Increased Production of Paired Helical Filament Epitopes in a Cell Culture System Reduces the Turnover of τ

Abstract: To investigate the regulation of posttranslational modifications of τ that might be pertinent to the production of the paired helical filament (PHF) of Alzheimer's disease, we incubated human neuroblastoma cells with the protein phosphatase inhibitor okadaic acid. This treatment results in increased immunoreactivity of τ with the monoclonal antibodies Alz-50, PHF-1, T3P, and NP8, a reduction in Tau-1 immunoreactivity, and an elevation in apparent molecular weight of τ. Moreover, our data demonstrate that accumulation of phosphates in τ leads to a decrease in the turnover rate of τ in the neuroblastoma cells. It is suggested that similar build-up of hyperphosphorylated τ in the neuronal perikarya may represent an early event in PHF formation. The present system facilitates the investigation of regulatory mechanisms governing the occurrence of PHF epitopes, their effects on neuronal cell metabolism, and possible pharmacological intervention.     

Phosphorylation of elF-4E and initiation of protein synthesis in P19 embryonal carcinoma cells

Mitogenic stimulation of protein synthesis is accompanied by an increase in elF-4E phosphorylation. The effect on protein synthesis by induction of differentiation is less well known. We treated P19 embryonal carcinoma cells with the differentiating agent retinoic acid and found that protein synthesis increased during the first hour of addition. However, the phosphorylation state, as well as the turnover of phosphate on elF-4E, remained unchanged. Apparently, the change in protein synthesis after RA addition is regulated by another mechanism than elF-4E phosphorylation. By using P19 cells overexpressing the EGF receptor, we show that the signal transduction pathway that leads to phosphorylation of elF-4E is present in P19 cells; the EGF-induced change in phosphorylation of elF-4E in these cells is likely to be regulated by a change in elF-4E phosphatase activity. These results suggest that the onset of retinoic acid-induced differentiation is triggered by a signal transduction pathway which involves changes in protein synthesis, but not elF-4E phosphorylation. © 1995 Wiley-Liss, Inc.     

Evidence against a role for alkaline phosphatase in the dephosphorylation of plasma membrane proteins: Hypophosphatasia fibroblast study

A major impasse to understanding the physiologic role(s) of alkaline phosphatase (ALP) is uncertainty as to its natural substrates. Various in vitro studies have led other investigators to suggest that ALP functions as a plasma membrane phosphoprotein phosphatase, consistent with our demonstration of ecto-topography of ALP in a variety of cell types. Thus, we compared the phosphorylation of plasma membrane proteins from control fibroblasts to those from profoundly ALP-deficient fibroblasts of hypophosphatasia patients. Fibroblasts from 3 controls and 3 hypophosphatasia patients (ALP activity < 4% of control) were biosynthetically labeled with ³²P_i for 2 h. ³²P incorporation into total trichloracetic acid (TCA)-precipitable material was not significantly different in control and patient cells. Plasma membranes were prepared from these cells by hypotonic shock, solubilized, and subjected to two-dimensional (2-D) gel electrophoretic separation. Video densitometric analysis of silver-stained 2-D gels failed to reveal any consistent difference in the protein profile between patient vs. control fibroblasts (i.e., unique species, altered pls, or increased abundance). Autoradiography of individual 2-D gels demonstrated 63 plasma membrane phosphoproteins with molecular weights ranging from 15 to 152 kDa and predominantly acidic pls. Although several of these phosphoproteins appeared to have had donor-specific labeling, none was unique or especially abundant in the hypophosphatasia group. Thus, in ALP-deficient fibroblasts, normal incorporation of ³²P into total cellular protein and into all identifiable plasma membrane phosphoproteins indicates that ALP does not modulate the phosphorylation of plasma membrane proteins.     

Raf-independent, PP2A-dependent MEK activation in response to ERK silencing

Biological roles of ERK and MEK in signal transduction have been controversial. The aim of the current study was to determine the role of ERK1/2 in signaling through the ERK-MAPK cascade by using RNAi methodology. Transient transfection of erk1 or erk2 siRNA decreased the respective protein level to 3-8% in human lung fibroblasts. Interestingly, individual ERK isoform silencing resulted in a 2-fold reciprocal increase in phosphorylation of the alternate ERK isoform, with no change in respective total protein expression. Moreover, MEK was hyperphosphorylated as a result of combined ERK1 and ERK2 silencing, but was unaffected in individual ERK1 or ERK2 silenced cells. This hyperactivation of MEK was not due to activation of Raf family members, but rather was associated with PP2A downregulation. These data highlight the existence of a feedback loop in normal cells whereby ERK silencing is associated with decreased PP2A activity and consequent MEK activation.     

Prdx1 inhibits tumorigenesis via regulating PTEN/AKT activity

It is widely accepted that reactive oxygen species (ROS) promote tumorigenesis. However, the exact mechanisms are still unclear. As mice lacking the peroxidase peroxiredoxin1 (Prdx1) produce more cellular ROS and die prematurely of cancer, they offer an ideal model system to study ROS‐induced tumorigenesis. Prdx1 ablation increased the susceptibility to Ras‐induced breast cancer. We, therefore, investigated the role of Prdx1 in regulating oncogenic Ras effector pathways. We found Akt hyperactive in fibroblasts and mammary epithelial cells lacking Prdx1. Investigating the nature of such elevated Akt activation established a novel role for Prdx1 as a safeguard for the lipid phosphatase activity of PTEN, which is essential for its tumour suppressive function. We found binding of the peroxidase Prdx1 to PTEN essential for protecting PTEN from oxidation‐induced inactivation. Along those lines, Prdx1 tumour suppression of Ras‐ or ErbB‐2‐induced transformation was mediated mainly via PTEN.     

In vivo phosphorylation of proteins in Clostridium sphenoides

Abstract Cell contents of Clostridium sphenoides , labeled with [³²P]orthophosphate under strict anaerobic conditions, were analyzed by two-dimensional gel electrophoresis. Autoradiography of these gels demonstrated the presence of at least 15 ³²P-labeled protein species, of which M _r and iso-electric point were determined. Treatment of the radioactively labeled cell contents with alkaline phosphatase and acid phosphatase showed that all these proteins were modified by phosphorylation. These findings demonstrate for the first time the presence of phosphorproteins in a strictly anaerobic bacterium.     

Crystal Structures of Wzb of Escherichia coli and CpsB of Streptococcus pneumoniae, Representatives of Two Families of Tyrosine Phosphatases that Regulate Capsule Assembly

Many Gram-positive and Gram-negative bacteria utilize polysaccharide surface layers called capsules to evade the immune system; consequently, the synthesis and export of the capsule are a potential therapeutic target. In Escherichia coli K-30, the integral membrane tyrosine autokinase Wzc and the cognate phosphatase Wzb have been shown to be key for both synthesis and assembly of capsular polysaccharides. In the Gram-positive bacterium Streptococcus pneumoniae, the CpsCD complex is analogous to Wzc and the phosphatase CpsB is the corresponding cognate phosphatase. The phosphatases are known to dephosphorylate their corresponding autokinases, yet despite their functional equivalence, they share no sequence homology. We present the structure of Wzb in complex with phosphate and high-resolution structures of apo-CpsB and a phosphate-complexed CpsB. We show that both proteins are active toward Wzc and thereby demonstrate that CpsB is not specific for CpsCD. CpsB is a novel enzyme and represents the first solved structure of a tyrosine phosphatase from a Gram-positive bacterium. Wzb and CpsB have completely different structures, suggesting that they must operate by very different mechanisms. Although the mechanism of Wzb can be inferred from previous studies, CpsB appears to have a tyrosine phosphatase mechanism not observed before. We propose a chemical mechanism for CpsB based on site-directed mutagenesis and structural data.