Preclinical Evidence That Trametinib Enhances the Response to Antiangiogenic Tyrosine Kinase Inhibitors in Renal Cell Carcinoma

Sunitinib and pazopanib are antiangiogenic tyrosine kinase inhibitors (TKI) used to treat metastatic renal cell carcinoma (RCC). However, the ability of these drugs to extend progression-free and overall survival in this patient population is limited by drug resistance. It is possible that treatment outcomes in RCC patients could be improved by rationally combining TKIs with other agents. Here, we address whether inhibition of the Ras-Raf-MEK-ERK1/2 pathway is a rational means to improve the response to TKIs in RCC. Using a xenograft model of RCC, we found that tumors that are resistant to sunitinib have a significantly increased angiogenic response compared with tumors that are sensitive to sunitinib in vivo. We also observed significantly increased levels of phosphorylated ERK1/2 in the vasculature of resistant tumors, when compared with sensitive tumors. These data suggested that the Ras-Raf-MEK-ERK1/2 pathway, an important driver of angiogenesis in endothelial cells, remains active in the vasculature of TKI-resistant tumors. Using an in vitro angiogenesis assay, we identified that the MEK inhibitor (MEKI) trametinib has potent antiangiogenic activity. We then show that, when trametinib is combined with a TKI in vivo, more effective suppression of tumor growth and tumor angiogenesis is achieved than when either drug is utilized alone. In conclusion, we provide preclinical evidence that combining a TKI, such as sunitinib or pazopanib, with a MEKI, such as trametinib, is a rational and efficacious treatment regimen for RCC.

Essential role for endocytosis in the growth factor-stimulated activation of ERK1/2 in endothelial cells

Vascular endothelial growth factor (VEGF) stimulates angiogenesis by binding to VEGF receptor 2 (VEGFR2) on endothelial cells (ECs). Downstream activation of the extracellular related kinases 1/2 (ERK1/2) is important for angiogenesis to proceed. Receptor internalization has been implicated in VEGFR2 signaling, but its role in the activation of ERK1/2 is unclear. To explore this question we utilized pitstop and dynasore, two small molecule inhibitors of endocytosis. First, we confirmed that both inhibitors block the internalization of VEGFR2 in ECs. We then stimulated ECs with VEGF in the presence and absence of the inhibitors and examined VEGFR2 signaling to ERK1/2. Activation of VEGFR2 and C-Raf still occurred in the presence of the inhibitors, whereas the activation of MEK1/2 and ERK1/2 was abrogated. Therefore, although internalization is not required for activation of either VEGFR2 or C-Raf in ECs stimulated with VEGF, internalization is necessary to activate the more distal kinases in the cascade. Importantly, inhibition of internalization also prevented activation of ERK1/2 when ECs were stimulated with other pro-angiogenic growth factors, namely fibroblast growth factor 2 and hepatocyte growth factor. In contrast, the same inhibitors did not block ERK1/2 activation in fibroblasts or cancer cells stimulated with growth factors. Finally, we show that these small molecule inhibitors of endocytosis block angiogenesis in vitro and in vivo. Therefore, receptor internalization may be a generic requirement for pro-angiogenic growth factors to activate ERK1/2 signaling in human ECs, and targeting receptor trafficking may present a therapeutic opportunity to block tumor angiogenesis.

Lysyl oxidase plays a critical role in endothelial cell stimulation to drive tumor angiogenesis

Identification of key molecules that drive angiogenesis is critical for the development of new modalities for the prevention of solid tumor progression. Using multiple models of colorectal cancer, we show that activity of the extracellular matrix-modifying enzyme lysyl oxidase (LOX) is essential for stimulating endothelial cells in vitro and angiogenesis in vivo. We show that LOX activates Akt through platelet-derived growth factor receptor β (PDGFRβ) stimulation, resulting in increased VEGF expression. LOX-driven angiogenesis can be abrogated through targeting LOX directly or using inhibitors of PDGFRβ, Akt, and VEGF signaling. Furthermore, we show that LOX is clinically correlated with VEGF expression and blood vessel formation in 515 colorectal cancer patient samples. Finally, we validate our findings in a breast cancer model, showing the universality of these observations. Taken together, our findings have broad clinical and therapeutic implications for a wide variety of solid tumor types.

Lysyl oxidase plays a critical role in endothelial cell stimulation to drive tumor angiogenesis

Identification of key molecules that drive angiogenesis is critical for the development of new modalities for the prevention of solid tumor progression. Using multiple models of colorectal cancer, we show that activity of the extracellular matrix-modifying enzyme lysyl oxidase (LOX) is essential for stimulating endothelial cells in vitro and angiogenesis in vivo. We show that LOX activates Akt through platelet-derived growth factor receptor β (PDGFRβ) stimulation, resulting in increased VEGF expression. LOX-driven angiogenesis can be abrogated through targeting LOX directly or using inhibitors of PDGFRβ, Akt, and VEGF signaling. Furthermore, we show that LOX is clinically correlated with VEGF expression and blood vessel formation in 515 colorectal cancer patient samples. Finally, we validate our findings in a breast cancer model, showing the universality of these observations. Taken together, our findings have broad clinical and therapeutic implications for a wide variety of solid tumor types.

FGFR signaling promotes the growth of triple-negative and basal-like breast cancer cell lines both in vitro and in vivo

PURPOSE

The oncogenic drivers of triple-negative (TN) and basal-like breast cancers are largely unknown. Substantial evidence now links aberrant signaling by the fibroblast growth factor receptors (FGFR) to the development of multiple cancer types. Here, we examined the role of FGFR signaling in TN breast cancer.

EXPERIMENTAL DESIGN

We examined the sensitivity of a panel of 31 breast cancer cell lines to the selective FGFR inhibitor PD173074 and investigated the potential mechanisms underlying sensitivity.

RESULTS

TN breast cancer cell lines were more sensitive to PD173074 than comparator cell lines (P = 0.011), with 47% (7/15) of TN cell lines showing significantly reduced growth. The majority of TN cell lines showed only modest sensitivity to FGFR inhibition in two-dimensional growth but were highly sensitive in anchorage-independent conditions. PD173074 inhibited downstream mitogen-activated protein kinase and PI3K-AKT signaling and induced cell-cycle arrest and apoptosis. Basal-like breast cancer cell lines were found to express FGF2 ligand (11/21 positive) and, similarly, 62% of basal-like breast cancers expressed FGF2, as assessed by immunohistochemistry compared with 5% of nonbasal breast cancers (P < 0.0001). RNA interference targeting of FGF2 in basal-like cell lines significantly reduced growth in vitro and reduced down stream signaling, suggesting an autocrine FGF2 signaling loop. Treatment with PD173074 significantly reduced the growth of CAL51 basal-like breast cancer cell line xenografts in vivo.

CONCLUSIONS

Basal-like breast cancer cell lines, and breast cancers, express autocrine FGF2 and show sensitivity to FGFR inhibitors, identifying a potential novel therapeutic approach for these cancers.

Whole-mount assays for gene induction and barrier formation in the developing epidermis

The skin as a surface organ is uniquely accessible for whole embryo/foetal analyses of developmental changes, such as gene induction, protein expression, formation of epidermal-derived appendages such as hair follicles and formation of the protective barrier. Such analyses have emphasised the heterogeneous nature of skin development, perhaps not surprisingly because epidermal development is programmed by heterogeneous underlying mesenchyme. It is necessary to account for this heterogeneity by precisely matching body sites when correlating sequential events during development, for example, the activation of gene expression, or comparing wild-type with mutant/knockout animals. In this chapter protocols designed to assay whole-mount in situ hybridisation and whole-mount barrier formation are presented. Formation of the protective barrier is the endpoint of epidermal terminal differentiation and defects in this process are reflected in failure, acceleration, or delay in barrier formation. Hence, these latter assays are of particular value as a rapid initial assay for epidermal developmental defects in genetically modified mice.

Akt-dependent Pp2a activity is required for epidermal barrier formation during late embryonic development

Acquisition of epidermal barrier function occurs late in mouse gestation. Several days before birth a wave of barrier acquisition sweeps across murine fetal skin, converging on dorsal and ventral midlines. We investigated the molecular pathways active during epidermal barrier formation. Akt signaling increased as the barrier wave crossed epidermis and Jun was transiently dephosphorylated. Inhibitor experiments on embryonic explants showed that the dephosphorylation of Jun was dependent on both Akt and protein phosphatase 2A (Pp2a). Inhibition of Pp2a and Akt signaling also caused defects in epidermal barrier formation. These data are compatible with a model for developmental barrier acquisition mediated by Pp2a regulation of Jun dephosphorylation, downstream of Akt signaling. Support for this model was provided by siRNA-mediated knockdown of Ppp2r2a (Pr55alpha or B55alpha), a regulatory subunit of Pp2a expressed in an Akt-dependent manner in epidermis during barrier formation. Ppp2r2a reduction caused significant increase in Jun phosphorylation and interfered with the acquisition of barrier function, with barrier acquisition being restored by inhibition of Jun phosphorylation. Our data provide strong evidence that Ppp2r2a is a regulatory subunit of Pp2a that targets this phosphatase to Jun, and that Pp2a action is necessary for barrier formation. We therefore describe a novel Akt-dependent Pp2a activity that acts at least partly through Jun to affect initial barrier formation during late embryonic epidermal development.

Stimulation of tumor growth and angiogenesis by low concentrations of RGD-mimetic integrin inhibitors

Inhibitors of alpha(v)beta(3) and alpha(v)beta(5) integrin have entered clinical trials as antiangiogenic agents for cancer treatment but generally have been unsuccessful. Here we present in vivo evidence that low (nanomolar) concentrations of RGD-mimetic alpha(v)beta(3) and alpha(v)beta(5) inhibitors can paradoxically stimulate tumor growth and tumor angiogenesis. We show that low concentrations of these inhibitors promote VEGF-mediated angiogenesis by altering alpha(v)beta(3) integrin and vascular endothelial growth factor receptor-2 trafficking, thereby promoting endothelial cell migration to VEGF. The proangiogenic effects of low concentrations of RGD-mimetic integrin inhibitors could compromise their efficacy as anticancer agents and have major implications for the use of RGD-mimetic compounds in humans.

AKT-dependent HspB1 (Hsp27) activity in epidermal differentiation

AKT activity has been reported in the epidermis associated with keratinocyte survival and differentiation. We show in developing skin that Akt activity associates first with post-proliferative, para-basal keratinocytes and later with terminally differentiated keratinocytes that are forming the fetal stratum corneum. In adult epidermis the dominant Akt activity is in these highly differentiated granular keratinocytes, involved in stratum corneum assembly. Stratum corneum is crucial for protective barrier activity, and its formation involves complex and poorly understood processes such as nuclear dissolution, keratin filament aggregation, and assembly of a multiprotein cell cornified envelope. A key protein in these processes is filaggrin. We show that one target of Akt in granular keratinocytes is HspB1 (heat shock protein 27). Loss of epidermal HspB1 caused hyperkeratinization and misprocessing of filaggrin. Akt-mediated HspB1 phosphorylation promotes a transient interaction with filaggrin and intracellular redistribution of HspB1. This is the first demonstration of a specific interaction between HspB1 and a stratum corneum protein and indicates that HspB1 has chaperone activity during stratum corneum formation. This work demonstrates a new role for Akt in epidermis.