hMENA(11a) contributes to HER3-mediated resistance to PI3K inhibitors in HER2-overexpressing breast cancer cells.

Human Mena (hMENA), an actin regulatory protein of the ENA/VASP family, cooperates with ErbB receptor family signaling in breast cancer. It is overexpressed in high-risk preneoplastic lesions and in primary breast tumors where it correlates with HER2 overexpression and an activated status of AKT and MAPK. The concomitant overexpression of hMENA and HER2 in breast cancer patients is indicative of a worse prognosis. hMENA is expressed along with alternatively expressed isoforms, hMENA(11a) and hMENAΔv6 with opposite functions. A novel role for the epithelial-associated hMENA(11a) isoform in sustaining HER3 activation and pro-survival pathways in HER2-overexpressing breast cancer cells has been identified by reverse phase protein array and validated in vivo in a series of breast cancer tissues. As HER3 activation is crucial in mechanisms of cell resistance to PI3K inhibitors, we explored whether hMENA(11a) is involved in these resistance mechanisms. The specific hMENA(11a) depletion switched off the HER3-related pathway activated by PI3K inhibitors and impaired the nuclear accumulation of HER3 transcription factor FOXO3a induced by PI3K inhibitors, whereas PI3K inhibitors activated hMENA(11a) phosphorylation and affected its localization. At the functional level, we found that hMENA(11a) sustains cell proliferation and survival in response to PI3K inhibitor treatment, whereas hMENA(11a) silencing increases molecules involved in cancer cell apoptosis. As shown in three-dimensional cultures, hMENA(11a) contributes to resistance to PI3K inhibition because its depletion drastically reduced cell viability upon treatment with PI3K inhibitor BEZ235. Altogether, these results indicate that hMENA(11a) in HER2-overexpressing breast cancer cells sustains HER3/AKT axis activation and contributes to HER3-mediated resistance mechanisms to PI3K inhibitors. Thus, hMENA(11a) expression can be proposed as a marker of HER3 activation and resistance to PI3K inhibition therapies, to select patients who may benefit from these combined targeted treatments. hMENA(11a) activity could represent a new target for antiproliferative therapies in breast cancer.

Gene expression of 70 kDa heat shock protein of Candida albicans: transcriptional activation and response to heat shock.

CaHSP70 (70 kDa heat shock protein) is a highly immunogenic protein of Candida albicans. We have studied heat shock-induced expression of the CaHSP70 gene under germ tube-inductive and non-inductive conditions. The CaHSP70 upstream regulatory region was cloned and sequenced. It contains at least three heat shock elements (HSEs), specific DNA sequences that are bound by the heat shock transcription factor (HSF), and one stress response element (STRE), which is an upstream activator sequence (UAS) that causes transcription activation under stress. The binding of HSF to HSE in the CaHSP70 promoter region is constitutive, although the mobility of protein/DNA complexes is altered after heat shock. The CaHSP70 promoter was cloned into a lacZ reporter plasmid, and was able to respond to heat shock in C. albicans as well as in Saccharomyces cerevisiae.

Human Mus81-associated endonuclease cleaves Holliday junctions in vitro.

Mus81, a protein with homology to the XPF subunit of the ERCC1-XPF endonuclease, is important for replicational stress tolerance in both budding and fission yeast. Human Mus81 has associated endonuclease activity against structure-specific oligonucleotide substrates, including synthetic Holliday junctions. Mus81-associated endonuclease resolves Holliday junctions into linear duplexes by cutting across the junction exclusively on strands of like polarity. In addition, Mus81 protein abundance increases in cells following exposure to agents that block DNA replication. Taken together, these findings suggest a role for Mus81 in resolving Holliday junctions that arise when DNA replication is blocked by damage or by nucleotide depletion. Mus81 is not related by sequence to previously characterized Holliday junction resolving enzymes, and it has distinct enzymatic properties that suggest it uses a novel enzymatic strategy to cleave Holliday junctions.

Threonine 68 is required for radiation-induced phosphorylation and activation of Cds1.

In response to DNA damage, eukaryotic cells use a system of checkpoint controls to delay cell-cycle progression. Checkpoint delays provide time for repair of damaged DNA before its replication in S phase and before segregation of chromatids in M phase. The Cds1 (Chk2) tumour-suppressor protein has been implicated in certain checkpoint responses in mammalian cells. It directly phosphorylates and inactivates the mitosis-inducing phosphatase Cdc25 in vitro and is required to maintain the G2 arrest that is observed in response to gamma-irradiation. Cds1 also directly phosphorylates p53 in vitro at a site that is implicated in its stabilization, and is required for stabilization of p53 and induction of p53-dependent transcripts in vivo upon gamma-ionizing radiation. Thus, Cds1 functions in both the G1 and G2 checkpoint responses. Like Cds1, the checkpoint protein kinase ATM (ataxia-telangiectasia-mutated) is required for correct operation of both the G1 and G2 damage checkpoints. ATM is necessary for phosphorylation and activation of Cds1 in vivo and can phosphorylate Cds1 in vitro, although evidence that the sites that are phosphorylated by ATM are required for activation is lacking. Here we show that threonine 68 of Cds1 is the preferred site of phosphorylation by ATM in vitro, and is the principal irradiation-induced site of phosphorylation in vivo. The importance of this phosphorylation site is demonstrated by the failure of a mutant, non-phosphorylatable form of Cds1 to be fully activated, and by its reduced ability to induce G1 arrest in response to ionising radiation.

TGF-beta autocrine loop regulates cell growth and myogenic differentiation in human rhabdomyosarcoma cells.

Transforming growth factor beta (TGF) is a well-known inhibitor of myogenic differentiation as well as an autocrine product of rhabdomyosarcoma cells. We studied the role of the TGF-beta autocrine loop in regulating growth and myogenic differentiation in the human rhabdomyosarcoma cell line, RD. We previously reported that the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) induces growth arrest and myogenic differentiation in these cells, which constitutively express muscle regulatory factors. We show that TPA inhibits the activation of secreted latent TGF-beta, thus decreasing the concentration of active TGF-beta to which the cells are exposed. This event is mediated by the TPA-induced alteration of the uPA/PAI serine-protease system. Complete removal of TGF-beta, mediated by the ectopic expression of a soluble type II TGF-beta receptor dominant negative cDNA, induces growth arrest, but does not trigger differentiation. In contrast, a reduction in the TGF-beta concentration, to a range of 0.14-0.20 x 10(-2) ng/ml (which is similar to that measured in TPA-treated cells), mimics TPA-induced differentiation. Taken together, these data demonstrate that cell growth and suppression of differentiation in rhabdomyosarcoma cells require overproduction of active TGF-beta; furthermore, they show that a 'critical' concentration of TGF-beta is necessary for myogenic differentiation to occur, whereas myogenesis is abolished below and above this concentration. By impairing the TGF-beta autocrine loop, TPA stabilizes the factor concentration within the range compatible for differentiation to occur. In contrast, in human primary muscle cells a much higher concentration of exogenous TGF-beta is required for the differentiation inhibitory effect and TPA inhibits differentiation in these cells probably through a TGF-beta independent mechanism. These data thus clarify the mechanism underlying the multiple roles of TGF-beta in the regulation of both the transformed and differentiated phenotype.

Differential expression of the Wnt putative receptors Frizzled during mouse somitogenesis.

The expression of eight murine Frizzled (1,3-9) genes was studied during mouse somitogenesis, in order to correlate the Wnt-dependent activation of myogenesis with the expression of specific Frizzled putative receptors. Frizzled 1, 3, 6, 7, 8, and 9 have specific expression in the forming and differentiating somites. The genes analyzed have a complex and partly overlapping pattern of expression in other regions of the embryo.

Generation of a highly immunogenic recombinant enolase of the human opportunistic pathogen Candida albicans.

Enolase, a 46 kDa glycolytic enzyme, is an immunodominant antigen of Candida albicans, an important human opportunistic pathogen. The full-length coding sequence of C. albicans enolase gene was subcloned into the prokaryotic expression vector pDS56/RBSII,His6/E- under the control of an inducible promoter to produce a His6-tagged enolase. The recombinant protein was purified to homogeneity by one-step nickel-chelate affinity chromatography. It was recognized by a monoclonal antibody specific for C. albicans enolase, as well as by anti-enolase antibodies present in human sera (IgG). The recombinant protein promptly elicited antibody in mice and detected immune responses in normal human subjects that were comparable to those generated by the native C. albicans enolase. Thus this new recombinant enolase constitutes a valuable reagent for studying the possible role of this protein in anti-Candida immune response.

Inhibition of myogenesis by transforming growth factor beta is density-dependent and related to the translocation of transcription factor MEF2 to the cytoplasm.

Transforming growth factor beta (TGF-beta) was found to inhibit differentiation of myogenic cells only when they were grown to high density. Inhibition also occurred when myogenic cells were cocultured with other types of mesenchymal cells but not when they were cocultured with epithelial cells. It is therefore possible that some density-dependent signaling mediates the intracellular response to TGF-beta. Within 30 min of treatment, TGF-beta induced translocation of MEF2, but not MyoD, myogenin, or p21, to the cytoplasm of myogenic cells grown to high density. Translocation was reversible on withdrawal of TGF-beta. By using immune electron microscopy and Western blot analysis on subcellular fractions, MEF2 was shown to be tightly associated with cytoskeleton membrane components. To test whether MEF2 export from the nucleus was causally related to the inhibitory action of TGF-beta, we transfected C2C12 myoblasts with MEF2C containing the nuclear localization signal of simian virus 40 large T antigen (nlsSV40). Myogenic cells expressing the chimerical MEF2C/nlsSV40, but not wild-type MEF2C, retained this transcription factor in the nucleus and were resistant to the inhibitory action of TGF-beta. We propose a mechanism in which the inhibition of myogenesis by TGF-beta is mediated through MEF2 localization to the cytoplasm, thus preventing it from participating in an active transcriptional complex.

Negative regulation of beta enolase gene transcription in embryonic muscle is dependent upon a zinc finger factor that binds to the G-rich box within the muscle-specific enhancer.

We have previously identified a muscle-specific enhancer within the first intron of the human beta enolase gene. Present in this enhancer are an A/T-rich box that binds MEF-2 protein(s) and a G-rich box (AGTGGGGGAGGGGGCTGCG) that interacts with ubiquitously expressed factors. Both elements are required for tissue-specific expression of the gene in skeletal muscle cells. Here, we report the identification and characterization of a Kruppel-like zinc finger protein, termed beta enolase repressor factor 1, that binds in a sequence-specific manner to the G-rich box and functions as a repressor of the beta enolase gene transcription in transient transfection assays. Using fusion polypeptides of beta enolase repressor factor 1 and the yeast GAL4 DNA-binding domain, we have identified an amino-terminal region responsible for the transcriptional repression activity, whereas a carboxyl-terminal region was shown to contain a potential transcriptional activation domain. The expression of this protein decreases in developing skeletal muscles, correlating with lack of binding activity in nuclear extract from adult skeletal tissue, in which novel binding activities have been detected. These results suggest that in addition to the identified factor, which functionally acts as a negative regulator and is enriched in embryonic muscle, the G-rich box binds other factors, presumably exerting a positive control on transcription. The interplay between factors that repress or activate transcription may constitute a developmentally regulated mechanism that modulates beta enolase gene expression in skeletal muscle.

A temperature-conditional mutant of simian virus 40 large T antigen requires serum to inhibit myogenesis and does not induce DNA synthesis in myotubes.

The temperature-conditional mutant tsA58 of SV40 large T antigen (Tag) increases the proliferation rate and the number of cell divisions in primary murine and human myogenic cells when expressed under permissive conditions (i.e., at 33 degrees C in medium containing high levels of serum). Under these conditions, Tag also prevents terminal differentiation. Under nonpermissive conditions (i.e., at 39 degrees C in medium containing low levels of serum) in which Tag is largely inactive, proliferation is arrested, and differentiation occurs. However, even at a permissive temperature, the removal of serum induced myosin expression and the fusion of myogenic cells, which continued to express functional Tag. Although Tag was complexed with pRb, as expected from a functional protein, proliferation was nevertheless arrested, and differentiation was induced. Consistent with these findings, the exposure of Tag-expressing differentiated myotubes to serum at 33 degrees C did not reinduce DNA synthesis in these cells. Thus, in myogenic cells, temperature-conditional mutants of Tag stimulate proliferation in the presence of serum but neither prevent terminal differentiation in the absence of serum nor induce DNA synthesis once complete withdrawal from the cycle has occurred.

Absence of MEF2 binding to the A/T-rich element in the muscle creatine kinase (MCK) enhancer correlates with lack of early expression of the MCK gene in embryonic mammalian muscle.

During skeletal muscle development, different types of muscle fibers are generated, which express different combinations of muscle-specific gene products. For example, the muscle creatine kinase gene (MCK) is highly expressed in fetal but not embryonic myotubes. We performed transient transfections of CAT reporter constructs, driven by the MCK promoter with variable lengths of 5'-flanking sequence, into primary cultures of embryonic and fetal muscle cells. Reporter activity was observed in fetal but not embryonic muscle cells. We assayed the ability of nuclear extracts prepared from embryonic and fetal muscle and C2C12 myotubes to bind specific regulatory elements in the MCK enhancer. The profile of DNA/protein complexes resulting from electrophoretic mobility shift assays was qualitatively the same with all extracts used when the oligonucleotide probes represented the MCK-E-box, MHox site, CArG-box, and AP2 site. In contrast, no binding activity to the MEF2 site was observed with embryonic nuclear extract. Interestingly, MEF2 mRNAs and proteins were detected in both fetal and embryonic muscle, with the exception of the MEF2D1b isoform, which is restricted to fetal muscle. Furthermore, we found that protein phosphatase inhibitors included in the preparation of embryonic nuclear extracts or added to the medium of transfected embryonic myotubes can restore MEF2 DNA binding activity, as well as reporter activity driven by the MCK promoter and partial transcriptional activation of the endogenous MCK gene. We propose that phosphorylation of MEF2 regulates its activity and represents an important aspect of the mechanism controlling stage-specific transcription during skeletal myogenesis.