Regulation of protein phosphatase 2A activity by caspase-3 during apoptosis

Although the available evidence suggests that whereas the caspase family plays a major role in apoptosis, they are not the sole stimulators of death. A random yeast two-hybrid screen of a lymphocyte cDNA library (using caspase-3 as the bait) found an interaction between caspase-3 and the regulatory subunit Aalpha of protein phosphatase 2A. This protein was found to be a substrate for caspase-3, but not caspase-1, and could compete effectively against either a protein or synthetic peptide substrate. In Jurkat cells induced to undergo apoptosis with anti-Fas antibody, protein phosphatase 2A (PP2A) activity increased 4.5-fold after 6 h. By 12 h, the regulatory Aalpha subunit could no longer be detected in cell lysates. There was no change in the amount of the catalytic subunit. The effects on PP2A could be prevented by the caspase family inhibitors acetyl-Asp-Glu-Val-Asp (DEVD) aldehyde or Ac-DEVD fluoromethyl ketone. The mitogen-activated protein (MAP) kinase pathway is regulated by PP2A. At 12 h after the addition of anti-Fas antibody, a decrease in the amount of the phosphorylated forms of MAP kinase was observed. Again, this loss of activated MAP kinase could be prevented by the addition of DEVD-cho or DEVD-fmk. These data are consistent with a pathway whereby induction of apoptosis activates caspase-3. This enzyme then cleaves the regulatory Aalpha subunit of PP2A, increasing its activity. These data show that the activated PP2A will then effect a change in the phosphorylation state of the cell. These data provide a link between the caspases and signal transduction pathways.

Substrate specificities of caspase family proteases

The caspase family represents a new class of intracellular cysteine proteases with known or suspected roles in cytokine maturation and apoptosis. These enzymes display a preference for Asp in the P1 position of substrates. To clarify differences in the biological roles of the interleukin-1beta converting enzyme (ICE) family proteases, we have examined in detail the specificities beyond the P1 position of caspase-1, -2, -3, -4, -6, and -7 toward minimal length peptide substrates in vitro. We find differences and similarities between the enzymes that suggest a functional subgrouping of the family different from that based on overall sequence alignment. The primary specificities of ICE homologs explain many observed enzyme preferences for macromolecular substrates and can be used to support predictions of their natural function(s). The results also suggest the design of optimal peptidic substrates and inhibitors.

Preparation of an autolysis-resistant interleukin-1 beta converting enzyme mutant

We describe the expression, purification, and characterization of human interleukin-1 beta converting enzyme (ICE) containing an affinity tag and modified to resist autoproteolysis. The point mutation Asp381 to Glu was added to eliminate the major site of autolytic degradation while maintaining catalytic activity, and an N-terminal polyhistidine tag was added in place of the ICE pro-region to facilitate purification. N-His (D381E) ICE was expressed in Escherichia coli and purified by nickel-chelating Sepharose and size-exclusion chromatography (SEC). The enzyme was stabilized greater than 80-fold against autolytic degradation relative to wild-type N-His ICE. SDS-PAGE analysis with silver-staining revealed no impurities, and 85% of the protein was catalytically active as determined by titration with a novel titrant, PD 163594 (3-[2-(2-benzyloxycarbonylamino-3-methylbutyrylamino)prop ionylamino]-4- oxo-5-(2-oxo-2H-chromen-7-yloxypentanoic acid). An oxidized adduct of ICE with glutathione, formed by disulfide rearrangement with oxidized glutathione to inhibit and stabilize the enzyme during purification, was rapidly reduced upon exposure to 5 mM DTT. One mole of glutathione was released per mole of active enzyme. Of the nine cysteines in ICE, eight were present in their reduced form in the glutathione adduct. N-His (D381E) ICE cleaved Ac-YVAD-Amc with the Michaelis-Menten parameters K(M) = 14 microM and Kcat = 0.7 s-1, values essentially identical to those reported for enzyme from natural sources.

Stability and oligomeric equilibria of refolded interleukin-1beta converting enzyme

We report the preparation and characterization of interleukin-1beta converting enzyme (ICE) refolded from its p20 and p10 protein fragments. Refolded ICE heterodimer (p20p10) was catalytically active but unstable, and in size exclusion chromatography eluted at an apparent molecular mass of 30 kDa. The mechanisms of the observed instability were pH-dependent dissociation at low enzyme concentrations, and autolytic degradation of the p10 subunit at high concentrations. Binding and subsequent removal of a high affinity peptidic inhibitor increased the apparent molecular mass to 43 kDa (by size exclusion chromatography), and significantly increased its stability and specific activity. Chemical cross-linking and SDS-polyacrylamide gel electrophoresis analysis of the 43-kDa size exclusion chromatography conformer revealed a 60-kDa species, which was absent in the 30-kDa conformer, suggesting that inhibitor binding caused formation of a (p20p10)2 homodimer. The observation of a reversible equilibrium between ICE (p20p10) and (p20p10)2 suggests that analogous associations, possibly between ICE and ICE homologs, can occur in vivo, resulting in novel oligomeric protease species.

Competitive strategy for providers

National Health Service (NHS) Trusts are struggling to determine a long-term strategic direction for their organizations in response to the competitive pressures generated by the NHS reforms. The development of long-term strategic direction and the methods to implement this are presenting real challenges to the Trusts which have inherited service configurations based on bureaucratic planning frameworks rather than service configurations suited to a more competitive environment. Examines the strategic choices available to these organizations; explores the importance of identifying positive strategic choices; and discusses the advantages and disadvantages in the context of the NHS internal market.

Are there alternatives to merger?

The reconfiguration of the supply side of the National Health Service (NHS) has stimulated an extensive debate about the need to merge NHS Trusts to maintain viable organizations. The result has been a number of mergers across the UK. Considers the underlying advantages of mergers and their drawbacks and argues that consideration must be made to addressing the long-term strategic direction of an organization rather than regarding mergers as ends in themselves. Undertakes an appraisal of alliances and joint partnerships as alternatives to merger. Considers the conditions necessary to create effective alliances and joint partnerships and the factors which will sustain them. Appraises these requirements in the context of the NHS Trusts, in which little detailed work has been undertaken on classifying, understanding and developing what effective alliances mean for these organizations.

Crystal structure of the cysteine protease interleukin-1 beta-converting enzyme: a (p20/p10)2 homodimer

Interleukin-1 beta-converting enzyme (ICE) proteolytically cleaves pro-IL-1 beta to its mature, active form. The crystal structure at 2.5 A resolution of a recombinant human ICE-tetrapeptide chloromethylketone complex reveals that the holoenzyme is a homodimer of catalytic domains, each of which contains a p20 and a p10 subunit. The spatial separation of the C-terminus of p20 and the N-terminus of p10 in each domain suggests two alternative pathways of assembly and activation in vivo. ICE is homologous to the C. elegans cell death gene product, CED-3, and these may represent a novel class of cytoplasmic cysteine proteases that are important in programmed cell death (apoptosis). Conservation among members of the ICE/CED-3 family of the amino acids that form the active site region of ICE supports the hypothesis that they share functional similarities.