Development of a p38 Kinase Binding Assay for High Throughput Screening

Histone deacetylase activity selectively regulates notch-mediated smooth muscle differentiation in human vascular cells

Background

Histone deacetylases (HDACs) modify smooth muscle cell (SMC) proliferation and affect neointimal lesion formation by regulating cell cycle progression. HDACs might also regulate SMC differentiation, although this is not as well characterized.

Methods and Results

Notch signaling activates SMC contractile markers and the differentiated phenotype in human aortic SMCs. Using this model, we found that HDAC inhibition antagonized the ability of Notch to increase levels of smooth muscle α-actin, calponin1, smooth muscle 22α, and smooth muscle myosin heavy chain. However, inhibition of HDAC activity did not suppress Notch activation of the HRT target genes. In fact, HDAC inhibition increased activation of the canonical C-promoter binding factor-1 (CBF-1)–mediated Notch pathway, which activates HRT transcription. Although CBF-1–mediated Notch signaling was increased by HDAC inhibition in human SMCs and in a C3H10T1/2 model, SMC differentiation was inhibited in both cases. Further characterization of downstream Notch signaling pathways showed activation of the c-Jun N-terminal kinase, p38 mitogen-activated protein kinase, and PI3K/Akt pathways. The activation of these pathways was sensitive to HDAC inhibition and was positively correlated with the differentiated phenotype.

Conclusions

Our studies define novel signaling pathways downstream of Notch signaling in human SMCs. In addition to the canonical CBF-1 pathway, Notch stimulates c-Jun N-terminal kinase, mitogen-activated protein kinase, and PI3K cascades. Both canonical and noncanonical pathways downstream of Notch promote a differentiated, contractile phenotype in SMCs. Although CBF-1–mediated Notch signaling is not suppressed by HDAC inhibition, HDAC activity is required for Notch differentiation signals through mitogen-activated protein kinase and PI3K pathways in SMCs. (J Am Heart Assoc. 2012;1:e000901 doi: 10.1161/JAHA.112.000901)

Mechanisms of TGF-β-induced differentiation in human vascular smooth muscle cells

BACKGROUND

Transforming growth factor-β (TGF-β) plays an important role in vascular homeostasis through effects on vascular smooth muscle cells (SMC). Fine-tuning of TGF-β signaling occurs at the level of ALK receptors or Smads, and is regulated with cell type specificity.

METHODS

Our goal was to understand TGF-β signaling in regulating SMC differentiation marker expression in human SMC. Activation of Smads was characterized, and loss- and gain-of-function reagents used to define ALK pathways. In addition, Smad-independent mechanisms were determined.

RESULTS

TGF-β type I receptors, ALK1 and ALK5, are expressed in human SMC, and TGF-β1 phosphorylates Smad1/5/8 and Smad2/3 in a time- and dosage-dependent pattern. ALK5 activity, not bone morphogenetic protein type I receptors, is required for Smad phosphorylation. Endoglin, a TGF-β type III receptor, is a TGF-β1 target in SMC, yet endoglin does not modify TGF-β1 responsiveness. ALK5, not ALK1, is required for TGF-β1-induction of SMC differentiation markers, and ALK5 signals through an ALK5/Smad3- and MAP kinase-dependent pathway.

CONCLUSION

The definition of the specific signaling downstream of TGF-β regulating SMC differentiation markers will contribute to a better understanding of vascular disorders involving changes in SMC phenotype.

Three Mechanistically Distinct Kinase Assays Compared: Measurement of Intrinsic ATPase Activity Identified the Most Comprehensive Set of ITK Inhibitors

Numerous assay methods have been developed to identify small-molecule effectors of protein kinases, but no single method can be applied to all isolated kinases. The authors developed a set of 3 high-throughput screening (HTS)–compatible biochemical assays that can measure 3 mechanistically distinct properties of a kinase active site, with the goal that at least 1 of the 3 would be applicable to any kinase selected as a target for drug discovery efforts. Two assays measure catalytically active enzyme: A dissociation-enhanced lanthanide fluoroimmuno assay (DELFIA) uses an antibody to quantitate the generation of phosphorylated substrate; a second assay uses luciferase to measure the consumption of adenosine triphosphate (ATP) during either phosphoryl-transfer to a peptide substrate or to water (intrinsic ATPase activity). A third assay, which is not dependent on a catalytically active enzyme, measures the competition for binding to kinase between an inhibitor and a fluorescent ATP binding site probe. To evaluate the suitability of these assays for drug discovery, the authors compared their ability to identify inhibitors of a nonreceptor protein tyrosine kinase from the Tec family, interleukin-2-inducible T cell kinase (ITK). The 3 assays agreed on 57% of the combined confirmed hit set identified from screening a 10,208-compound library enriched with known kinase inhibitors and molecules that were structurally similar. Among the 3 assays, the one measuring intrinsic ATPase activity produced the largest number of unique hits, the fewest unique misses, and the most comprehensive hit set, missing only 2.7% of the confirmed inhibitors identified by the other 2 assays combined. Based on these data, all 3 assay formats are viable for screening and together provide greater options for assay design depending on the targeted kinase.

Cloning, characterization, and heat stress-induced redistribution of a protein homologous to human hsp27 in the zebrafish Danio rerio

Hsp27 is a small heat shock protein (shsp) regulating stress tolerance and increasingly thought to play roles in tissue homeostasis and differentiation. The zebrafish Danio rerio is an important model for the study of developmental processes, but little is known regarding shsps in this animal. Here, we report the sequence, expression, regulation, and function of a zebrafish protein (zfHsp27) homologous to human Hsp27. zfHsp27 contains three conserved phosphorylatable serines and a cysteine important for regulation of apoptosis, but it lacks much of a C-terminal tail domain and shows low homology in two putative actin interacting domains that are features of mammalian Hsp27. zfHsp27 mRNA is most abundant in adult skeletal muscle and heart and is upregulated during early embryogenesis. zfHsp27 expressed in mammalian fibroblasts was phosphorylated in response to heat stress and anisomycin, and this phosphorylation was prevented by treatment with SB202190, an inhibitor of p38 MAPK. Expression of zfHsp27 and human Hsp27 in mammalian fibroblasts promoted a similar degree of tolerance to heat stress. zfHsp27 fusion proteins entered the nucleus and associated with the cytoskeleton of heat stressed cells in vitro and in zebrafish embryos. These results reveal conservation in regulation and function of mammalian and teleost Hsp27 proteins and define zebrafish as a new model for the study of Hsp27 function.

Expression, purification, and enzymatic characterization of the dual specificity mitogen-activated protein kinase phosphatase, MKP-4

Mitogen-activated protein kinase phosphatase-4 (MKP-4) is a dual specificity phosphatase, which acts as a negative regulator of insulin-stimulated pathways. Here, we describe expression, purification, and biochemical characterization of MKP-4. We used the Baculovirus expression system and purification with a combination of affinity and gel filtration chromatography to generate pure MKP-4 and MKP-4/p38 complex. Both MKP-4 and the MKP-4/p38 complex exhibited moderate activity toward the surrogate substrates p-nitrophenyl phosphate, 6, 8-difluoro-4-methylumbelliferyl phosphate, and 3-O-methylfluorescein phosphate. The phosphatase activity could be inhibited by peroxovanate, a potent inhibitor of protein tyrosine phosphatases. We further determined kinetic parameters for the MKP-4 and the MKP-4/p38 by using spectrophotometric and fluorescence intensity methods. The MKP-4/p38 complex was found to provide substantially higher phosphatase activity than MKP-4 alone, similar to what has been shown for MKP-3. Our data allow the configuration of screens for modulators of MKP-4 activity.

Data concordance from a comparison between filter binding and fluorescence polarization assay formats for identification of ROCK-II inhibitors

The Rho-associated coiled-coil-containing protein serine/threonine kinases ROCK-I and ROCK-II are thought to play a major role in cytoskeletal dynamics by serving as downstream effectors of the Rho/Rac family of cytokine- and growth factor-activated small GTPases. As such, the ROCK family members are attractive intervention targets for a variety of pathologies, including cancer and cardiovascular disease. The authors developed a high-throughput screen to identify ROCK-II inhibitors and report results from a direct comparison of 2 screening campaigns for ROCK-II inhibitors using fluorescence polarization (FP) and filter binding (FB). Screening protocols to identify inhibitors of ROCK-II were developed in FB and FP formats under similar assay and kinetic conditions. A 30,000-member compound library was screened using FB ((33)P) and FP detection systems, and compounds that were active in either assay were retested in 5-point curve confirmation assays. Analysis of these data showed an approximate 95% agreement of compounds identified as active in both assay formats. Also, compound potency determinations from FB and FP had a high degree of correlation and were considered equivalent. These data suggest that the assay methodology has little impact on the quality and productivity of the screen, provided that the assays are developed to standardize kinetic conditions.