Glioblastoma stem cells exploit the αvß8 integrin-TGFß1 signaling axis to drive tumor initiation and progression.

Glioblastoma (GBM) is a primary brain cancer that contains populations of stem-like cancer cells (GSCs) that home to specialized perivascular niches. GSC interactions with their niche influence self-renewal, differentiation and drug resistance, although the pathways underlying these events remain largely unknown. Here, we report that the integrin αvß8 and its latent transforming growth factor ß1 (TGFß1) protein ligand have central roles in promoting niche co-option and GBM initiation. αvß8 integrin is highly expressed in GSCs and is essential for self-renewal and lineage commitment in vitro. Fractionation of ß8high cells from freshly resected human GBM samples also reveals a requirement for this integrin in tumorigenesis in vivo. Whole-transcriptome sequencing reveals that αvß8 integrin regulates tumor development, in part, by driving TGFß1-induced DNA replication and mitotic checkpoint progression. Collectively, these data identify the αvß8 integrin-TGFß1 signaling axis as crucial for exploitation of the perivascular niche and identify potential therapeutic targets for inhibiting tumor growth and progression in patients with GBM.

Loss of epithelial p53 and αv integrin cooperate through Akt to induce squamous cell carcinoma yet prevent remodeling of the tumor microenvironment.

Most of the squamous cell carcinomas (SCCs) of the skin and head and neck contain p53 mutations. The presence of p53 mutations in premalignant lesions suggests that they represent early events during tumor progression and additional alterations may be required for SCC development. Here we show that codeletion of the p53 and αv integrin genes in mouse stratified epithelia induced SCCs in 100% of the mice, more frequently and with much shorter latency than deletion of either gene alone. The SCCs that lacked p53 and αv in the epithelial tumor cells exhibited high Akt activity, lacked multiple types of infiltrating immune cells, contained a defective vasculature and grew slower than tumors that expressed p53 or αv. These results reveal that loss of αv in epithelial cells that lack p53 promotes SCC development, but also prevents remodeling of the tumor microenvironment and delays tumor growth. We observed that Akt inactivation in SCC cells that lack p53 and αv promoted anoikis. Thus, tumors may arise in these mice as a result of the increased cell survival induced by Akt activation triggered by loss of αv and p53, and by the defective recruitment of immune cells to these tumors, which may allow immune evasion. However, the defective vasculature and lack of a supportive stroma create a restrictive microenvironment in these SCCs that slows their growth. These mechanisms may underlie the rapid onset and slow growth of SCCs that lack p53 and αv.

A mosaic mouse model of astrocytoma identifies alphavbeta8 integrin as a negative regulator of tumor angiogenesis.

Angiogenesis involves a complex set of cell-cell and cell-extracellular matrix (ECM) interactions that coordinately promote and inhibit blood vessel growth and sprouting. Although many factors that promote angiogenesis have been characterized, the identities and mechanisms of action of endogenous inhibitors of angiogenesis remain unclear. Furthermore, little is known about how cancer cells selectively circumvent the actions of these inhibitors to promote pathological angiogenesis, a requisite event for tumor progression. Using mosaic mouse models of the malignant brain cancer, astrocytoma, we report that tumor cells induce pathological angiogenesis by suppressing expression of the ECM protein receptor alphavbeta8 integrin. Diminished integrin expression in astrocytoma cells leads to reduced activation of latent TGFbetas, resulting in impaired TGFbeta receptor signaling in tumor-associated endothelial cells. These data reveal that astrocytoma cells manipulate their angiogenic balance by selectively suppressing alphavbeta8 integrin expression and function. Finally, these results show that an adhesion and signaling axis normally involved in developmental brain angiogenesis is pathologically exploited in adult brain tumors.

The TrkB receptor tyrosine kinase regulates cellular proliferation via signal transduction pathways involving SHC, PLCgamma, and CBL.

The TrkB protein tyrosine kinase is a high affinity receptor for brain derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4). TrkB autophosphorylation occurs on five cytoplasmic tyrosines: Y484, Y670, Y674, Y675, and Y785. Using site directed mutagenesis, we have assessed the importance of TrkB tyrosines 484 and 785 in affecting TrkB-mediated signaling events leading to NIH 3T3 cell mitogenesis and survival. Mutation of TrkB tyrosine 484, while having no affect on BDNF-inducible PLCgamma and Cbl tyrosine phosphorylation, is essential for the phosphorylation of Shc, the complete activation of extracellular regulated kinase 1/2 (ERK1/2) and the induction of c-fos protein synthesis. In contrast, mutation of Y785 does not significantly affect BDNF-inducible Shc phosphorylation, ERK1/2 activation, or c-fos protein synthesis, but completely inhibits the tyrosine phosphorylation of PLCgamma and Cbl. These data indicate that both ERK-dependent and ERK-independent signaling pathways lead to BDNF-inducible mitogenesis and survival.

Activation loop tyrosines contribute varying roles to TrkB autophosphorylation and signal transduction.

The TrkB receptor tyrosine kinase (RTK) is a high affinity receptor for the neurotrophins brain derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5). Following exposure to BDNF or NT-4/5, TrkB is autophosphorylated on five cytoplasmic tyrosines: Y484, Y670, Y674, Y675, and Y785. Based on crystallographic analyses for others RTKs, TrkB tyrosines Y670, Y674, and Y675 are expected to lie within a putative kinase activation loop. Phosphorylation of these activation loop tyrosines is postulated to be a conserved event required for complete RTK activation. Here, we have assessed the importance these activation loop tyrosines play in regulating TrkB autophosphorylation, cytoplasmic signal transduction, and cell proliferation. We show that while tyrosine 670 is dispensable for BDNF-inducible TrkB autophosphorylation and the activation of certain signal transduction events, it is required for complete TrkB-mediated cellular proliferation. Combinatorial mutagenesis of tyrosines 674 and 675 only moderately affects TrkB autophosphorylation, but significantly impairs the BDNF-inducible stimulation of cytoplasmic signaling events and cellular proliferation. The combined mutation of all three activation loop tyrosines results in an inactive receptor, which is unable to autophosphorylate, stimulate signaling events, or induce mitogenesis. The data highlight the varying degrees of importance of the three activation loop tyrosines in TrkB mediated biological responses.

The Nck SH2/SH3 adaptor protein: a regulator of multiple intracellular signal transduction events.

The process generally termed signal transduction involves the coordinated relay of information from extracellular cues to intracellular effectors, subsequently leading to a specified cellular response. The formation of multimeric protein complexes is a critical step in the activation of most intracellular signal transduction cascades. In many cases, these processes are initiated by a family of molecules consisting of protein association motifs known as src homology 2 and 3 (SH2 and SH3) domains. This review focuses on a group of proteins within this family that lack intrinsic enzymatic functions and consist almost entirely of SH2 and SH3 domains. Termed "adaptors," these proteins serve to physically bridge activated cell surface receptors to various intracellular signal transduction pathways. Here, I briefly summarize current knowledge concerning the various adaptor proteins and place a particular emphasis on Nck. Various data are discussed which collectively support a role for Nck in the regulation of multiple intracellular signaling events.

Sense and antisense transfection analysis of tau function: tau influences net microtubule assembly, neurite outgrowth and neuritic stability.

Microtubules are fundamental elements participating in many aspects of cell behavior and maintenance, yet the factors regulating microtubule behavior in vivo remain poorly understood. Employing the nerve growth factor (NGF)-responsive cell line, PC12, we have used sense and antisense DNA transfection strategies to examine the role of the microtubule-associated protein (MAP) tau in several aspects of neuronal cell behavior. Stable transfectants over-expressing tau accumulate more microtubule mass and extend neurites more rapidly than control cells, while transfectants under-expressing tau exhibit reduced microtubule levels and slower neurite outgrowth. Further, tau over-expressing cells are markedly more resistant to nocodazole-induced neuritic degeneration when compared to wild-type or tau under-expressing cells. These observations provide direct support for the model that tau is capable of influencing: (i) net microtubule assembly, (ii) the rate of neurite elongation and (iii) neuritic stability. These capabilities suggest that tau plays crucial roles in the development and maintenance of neuronal cells.