results provide further evidence that IGFIR is an attractive therapeutic target in EFTs and that EGCG is an effective inhibitor of this receptor tyrosine kinase. EGCG may be a useful agent for targeting IGFIR, either alone or in combination, with other potentially more toxic IGFIR inhibitors for the management of EFTs.

IGF-IR and its inhibitors in gastrointestinal carcinomas (Review)

The type I insulin-like growth factor receptor (IGF-IR) and its associated signaling system play a significant role in tumorigenesis, tumor survival and progression, and cancer therapeutic resistance, and thus has provoked great interest as a promising target for cancer treatment. In this report we present the role of IGF-IR in gastrointestinal carcinomas whose pathology has been identified as tightly correlated with an abnormal expression and activation of IGF-IR. Reported data from experimental studies suggest the feasibility of targeted IGF-IR therapy in gastrointestinal carcinomas. Many types of inhibitors against IGF-IR have been developed. Inhibitors with anti-IGF-IR monoclonal antibodies and tyrosine kinase inhibitors currently undergoing preclinical and clinical evolution are also reviewed.

Targeting the insulin-like growth factor-1 receptor by picropodophyllin as a treatment option for glioblastoma

Glioblastoma (GB) is the most common malignant brain tumor in adults. It has limited treatment opportunities and is almost exclusively fatal. Owing to the central role the insulin-like growth factor-1 receptor (IGF-1R) plays in malignant cells, it has been suggested as a target for anticancer therapy including GB. The cyclolignan picropodophyllin (PPP) inhibits IGF-1R without affecting the highly homologous insulin receptor. Here, we show that PPP inhibits growth of human GB cell lines along with reduced phosphorylation of IGF-1R and AKT. In vivo, PPP-treatment causes dramatic tumor regression not only in subcutaneous xenografts but also in intracerebral xenografts, indicating passage of PPP across the blood-brain barrier.

Insulin-like growth factor-I receptor tyrosine kinase inhibitor cyclolignan picropodophyllin inhibits proliferation and induces apoptosis in multidrug resistant osteosarcoma cell lines

Insulin-like growth factor-I receptor (IGF-IR) is an important mediator of tumor cell survival and shows prognostic significance in sarcoma. To explore potential therapeutic strategies for interrupting signaling through this pathway, we assessed the ability of cyclolignan picropodophyllin (PPP), a member of the cyclolignan family, to selectively inhibit the receptor tyrosine kinase activity of IGF-IR in several sarcoma cell line model systems. Of the diverse sarcoma subtypes studied, osteosarcoma cell lines were found to be particularly sensitive to IGF-IR inhibition, including several multidrug resistant osteosarcoma cell lines with documented resistance to various conventional anticancer drugs. PPP shows relatively little toxicity in human osteoblast cell lines when compared with osteosarcoma cell lines. These studies show that PPP significantly inhibits IGF-IR expression and activation in both chemotherapy-sensitive and chemotherapy-resistant osteosarcoma cell lines. This inhibition of the IGF-IR pathway correlates with suppression of proliferation of osteosarcoma cell lines and with apoptosis induction as measured by monitoring of poly(ADP-ribose) polymerase and its cleavage product and by quantitative measurement of apoptosis-associated CK18Asp396. Importantly, PPP increases the cytotoxic effects of doxorubicin in doxorubicin-resistant osteosarcoma cell lines U-2OS(MR) and KHOS(MR). Furthermore, small interfering RNA down-regulation of IGF-IR expression in drug-resistant cell lines also caused resensitization to doxorubicin. Our data suggest that inhibition of IGF-IR with PPP offers a novel and selective therapeutic strategy for ostosarcoma, and at the same time, PPP is effective at reversing the drug-resistant phenotype in osteosarcoma cell lines.

The role of the insulin-like growth factor 1 receptor axis in multiple myeloma

Multiple myeloma remains a fatal B cell malignancy with severe clinical features such as anaemia and bone fractures, caused by the predominant localization of the myeloma cells in the bone marrow (BM). The MM cells first migrate towards the BM, followed by their clonal expansion and induction of angiogenesis and osteolysis. Insulin-like growth factor 1 or IGF-1 is a cytokine which plays a role in myeloma development. Besides serving as a growth and survival factor, it attracts the cells towards the BM, and is involved in the angiogenesis process. This makes the IGF-1R an interesting target for therapeutical interventions. Apart from mediating aspects of the malignant phenotype, it also appears not to be an absolute requirement for normal cell homeostasis. Various strategies targeting the IGF-1R have emerged with the two main strategies being blocking antibodies and small molecule inhibitors. After encouraging preclinical results both strategies are now in clinical trials.

Igf1r as a therapeutic target in a mouse model of basal-like breast cancer

Considering the strong association between dysregulated insulin-like growth factor (IGF) signaling and various human cancers, we have used an expedient combination of genetic analysis and pharmacological treatment to evaluate the potential of the type 1 IGF receptor (Igf1r) for targeted anticancer therapy in a mouse model of mammary tumorigenesis. In this particular strain of genetically modified animals, histopathologically heterogeneous invasive carcinomas exhibiting up-regulation of the Igf1r gene developed extremely rapidly by mammary gland-specific overexpression of constitutively active oncogenic Kras* (mutant Kras(G12D)). Immunophenotyping data and expression profiling analyses showed that, except for a minor luminal component, these mouse tumors resembled basal-like human breast cancers. This is a group of aggressive tumors of poor prognosis for which there is no targeted therapy currently available, and it includes a subtype correlating with KRAS locus amplification. Conditional ablation of Igf1r in the mouse mammary epithelium increased the latency of Kras*-induced tumors very significantly (approximately 11-fold in comparison with the intact model), whereas treatment of tumor-bearing animals by administration of picropodophyllin (PPP), a specific Igf1r inhibitor, resulted in a dramatic decrease in tumor mass of the main forms of basal-like carcinomas. PPP also was effective against xenografts of the human basal-like cancer cell line MDA-MB-231, which carries a KRAS(G13D) mutation.

Targeted therapies in multiple myeloma

Increasing knowledge of the biology of multiple myeloma led the way for the development of novel drugs that have changed the management of the disease. New treatments target not only to the malignant plasma cell but also target the interactions of myeloma cells with their microenvironment. Several preclinical studies have identified potential targets and drugs are developed that act on pathways crucial for myeloma cell survival, proliferation, migration and drug resistance. The identification of active agents in the laboratory is followed by rationally designed clinical studies that validate these drugs, either as single agents or in combinations with other active drugs. These novel agents may be either small molecules or monoclonal antibodies targeting receptors, kinase activity of receptors or key molecules within critical pathways, intracellular maintenance mechanisms and immune modulation.

Oral picropodophyllin (PPP) is well tolerated in vivo and inhibits IGF-1R expression and growth of uveal melanoma

PURPOSE

The cyclolignan picropodophyllin (PPP) efficiently blocks the activity of insulin-like growth factor-1 receptor (IGF-1R) and inhibits growth of uveal melanoma cells in vitro and in vivo. In this study, we aimed to investigate the efficiency of orally administered PPP on growth of uveal melanoma xenografts. Further, we focused on the effect of PPP on vascular endothelial growth factor (VEGF) in vivo and evaluated its effects in combination with other established anti-tumor agents in vitro.

METHODS

Four different uveal melanoma cell lines (OCM-1, OCM-3, OCM-8, 92-1) were treated with PPP alone and in combination with imatinib mesylate, cisplatin, 5-FU and doxorubicin. Cell viability was determined by XTT assay. SCID mice xenografted with uveal melanoma cells were used to determine anti-tumor efficacy of oral PPP in vivo. Tumor samples obtained from the in vivo experiments were analyzed for VEGF and IGF-1R expression by western blotting.

RESULTS

PPP was found to be superior to the other anti-tumor agents in killing uveal melanoma cells. Oral PPP inhibited uveal melanoma growth in vivo and was well tolerated by the animals. PPP decreased VEGF expression in the tumors.

CONCLUSIONS

Oral PPP is well tolerated in vivo and caused total growth inhibition of uveal melanoma xenografts as well as it decreased the levels of VEGF in the tumors.

Inhibition of VEGF secretion and experimental choroidal neovascularization by picropodophyllin (PPP), an inhibitor of the insulin-like growth factor-1 receptor

INTRODUCTION

Choroidal neovascularization (CNV) is a debilitating complication of age-related macular degeneration (AMD) and a leading cause of vision loss. Along with other angiogenic factors like vascular endothelial growth factor (VEGF), insulin-like growth factor (IGF-1) and its receptor, IGF-1R, have been implicated in CNV.

PURPOSE

We have previously shown that the cyclolignan picropodophyllin (PPP) efficiently blocks the insulin-like growth factor-1 receptor (IGF-1R) activity and causes cell death in uveal melanoma cell lines and in an in-vivo model. In this study we investigated the effect of PPP on VEGF expression both in vitro and in vivo and whether this effect has anti-angiogenic consequences in a murine CNV model.

MATERIALS AND METHODS

C57BL/6J mice with laser-induced CNVs were treated with PPP. Effects on CNV area were assayed by image analysis. VEGF levels in choroids and retinal pigment epithelial cells (APRE-19) were measured by Western blot or ELISA. Transcriptional activation of the VEGF promoter was determined by luciferase reporter gene assay.

RESULTS

Mice treated with PPP, administered intraperitoneally or orally, showed 22-32% (p = 0.002) decrease in CNV area. Furthermore, VEGF levels in the choroids were significantly reduced. In cultured APRE-19 cells, IGF-1 was shown to increase VEGF secretion. This increase was completely blocked by PPP. We could confirm that PPP reduced the level of transcriptional activity of VEGF promoter.

CONCLUSIONS

PPP reduces IGF-1 dependent VEGF expression and CNV in vivo. Accordingly, IGF-1R inhibitors may be useful tools in the therapy of conditions associated with CNV including neovascular AMD.

Arsenic trioxide and 2-methoxyestradiol reduce beta-catenin accumulation after proteasome inhibition and enhance the sensitivity of myeloma cells to Bortezomib

Beta-catenin, the key protein in canonical Wingless/int (Wnt) pathway, degrades via ubiquitin-proteasome pathway. Recently, it proved important roles in the proliferation of myeloma cells. But little is known about whether cytoplasmic beta-catenin content is associated with myeloma cell's sensitivity to Bortezomib. We examined the constitutive expression of beta-catenin in five myeloma cell lines and primary cells from patients. Meanwhile, the effect of Bortezomib combined with arsenic trioxide (As(2)O(3))/2-methoxyestradiol (2ME2) on beta-catenin accumulation, myeloma cells' survival, apoptosis and their sensitivity to Bortezomib were also investigated. Our study proved that beta-catenin protein levels are negatively associated with myeloma cells' sensitivity to Bortezomib. As(2)O(3)/2ME2 can reduce cytoplasmic beta-catenin accumulation after proteasome inhibition and enhance myeloma cells' sensitivity to Bortezomib. This will preliminarily help to optimize the new therapeutic regimens for MM treatment in the future.

Differential localization of MT1-MMP in human prostate cancer tissue: role of IGF-1R in MT1-MMP expression

BACKGROUND

MT1-MMP is a metalloproteinase involved in prostate cancer metastasis. The IGF-1R is a tyrosine kinase receptor involved with tumor progression and metastasis. The purpose of this investigation was to examine MT1-MMP and IGF-1R expression and localization in prostate cancer tissues and explore the role of IGF-1R in regulating MT1-MMP in prostate cancer cell lines.

METHODS

Immunohistochemistry was utilized to study MT1-MMP and IGF-1R expression in human prostate tissues. IGF-1R regulation of MT1-MMP expression was determined by gene promoter analysis, quantitative RT-PCR and Western blot analysis following pharmacological inhibition of the receptor in PC-3N cells and treatment of LNCaP cells with androgen and IGF-1.

RESULTS

MT1-MMP expression was high in the apical regions of the luminal cells in PIN and prostate cancer and less intense in the basalateral regions of benign tissues. IGF-1R was expressed primarily in the basal cells of normal glands and highly expressed in prostate cancer. Inhibition of IGF-1R in PC-3N cells decreased MT1-MMP expression and treatment of LNCaP cells with a synthetic androgen and IGF-1 increased MT1-MMP expression.

CONCLUSIONS

These data demonstrate that MT1-MMP is highly expressed in the apical cytoplasmic regions of the luminal cells in PIN and prostate cancer when compared to basalateral cytoplasmic membrane staining in benign glands. Additionally, we demonstrate that IGF-1R is highly expressed in human prostate carcinoma. These findings suggest that MT1-MMP localization and IGF-1R expression in prostate carcinoma could be predictive biomarkers for aggressive disease and support IGF-1R as a promising therapeutic target to decrease processes of prostate cancer metastasis.

Insulin-like growth factor type-I receptor-dependent phosphorylation of extracellular signal-regulated kinase 1/2 but not Akt (protein kinase B) can be induced by picropodophyllin

The initial event upon binding of insulin-like growth factor 1 to the insulin-like growth factor type-I receptor (IGF-1R) is auto-phosphorylation of tyrosine residues within the activation loop of the kinase domain followed by phosphorylation of other receptor tyrosine residues and the subsequent activation of the intracellular signaling cascades. We found recently that the cyclolignan picropodophyllin (PPP) inhibits phosphorylation of IGF-1R and phosphatidyl-3 kinase/Akt (protein kinase B) signaling molecules without interfering with the highly homologous insulin receptor. Furthermore, PPP causes regression of tumor grafts and substantially prolongs the survival of animals with systemic tumor disease. It is of interest that we show here that short treatments with PPP activate the intracellular extracellular signal-regulated kinase (ERK) signaling. Our data suggest that PPP induces IGF-1R ubiquitination and in turn activates ERK1/2. The PPP-induced ERK activation requires IGF-1R because PPP is not able to induce ERK phosphorylation in IGF-1R-negative cells or in cells in which the receptor is knocked down by small interfering RNA. Moreover, in the absence of Mdm2, an E3 ligase that has been shown previously to be involved in IGF-1R ubiquitination, the phosphorylation of ERK did not occur. Thus, apart from inhibiting the receptor activity, PPP can induce IGF-1R ubiquitination and stimulate ERK in an Mdm2-dependent manner. This response could contribute to the apoptotic effect of PPP.

From the bench to the bedside: emerging new treatments in multiple myeloma

Within the last decade, several novel classes of anti-myeloma therapeutics have become available. The clinical successes achieved by thalidomide, lenalidomide, and the proteasome inhibitor bortezomib, and in particular the ability of these agents to lead to major clinical responses in patients resistant to conventional or high-dose chemotherapy, have highlighted the importance of expanding further the spectrum of classes of agents utilized for the treatment of myeloma. Herein, we review the current status for the development of novel anti-myeloma agents, with emphasis on classes of therapeutics which have already translated into clinical trials or those in advanced stages of preclinical development. These include second-generation proteasome inhibitors (NPI-0052 and PR-171), heat shock protein 90 (hsp90) inhibitors, 2-methoxyestradiol, histone deacetylase (HDAC) inhibitors (e.g. SAHA and LBH589), fibroblast growth factor receptor 3 (FGF-R3) inhibitors, insulin-like growth factor 1 receptor (IGF-1R) inhibitors, mTOR inhibitors, monoclonal antibodies, and agents specifically targeting the tumor microenvironment, such as defibrotide.

Picropodophyllin induces downregulation of the insulin-like growth factor 1 receptor: potential mechanistic involvement of Mdm2 and β-arrestin1

The insulin-like growth factor 1 receptor (IGF-1R) is crucial for growth and survival of malignant cells. Experience in targeting IGF-1R in cancer models has shown that strategies promoting downregulation of the receptor are much more efficient in inducing apoptosis than those inhibiting the IGF-1R activity. Recently, we found that the cyclolignan picropodophyllin (PPP) inhibits phosphorylation of IGF-1R and activation of downstream signaling without interfering with the highly homologous insulin receptor (IR). Furthermore, PPP treatment caused strong regression of tumor grafts and prolonged survival of animals with systemic tumor disease. Here we demonstrate that PPP also downregulates the IGF-1R, whereas the IR and several other receptors were not affected. PPP-induced IGF-1R downregulation required expression of the MDM2 E3 ligase, which recently was found to ubiquitinate and cause degradation of the IGF-1R. In addition knockdown of beta-arrestin1, the adaptor molecule known to bridges MDM2 and IGF-1R, prevented downregulation of the receptor and significantly decreased PPP-induced cell death. All together these data suggest that PPP downregulates IGF-1R by interfering with the action of beta-arrestin1/MDM2 as well as the achieved receptor downregulation contributes to the apoptotic effect of PPP.