MET and ERBB2 are coexpressed in ERBB2+ breast cancer and contribute to innate resistance.

UNLABELLED: Breast cancer displays significant intratumoral heterogeneity, which has been shown to have a substantial impact on both innate and acquired resistance to tyrosine kinase inhibitors. The heterogeneous expression of multiple receptor tyrosine kinases (RTK) in cancers supports tumor signaling robustness and plays a significant role in resistance to targeted inhibition. Recent studies have revealed interactions between the MET receptor and the ERBB receptor family in the therapeutic resistance of several cancers. In this study, the relationship between MET expression/activity and the expression/activity of the ERBB receptor family in human breast cancer was interrogated. Importantly, a significant percentage of ERBB2(+) tumors coexpressing MET and ERBB2 were observed and displayed significant heterogeneity with subpopulations of cells that are MET(-)/ERBB2(+), MET(+)/ERBB2(-), and MET(+)/ERBB2(+). In a MET(+)/ERBB2(+) breast cancer cell line, MET depletion resulted in increased ERBB2 activation, and conversely, ERBB2 depletion resulted in increased MET activation. Neither EGFR nor ERBB3 compensated for MET or ERBB2 knockdown. The loss of either MET or ERBB2 led to a decrease in PI3K/AKT signaling and increased dependency on MAPK. These data show that a subset of ERBB2(+) breast cancers express MET and contain MET(+)/ERBB2(+) subpopulations. Moreover, analysis of RTK activation during ERBB2 knockdown indicated that MET signaling is a compensatory pathway of resistance. IMPLICATIONS: ERBB2(+) breast cancers with MET(+)/ERBB2(+) subpopulations may have an innate resistance to ERBB2 inhibition and may benefit from combined MET and ERBB2 inhibition.

Mitogen-activated protein kinase kinase signaling promotes growth and vascularization of fibrosarcoma.

We hypothesized that signaling through multiple mitogen-activated protein kinase (MAPK) kinase (MKK) pathways is essential for the growth and vascularization of soft-tissue sarcomas, which are malignant tumors derived from mesenchymal tissues. We tested this using HT-1080, NCI, and Shac fibrosarcoma-derived cell lines and anthrax lethal toxin (LeTx), a bacterial toxin that inactivates MKKs. Western blots confirmed that LeTx treatment reduced the levels of phosphorylated extracellular signal-regulated kinase and p38 MAPK in vitro. Although short treatments with LeTx only modestly affected cell proliferation, sustained treatment markedly reduced cell numbers. LeTx also substantially inhibited the extracellular release of angioproliferative factors including vascular endothelial growth factor, interleukin-8, and basic fibroblast growth factor. Similar results were obtained with cell lines derived from malignant fibrous histiocytomas, leiomyosarcomas, and liposarcomas. In vivo, LeTx decreased MAPK activity and blocked fibrosarcoma growth. Growth inhibition correlated with decreased cellular proliferation and extensive necrosis, and it was accompanied by a decrease in tumor mean vessel density as well as a reduction in serum expression of angioproliferative cytokines. Vital imaging using high-resolution ultrasound enhanced with contrast microbubbles revealed that the effects of LeTx on tumor perfusion were remarkably rapid (<24 h) and resulted in a marked reduction of perfusion within the tumor but not in nontumor tissues. These results are consistent with our initial hypothesis and lead us to propose that MKK inhibition by LeTx is a broadly effective strategy for targeting neovascularization in fibrosarcomas and other similar proliferative lesions.

Anthrax lethal toxin inhibits growth of and vascular endothelial growth factor release from endothelial cells expressing the human herpes virus 8 viral G protein coupled receptor.

PURPOSE: In this study, we tested the hypothesis that inhibition of mitogen-activated protein kinase kinases (MKK) inhibits tumor growth by acting on angiogenic signaling and by extension may form the basis of an effective strategy for treatment of Kaposi's sarcoma. EXPERIMENTAL DESIGN: Murine endothelial cells expressing the human herpes virus 8 G protein-coupled receptor (vGPCR-SVEC) were treated with anthrax lethal toxin (LeTx). LeTx is a binary toxin ordinarily secreted by Bacillus anthracis and is composed of two proteins: protective antigen (the binding moiety) and lethal factor (the active moiety). Lethal factor is a protease that cleaves and inactivates MKKs. RESULTS: In vitro, treatment of vGPCR-SVEC with LeTx inhibited MKK signaling, moderately inhibited cell proliferation, and blocked the ability of these cells to form colonies in soft agar. Treatment with LeTx also blocked the ability of these cells to release several angioproliferative cytokines, notably vascular endothelial growth factor (VEGF). In contrast, inhibition of mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2 with U0126 caused a substantial inhibition of proliferation but only modestly inhibited VEGF release. In xenograft models, i.v. injection of LeTx caused reduced tumor growth characterized immunohistochemically by inhibition of MKK signaling, decreased rates of proliferation, and reduced levels of VEGF and VEGF receptor 2, with a corresponding decrease in vascular density. CONCLUSIONS: These data support a role for MKK signaling in tumor growth and vascularization and are consistent with the hypothesis that inhibition of MKK signaling by LeTx or a similar agent may be an effective strategy for the treatment of Kaposi's sarcoma as well as other vascular tumors.

Inactivation of Apc in the mouse prostate causes prostate carcinoma.

Alterations of the Wnt/beta-catenin signaling pathway are positively associated with the development and progression of human cancer, including carcinoma of the prostate. To determine the role of activated Wnt/beta-catenin signaling in mouse prostate carcinogenesis, we created a mouse prostate tumor model using probasin-Cre-mediated deletion of Apc. Prostate tumors induced by the deletion of Apc have elevated levels of beta-catenin protein and are highly proliferative. Tumor formation is fully penetrant and follows a consistent pattern of progression. Hyperplasia is observed as early as 4.5 weeks of age, and adenocarcinoma is observed by 7 months. Continued tumor growth usually necessitated sacrifice between 12 and 15 months of age. Despite the high proliferation rate, we have not observed metastasis of these tumors to the lymph nodes or other organs. Surgical castration of 6-week-old mice inhibited tumor formation, and castration of mice with more advanced tumors resulted in the partial regression of specific prostate glands. However, significant areas of carcinoma remained 2 months postcastration, suggesting that tumors induced by Apc loss of function are capable of growth under conditions of androgen depletion. We conclude that the prostate-specific deletion of Apc and the increased expression of beta-catenin associated with prostate carcinoma suggests a role for beta-catenin in prostate cancer and offers an appropriate animal model to investigate the interaction of Wnt signaling with other genetic and epigenetic signals in prostate carcinogenesis.

Decreased BMD and limb deformities in mice carrying mutations in both Lrp5 and Lrp6.

UNLABELLED: Humans and mice lacking Lrp5 have low BMD. To evaluate whether Lrp5 and Lrp6 interact genetically to control bone or skeletal development, we created mice carrying mutations in both Lrp5 and the related gene Lrp6. We found that compound mutants had dose-dependent deficits in BMD and limb formation, suggesting functional redundancy between these two genes in bone and limb development. INTRODUCTION: Lrp5 and Lrp6 are closely related members of the low density lipoprotein receptor family and are co-receptors for Wnt ligands. While Lrp5 mutations are associated with low BMD in humans and mice, the role of Lrp6 in bone formation has not been analyzed. MATERIALS AND METHODS: To address whether Lrp5 and Lrp6 play complimentary roles in bone and skeletal development, we created mice with mutations in both genes. We inspected limbs of mice from the different genotypic classes of compound mutants to identify abnormalities. DXA and muCT were used to evaluate the effect of mutations in Lrp5 and Lrp6 on BMD and microarchitecture. RESULTS: Mice heterozygous for mutations in Lrp6 and either heterozygous or homozygous for a mutation in Lrp5 (Lrp6(+/-);Lrp5(+/-) or Lrp6(+/-);Lrp5(-/-)) display limb defects with incomplete penetrance and variable expression. DXA analysis showed that BMD decreased as mice progressively were more deficient in Lrp5 and Lrp6. Lrp6(+/-);Lrp5(-/-) mice were more severely affected than Lrp6(+/+);Lrp5(-/-) mice, whereas Lrp6(+/-);Lrp5(+/-) mice had statistically higher BMD than Lrp6(+/+);Lrp5(-/-) mice and lower BMD compared with wildtype mice and mice heterozygous for either mutation alone. CONCLUSIONS: Lrp6 and Lrp5 genetically interact in limb development in mice. Furthermore, heterozygosity for an inactivating mutation in Lrp6 further reduces BMD in both male and female mice lacking Lrp5.