Low expression of PP2A regulatory subunit B55α is associated with T308 phosphorylation of AKT and shorter complete remission duration in acute myeloid leukemia patients.

The regulation of protein kinase B (AKT) is a dynamic process that depends on the balance between phosphorylation by upstream kinases for activation and inactivation by dephosphorylation by protein phosphatases. Phosphorylated AKT is commonly found in acute myeloid leukemia (AML) and confers an unfavorable prognosis. Understanding the relative importance of upstream kinases and AKT phosphatase in the activation of AKT is relevant for the therapeutic targeting of this signaling axis in AML. The B55α subunit of protein phosphatase 2A (PP2A) has been implicated in AKT dephosphorylation, but its role in regulating AKT in AML is unknown. We examined B55α protein expression in blast cells derived from 511 AML patients using reverse phase protein analysis. B55α protein expression was lower in AML cells compared with normal CD34+ cells. B55α protein levels negatively correlated with threonine 308 phosphorylation levels. Low levels of B55α were associated with shorter complete remission duration, demonstrating that decreased expression is an adverse prognostic factor in AML. These findings suggest that decreased B55α expression in AML is at least partially responsible for increased AKT signaling in AML and suggests that therapeutic targeting of PP2A could counteract this.

Expression and prognostic significance of IAP-family genes in human cancers and myeloid leukemias.

Expression of several inhibitor of apoptosis proteins (IAPs) was investigated in the National Cancer Institute panel of 60 human tumor cell lines, and the expression and prognostic significance of one of these, XIAP, was evaluated in 78 previously untreated patients with acute myelogenous leukemia (AML). XIAP and cIAP1 were expressed in most cancer lines analyzed, with substantial variability in their relative levels. In contrast, NAIP mRNA was not detectable, and cIAP2 was found at the mRNA and protein levels in only 34 (56%) and 5 (8%) of the 60 tumor cell lines analyzed, respectively. Interestingly, XIAP, cIAP1, and cIAP2 mRNA levels did not correlate with protein levels in the tumor lines, indicating posttranscriptional regulation of expression. High levels of XIAP protein in tumor cell lines were unexpectedly correlated with sensitivity to some anticancer drugs, particularly cytarabine and other nucleosides, whereas higher levels of cIAP1 protein levels were associated with resistance to several anticancer drugs. The relevance of XIAP to in vivo responses to cytarabine was explored in AML, making correlations with patient outcome (n = 78). Patients with lower levels of XIAP protein had significantly longer survival (median, 133 versus 52.5 weeks; P = 0.05) and a tendency toward longer remission duration (median, 87 versus 52.5 weeks; P = 0.13) than those with higher levels of XIAP. Altogether, these findings show that IAPs are widely but differentially expressed in human cancers and leukemias and suggest that higher XIAP protein levels may have adverse prognostic significance for patients with AML.

Neutralization epitopes on poliovirus type 3 particles: an analysis using monoclonal antibodies.

Monoclonal antibodies to poliovirus type 3 secreted by 51 hybridoma cell clones have been characterized in terms of (i) virus-neutralizing properties, (ii) reactivity in antigen-blocking tests with infectious, 155S ('D' antigen) and empty 80S ('C' antigen) poliovirus particles and (iii) reactivity in immunoblot tests with the isolated protein components of the poliovirus capsid. The antibodies could be separated into three groups on the basis of their reactivities with 'D' and 'C' antigens. All antibodies that reacted with both 'D' and 'C' antigen had potent neutralizing activity. Only a proportion of antibodies that reacted uniquely with 'D' antigen possessed neutralizing activity. Unexpectedly, one of 24 'C' antigen-specific antibodies inhibited virus growth. None of the antibodies that possessed virus-neutralizing activity reacted with isolated poliovirus capsid proteins, although the majority of these have been shown in previous studies to be specific for VP1 on intact virus particles. These findings suggest that antigenic determinants involved in virus neutralization do not survive the denaturing conditions required for the isolation of poliovirus capsid proteins and consequently are likely to be specified by the structural conformation of VP1 rather than by amino acid sequence alone. However, several of the antibodies which bound uniquely to 'C' antigen reacted in immunoblot tests, five with VP1 and one with VP3. Some of these antibodies also possessed heterotypic reactivity with the corresponding capsid proteins separated from other poliovirus types.