Structural and dynamic changes in P-Rex1 upon activation by PIP3 and inhibition by IP4.

PIP3-dependent Rac exchanger 1 (P-Rex1) is abundantly expressed in neutrophils and plays central roles in chemotaxis and cancer metastasis by serving as a guanine-nucleotide exchange factor (GEF) for Rac. The enzyme is synergistically activated by PIP3 and heterotrimeric Gßγ subunits, but mechanistic details remain poorly understood. While investigating the regulation of P-Rex1 by PIP3, we discovered that Ins(1,3,4,5)P4 (IP4) inhibits P-Rex1 activity and induces large decreases in backbone dynamics in diverse regions of the protein. Cryo-electron microscopy analysis of the P-Rex1·IP4 complex revealed a conformation wherein the pleckstrin homology (PH) domain occludes the active site of the Dbl homology (DH) domain. This configuration is stabilized by interactions between the first DEP domain (DEP1) and the DH domain and between the PH domain and a 4-helix bundle (4HB) subdomain that extends from the C-terminal domain of P-Rex1. Disruption of the DH-DEP1 interface in a DH/PH-DEP1 fragment enhanced activity and led to a more extended conformation in solution, whereas mutations that constrain the occluded conformation led to decreased GEF activity. Variants of full-length P-Rex1 in which the DH-DEP1 and PH-4HB interfaces were disturbed exhibited enhanced activity during chemokine-induced cell migration, confirming that the observed structure represents the autoinhibited state in living cells. Interactions with PIP3-containing liposomes led to disruption of these interfaces and increased dynamics protein-wide. Our results further suggest that inositol phosphates such as IP4 help to inhibit basal P-Rex1 activity in neutrophils, similar to their inhibitory effects on phosphatidylinositol-3-kinase.

Angiopoietin-2-induced lymphatic endothelial cell migration drives lymphangiogenesis via the ß1 integrin-RhoA-formin axis.

Lymphangiogenesis is an essential physiological process but also a determining factor in vascular-related pathological conditions. Angiopoietin-2 (Ang2) plays an important role in lymphatic vascular development and function and its upregulation has been reported in several vascular-related diseases, including cancer. Given the established role of the small GTPase RhoA on cytoskeleton-dependent endothelial functions, we investigated the relationship between RhoA and Ang2-induced cellular activities. This study shows that Ang2-driven human dermal lymphatic endothelial cell migration depends on RhoA. We demonstrate that Ang2-induced migration is independent of the Tie receptors, but dependent on ß1 integrin-mediated RhoA activation with knockdown, pharmacological approaches, and protein sequencing experiments. Although the key proteins downstream of RhoA, Rho kinase (ROCK) and myosin light chain, were activated, blockade of ROCK did not abrogate the Ang2-driven migratory effect. However, formins, an alternative target of RhoA, were identified as key players, and especially FHOD1. The Ang2-RhoA relationship was explored in vivo, where lymphatic endothelial RhoA deficiency blocked Ang2-induced lymphangiogenesis, highlighting RhoA as an important target for anti-lymphangiogenic treatments.

SET protein accumulates in HNSCC and contributes to cell survival: antioxidant defense, Akt phosphorylation and AVOs acidification.

OBJECTIVES: Determination of the SET protein levels in head and neck squamous cell carcinoma (HNSCC) tissue samples and the SET role in cell survival and response to oxidative stress in HNSCC cell lineages. MATERIALS AND METHODS: SET protein was analyzed in 372 HNSCC tissue samples by immunohistochemistry using tissue microarray and HNSCC cell lineages. Oxidative stress was induced with the pro-oxidant tert-butylhydroperoxide (50 and 250µM) in the HNSCC HN13 cell lineage either with (siSET) or without (siNC) SET knockdown. Cell viability was evaluated by trypan blue exclusion and annexin V/propidium iodide assays. It was assessed caspase-3 and -9, PARP-1, DNA fragmentation, NM23-H1, SET, Akt and phosphorylated Akt (p-Akt) status. Acidic vesicular organelles (AVOs) were assessed by the acridine orange assay. Glutathione levels and transcripts of antioxidant genes were assayed by fluorometry and real time PCR, respectively. RESULTS: SET levels were up-regulated in 97% tumor tissue samples and in HNSCC cell lineages. SiSET in HN13 cells (i) promoted cell death but did not induced caspases, PARP-1 cleavage or DNA fragmentation, and (ii) decreased resistance to death induced by oxidative stress, indicating SET involvement through caspase-independent mechanism. The red fluorescence induced by siSET in HN13 cells in the acridine orange assay suggests SET-dependent prevention of AVOs acidification. NM23-H1 protein was restricted to the cytoplasm of siSET/siNC HN13 cells under oxidative stress, in association with decrease of cleaved SET levels. In the presence of oxidative stress, siNC HN13 cells showed lower GSH antioxidant defense (GSH/GSSG ratio) but higher expression of the antioxidant genes PRDX6, SOD2 and TXN compared to siSET HN13 cells. Still under oxidative stress, p-Akt levels were increased in siNC HN13 cells but not in siSET HN13, indicating its involvement in HN13 cell survival. Similar results for the main SET effects were observed in HN12 and CAL 27 cell lineages, except that HN13 cells were more resistant to death. CONCLUSION: SET is potential (i) marker for HNSCC associated with cancer cell resistance and (ii) new target in cancer therapy.

Over-expression of MAGED4B increases cell migration and growth in oral squamous cell carcinoma and is associated with poor disease outcome.

MAGE proteins have been shown to be good targets for cancer immunotherapy. We demonstrate that MAGED4B is over-expressed in more than 50% of Oral Squamous Cell Carcinoma (OSCC) tissues and the expression of MAGED4B is associated with lymph node metastasis and poor disease specific survival. OSCC cell lines that over-express MAGED4B promote migration in vitro, exhibit an increase in cell growth both in vitro and in vivo, and are more resistant to apoptosis compared to control cells. Our data suggest that MAGED4B over-expression is a driver in oral carcinogenesis and argues strongly that this protein may represent a potential therapeutic target in OSCC.

c-Abl activates p38 MAPK independently of its tyrosine kinase activity: Implications in cisplatin-based therapy.

Activation of p38 MAPK is a critical requisite for the therapeutics activity of the antitumor agent cisplatin. In this sense, a growing body of evidences supports the role of c-Abl as a major determinant of p38 MAPK activation, especially in response to genotoxic stress when triggered by cisplatin. Here, we demonstrate that p38 MAPK activation in response to cisplatin does not require the tyrosine kinase activity of c-Abl. Indeed, c-Abl can activate the p38 MAPK signaling pathway by a mechanism that is independent of its tyrosine kinase activity, but that instead involves the ability of c-Abl to increase the stability of MKK6. Similar results were obtained in chronic myeloid leukemia-derived cell lines, in which a chimeric Bcr/Abl protein mimics the effects of c-Abl overexpression on p38 MAPK activation. These findings may explain why a clinically used c-Abl inhibitor, imatinib mesylate, fails to inhibit the p38 MAPK pathway alone or in combination with cisplatin, and provide evidence of a novel signaling mechanism in which these antitumor agents act.

Molecular analysis of anoikis resistance in oral cavity squamous cell carcinoma.

Oral cavity squamous cell carcinoma (OSCC) is one of the leading causes of cancer deaths worldwide and most of these deaths result from local-regional recurrence and metastases. Evasion of apoptosis is an important hallmark of cancer development and progression, and previous studies have shown that evasion of anoikis, or detachment-induced apoptosis, correlates with a more aggressive phenotype of carcinoma cells in OSCC. To elucidate the cytogenetic and molecular characteristics of anoikis resistance, we generated several cell lines and clones that displayed this cellular phenotype. To test the hypothesis that chromosomal alterations may underlie this phenotypic transformation, we used karyotype analysis to observe changes in the chromosomal structure of anoikis-sensitive and anoikis-resistant cell lines. We further hypothesized that a unique pattern of gene expression was induced by cell-detachment of anoikis-resistant cell lines, and cDNA microarray analysis was performed using a panel of anoikis-resistant oral cancer cell lines grown under attached and detached growth conditions. We identified S100P, KLK6 and CTNNAL1 as genes whose expression levels were differentially regulated in the anoikis-resistant cell lines compared to the anoikis-sensitive cells under detached conditions. These results were verified using real-time RT-PCR. The anoikis-resistant phenotype of squamous cell carcinoma has a distinct genetic expression pattern that is marked by chromosomal alterations that may contribute to differential expression of genes involved in diverse cellular functions. Therapies targeting these potential mediators of anoikis resistance may prove to be beneficial in the treatment of metastatic squamous cell carcinoma.