Xiaolin Wan

Rapamycin induces feedback activation of Akt signaling through an IGF-1R-dependent mechanism

Rapamycin and several analogs, such as CCI-779 and RAD001, are currently undergoing clinical evaluation as anticancer agents. In this study, we show that inhibition of mammalian target of rapamycin (mTOR) signaling by rapamycin leads to an increase of Akt phosphorylation in Rh30 and RD human rhabdomyosarcoma cell lines and xenografts, and insulin-like growth factor (IGF)-II-treated C2C12 mouse myoblasts and IGF-II-overexpressing Chinese hamster ovary cells. RNA interference-mediated knockdown of S6K1 also results in an increase of Akt phosphorylation. These data suggest that mTOR/S6K1 inhibition either by rapamycin or small interfering RNA (siRNA) triggers a negative feedback loop, resulting in the activation of Akt signaling. We next sought to investigate the mechanism of this negative feedback regulation from mTOR to Akt. Suppression of insulin receptor substrate (IRS)-1 and tuberous sclerosis complex-1 by siRNAs failed to abrogate rapamycin-induced upregulation of Akt phosphorylation in both Rh30 and RD cells. However, pretreatment with h7C10 antibody directed against insulin-like growth factor-1 receptor (IGF-1R) led to a blockade of rapamycin-induced Akt activation. Combined mTOR and IGF-1R inhibition with rapamycin and h7C10 antibody, respectively, resulted in additive inhibition of cell growth and survival. These data suggest that rapamycin mediates Akt activation through an IGF-1R-dependent mechanism. Thus, combining an mTOR inhibitor and an IGF-1R antibody/inhibitor may be an appropriate strategy to enhance mTOR-targeted anticancer therapy.

The membrane-cytoskeleton linker ezrin is necessary for osteosarcoma metastasis.

Metastatic cancers, once established, are the primary cause of mortality associated with cancer. Previously, we used a genomic approach to identify metastasis-associated genes in cancer. From this genomic data, we selected ezrin for further study based on its role in physically and functionally connecting the actin cytoskeleton to the cell membrane. In a mouse model of osteosarcoma, a highly metastatic pediatric cancer, we found ezrin to be necessary for metastasis. By imaging metastatic cells in the lungs of mice, we showed that ezrin expression provided an early survival advantage for cancer cells that reached the lung. AKT and MAPK phosphorylation and activity were reduced when ezrin protein was suppressed. Ezrin-mediated early metastatic survival was partially dependent on activation of MAPK, but not AKT. To define the relevance of ezrin in the biology of metastasis, beyond the founding mouse model, we examined ezrin expression in dogs that naturally developed osteosarcoma. High ezrin expression in dog tumors was associated with early development of metastases. Consistent with this data, we found a significant association between high ezrin expression and poor outcome in pediatric osteosarcoma patients.

Phosphoprotein Pathway Mapping: Akt/Mammalian Target of Rapamycin Activation Is Negatively Associated with Childhood Rhabdomyosarcoma Survival

Mapping of protein signaling networks within tumors can identify new targets for therapy and provide a means to stratify patients for individualized therapy. Despite advances in combination chemotherapy, the overall survival for childhood rhabdomyosarcoma remains approximately 60%. A critical goal is to identify functionally important protein signaling defects associated with treatment failure for the 40% nonresponder cohort. Here, we show, by phosphoproteomic network analysis of microdissected tumor cells, that interlinked components of the Akt/mammalian target of rapamycin (mTOR) pathway exhibited increased levels of phosphorylation for tumors of patients with short-term survival. Specimens (n = 59) were obtained from the Childrens Oncology Group Intergroup Rhabdomyosarcoma Study (IRS) IV, D9502 and D9803, with 12-year follow-up. High phosphorylation levels were associated with poor overall and poor disease-free survival: Akt Ser(473) (overall survival P < 0.001, recurrence-free survival P < 0.0009), 4EBP1 Thr(37/46) (overall survival P < 0.0110, recurrence-free survival P < 0.0106), eIF4G Ser(1108) (overall survival P < 0.0017, recurrence-free survival P < 0.0072), and p70S6 Thr(389) (overall survival P < 0.0085, recurrence-free survival P < 0.0296). Moreover, the findings support an altered interrelationship between the insulin receptor substrate (IRS-1) and Akt/mTOR pathway proteins (P < 0.0027) for tumors from patients with poor survival. The functional significance of this pathway was tested using CCI-779 in a mouse xenograft model. CCI-779 suppressed phosphorylation of mTOR downstream proteins and greatly reduced the growth of two different rhabdomyosarcoma (RD embryonal P = 0.00008; Rh30 alveolar P = 0.0002) cell lines compared with controls. These results suggest that phosphoprotein mapping of the Akt/mTOR pathway should be studied further as a means to select patients to receive mTOR/IRS pathway inhibitors before administration of chemotherapy.

Rapamycin inhibits ezrin-mediated metastatic behavior in a murine model of osteosarcoma.

Osteosarcoma is the most frequent primary malignant tumor of bone with a high propensity for metastasis. We have previously showed that ezrin expression is necessary for metastatic behavior in a murine model of osteosarcoma (K7M2). In this study, we found that a mechanism of ezrin-related metastatic behavior is linked to an Akt-dependent mammalian target of rapamycin (mTOR)/p70 ribosomal protein S6 kinase (S6K1)/eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) pathway. Suppression of ezrin expression either by antisense transfection or by small interfering RNAs or disruption of ezrin function by transfection of a dominant-negative ezrin-T567A mutant led to decreased expression and decreased phosphorylation of both S6K1 and 4E-BP1. Proteosomal inhibition by MG132 reversed antisense-mediated decrease of S6K1 and 4E-BP1 protein expression, but failed to affect the effect of ezrin on phosphorylation of S6K1 and 4E-BP1. Blockade of the mTOR pathway with rapamycin or its analog, cell cycle inhibitor-779 led to significant inhibition of experimental lung metastasis in vivo. These results suggest that blocking the mTOR/S6K1/4E-BP1 pathway may be an appropriate target for strategies to reduce tumor cell metastasis.

Addiction to Elevated Insulin-like Growth Factor I Receptor and Initial Modulation of the AKT Pathway Define the Responsiveness of Rhabdomyosarcoma to the Targeting Antibody

Insulin-like growth factor I receptor (IGF-IR) and its ligands are overexpressed by tumors, mediating proliferation and protecting against stress-induced apoptosis. Accordingly, there has been a considerable amount of interest in developing therapeutic agents against IGF-IR. IGF-IR is believed to be ubiquitously expressed without detectable mutation or amplification in cancer. We explored the determinants of cellular response to a humanized anti-IGF-IR antibody. Our results showed a large variation in IGF-IR levels in rhabdomyosarcoma tumor specimens that were comparable with those in rhabdomyosarcoma cell lines. In vitro analysis revealed a direct and very significant correlation between elevated IGF-IR levels and antiproliferative effects of the antibody and defined a receptor number that would predict sensitivity. Our data further suggested a strong dependence on IGF-IR for AKT signaling in cells with elevated IGF-IR. The sensitivity of the high IGF-IR-expressing cells was blocked with a constitutively active AKT. The extracellular signal-regulated kinase pathway was not affected by the antibody. In vivo studies showed that anti-IGF-IR had single-agent antitumor activity; furthermore, predictions of responses based on IGF-IR levels were accurate. In vivo biomarker analysis suggested that h7C10 down-regulated both IGF-IR and p-AKT initially, concordant with antitumor activity. Subsequent progression of tumors was associated with reactivation of p-AKT despite sustained suppression of IGF-IR. These results identified the first predictive biomarker for anti-IGF-IR therapies in cancer.

Levels of PTEN protein modulate Akt phosphorylation on serine 473, but not on threonine 308, in IGF-II-overexpressing rhabdomyosarcomas cells

Constitutive activation of Akt has been found in many types of human cancer, and is believed to promote proliferation and increased cell survival thereby contributing to cancer progression. In this study, we examined Akt phosphorylation on Ser473 and Thr308 in seven IGF-II-overexpressing rhabdomyosarcomas (RMS) cells. All the RMS cell lines tested had high levels of Akt phosphorylation on Thr308, whereas three cell lines (Rh5, Rh18, and CTR) had a much lower level of Akt phosphorylation on Ser473. To determine whether the difference in Akt phosphorylation on Ser473, but not on Thr308, observed among cell lines is a cell-specific phenomenon or due to other factors, which possibly downregulate Akt phosphorylation, we examined expression of PTEN protein, which acts as a negative regulator of the PI3K/Akt signaling pathway through its ability to dephosphorylate phosphatidylinositol 3,4,5-triphosphate (PIP3). The levels of PTEN expression inversely correlate with Akt phosphorylation on Ser473, but not on Thr308. Consistent with this finding, transfection of wild-type PTEN into RMS and mouse myoblast C2C12 cells resulted in reduced Akt phosphorylation on Ser473, but not on Thr308. Our data suggest that Ser473 may be a key target residue for PTEN to modulate the effects of IGF-II on activating the PI3K/Akt pathway in RMS cells. A better understanding of the pathway in RMS will likely contribute to insights into the biology of the RMS tumorigenesis and hopefully lead to novel therapeutic options.

Beta4 integrin promotes osteosarcoma metastasis and interacts with ezrin

The development of pulmonary metastasis is the major cause of death in osteosarcoma, and its molecular basis is poorly understood. In this study, we show that β4 integrin is highly expressed in human osteosarcoma cell lines and tumor samples. Furthermore, highly metastatic MNNG-HOS cells have increased levels of β4 integrin. Suppression of β4 integrin expression by shRNA and disruption of β4 integrin function by transfection of dominant-negative β4 integrin was sufficient to revert this highly metastatic phenotype in the MNNG-HOS model without significantly affecting primary tumor growth. These findings suggest a role for β4 integrin expression in the metastatic phenotype in human osteosarcoma cells. In addition, we identified a previously uncharacterized interaction between β4 integrin and ezrin, a membrane-cytoskeletal linker protein that is implicated in the metastatic behavior of osteosarcoma. The β4 integrin–ezrin interaction appears to be critical for maintenance of β4 integrin expression. These data begin to integrate ezrin and β4 integrin expression into a model of action for the mechanism of osteosarcoma metastases.

Loss-of-function screen in rhabdomyosarcoma identifies CRKL-YES as a critical signal for tumor growth

To identify novel signaling pathways necessary for rhabdomyosarcoma (RMS) survival, we performed a loss-of-function screen using an inducible small hairpin RNA (shRNA) library in an alveolar and an embryonal RMS cell line. This screen identified CRKL expression as necessary for growth of alveolar RMS and embryonal RMS both in vitro and in vivo. We also found that CRKL was uniformly highly expressed in both RMS cell lines and tumor tissue. As CRKL is a member of the CRK adapter protein family that contains an SH2 and two SH3 domains and is involved in signal transduction from multiple tyrosine kinase receptors, we evaluated CRKL interaction with multiple tyrosine kinase receptor signaling pathways in RMS cells. While we saw no interaction of CRKL with IGFIR, MET or PI3KAKT/mTOR pathways, we determined that CRKL signaling was associated with SRC family kinase (SFK) signaling, specifically with YES kinase. Inhibition of SFK signaling with dasatinib or another SFK inhibitor, sarcatinib, suppressed RMS cell growth in vitro and in vivo. These data identify CRKL as a novel critical component of RMS growth. This study also demonstrates the use of functional screening to identify a potentially novel therapeutic target and treatment approach for these highly aggressive pediatric cancers.

Identification of FoxM1/Bub1b Signaling Pathway as a Required Component for Growth and Survival of Rhabdomyosarcoma

We identified Bub1b as an essential element for the growth and survival of rhabdomyosarcoma (RMS) cells using a bar-coded, tetracycline-inducible short hairpin RNA (shRNA) library screen. Knockdown of Bub1b resulted in suppression of tumor growth in vivo, including the regression of established tumors. The mechanism by which this occurs is via postmitotic endoreduplication checkpoint and mitotic catastrophe. Furthermore, using a chromatin immunoprecipitation assay, we found that Bub1b is a direct transcriptional target of Forkhead Box M1 (FoxM1). Suppression of FoxM1 either by shRNA or the inhibitor siomycin A resulted in reduction of Bub1b expression and inhibition of cell growth and survival. These results show the important role of the Bub1b/FoxM1 pathway in RMS and provide potential therapeutic targets.

Targeting IGF-1R in the treatment of sarcomas: past, present and future

The use of multimodality therapy has resulted in markedly improved cure rates for patients with sarcomas in the past 25 years. However, for virtually all patients with metastatic or recurrent disease, survival remains dismal. The important role that members of the insulin-like growth factor (IGF) family play in tumorigenesis has been known for decades. But it has only been in the last five years, that humanized and fully human antibodies targeting insulin-like growth factor receptor 1 (IGF-1R) have been developed. The use of these agents in clinical trials has been accompanied by several dramatic responses in patients with recurrent and refractory sarcomas. In this review, we will focus on preclinical highlights in the past, current clinic trials and discuss some exciting research opportunities to foster advances in the future.