Kenneth D. Greis

Accumulation of Virion Tegument and Envelope Proteins in a Stable Cytoplasmic Compartment during Human Cytomegalovirus Replication: Characterization of a Potential Site of Virus Assembly

ABSTRACT The assembly of human cytomegalovirus (HCMV) is thought to be similar to that which has been proposed for alphaherpesviruses and involve envelopment of tegumented subviral particles at the nuclear membrane followed by export from the cell by a poorly defined pathway. However, several studies have shown that at least two tegument virion proteins remain in the cytoplasm during the HCMV replicative cycle, thereby suggesting that HCMV cannot acquire its final envelope at the nuclear envelope. We investigated the assembly of HCMV by determining the intracellular trafficking of the abundant tegument protein pp150 (UL32) in productively infected human fibroblasts. Our results indicated that pp150 remained within the cytoplasm throughout the replicative cycle of HCMV and accumulated in a stable, juxtanuclear structure late in infection. Image analysis using a variety of cell protein-specific antibodies indicated that the pp150-containing structure was not a component of the endoplasmic reticulum, (ER), ER-Golgi intermediate compartment, cis or medial Golgi, or lysosomes. Partial colocalization of the structure was noted with thetrans-Golgi network, and it appeared to lie in close proximity to the microtubule organizing center. Two additional tegument proteins (pp28 and pp65) and three envelope glycoproteins (gB, gH, and gp65) localized in this same structure late infection. This compartment appeared to be relatively stable since pp150, pp65, and the processed form of gB could be coisolated following cell fractionation. Our findings indicated that pp150 was expressed exclusively within the cytoplasm throughout the infectious cycle of HCMV and that the accumulation of the pp150 in this cytoplasmic structure was accompanied by at least five other virion proteins. These results suggested the possibility that this virus-induced structure represented a cytoplasmic site of virus assembly.

Mechanism of insulin sensitization by BMOV (bis maltolato oxo vanadium); unliganded vanadium (VO4) as the active component

Organovanadium compounds have been shown to be insulin sensitizers in vitro and in vivo. One potential biochemical mechanism for insulin sensitization by these compounds is that they inhibit protein tyrosine phosphatases (PTPs) that negatively regulate insulin receptor activation and signaling. In this study, bismaltolato oxovanadium (BMOV), a potent insulin sensitizer, was shown to be a reversible, competitive phosphatase inhibitor that inhibited phosphatase activity in cultured cells and enhanced insulin receptor activation in vivo. NMR and X-ray crystallographic studies of the interaction of BMOV with two different phosphatases, HCPTPA (human low molecular weight cytoplasmic protein tyrosine phosphatase) and PTP1B (protein tyrosine phosphatase 1B), demonstrated uncomplexed vanadium (VO(4)) in the active site. Taken together, these findings support phosphatase inhibition as a mechanism for insulin sensitization by BMOV and other organovanadium compounds and strongly suggest that uncomplexed vanadium is the active component of these compounds.

The von Hippel-Lindau Tumor Suppressor Protein and Egl-9-Type Proline Hydroxylases Regulate the Large Subunit of RNA Polymerase II in Response to Oxidative Stress

ABSTRACT Human renal clear cell carcinoma (RCC) is frequently associated with loss of the von Hippel-Lindau (VHL) tumor suppressor (pVHL), which inhibits ubiquitylation and degradation of the alpha subunits of hypoxia-inducible transcription factor. pVHL also ubiquitylates the large subunit of RNA polymerase II, Rpb1, phosphorylated on serine 5 (Ser5) within the C-terminal domain (CTD). A hydroxylated proline 1465 within an LXXLAP motif located N-terminal to the CTD allows the interaction of Rpb1 with pVHL. Here we report that in RCC cells, pVHL regulates expression of Rpb1 and is necessary for low-grade oxidative-stress-induced recruitment of Rpb1 to the DNA-engaged fraction and for its P1465 hydroxylation, phosphorylation, and nondegradative ubiquitylation. Egln-9-type prolyl hydroxylases, PHD1 and PHD2, coimmunoprecipitated with Rpb1 in the chromatin fraction of VHL+ RCC cells in response to oxidative stress, and PHD1 was necessary for P1465 hydroxylation while PHD2 had an inhibitory effect. P1465 hydroxylation was required for oxidative-stress-induced Ser5 phosphorylation of Rpb1. Importantly, overexpression of wild-type Rpb1 stimulated formation of kidney tumors by VHL+ cells, and this effect was abolished by P1465A mutation of Rpb1. These data indicate that through this novel pathway involving P1465 hydroxylation and Ser5 phosphorylation of Rbp1, pVHL may regulate tumor growth.

Peroxiredoxin-6 protects against mitochondrial dysfunction and liver injury during ischemia-reperfusion in mice

Hepatic ischemia-reperfusion (I/R) injury is an important complication of liver surgery and transplantation. Mitochondrial function is central to this injury. To examine alterations in mitochondrial function during I/R, we assessed the mitochondrial proteome in C57Bl/6 mice. Proteomic analysis of liver mitochondria revealed 234 proteins with significantly altered expression after I/R. From these, 13 proteins with the greatest expression differences were identified. One of these proteins, peroxiredoxin-6 (Prdx6), has never before been described in mitochondria. In hepatocytes from sham-operated mice, Prdx6 expression was found exclusively in the cytoplasm. After ischemia or I/R, Prdx6 expression disappeared from the cytoplasm and appeared in the mitochondria, suggesting mitochondrial trafficking. To explore the functional role of Prdx6 in hepatic I/R injury, wild-type and Prdx6-knockout mice were subjected to I/R injury. Prdx6-knockout mice had significantly more hepatocellular injury compared with wild-type mice. Interestingly, the increased injury in Prdx6-knockout mice occurred despite reduced inflammation and was associated with increased mitochondrial generation of H(2)O(2) and dysfunction. The mitochondrial dysfunction appeared to be related to complex I of the electron transport chain. These data suggest that hepatocyte Prdx6 traffics to the mitochondria during I/R to limit mitochondrial dysfunction as a protective mechanism against hepatocellular injury.

O-Linked N-Acetylglucosamine: The “Yin-Yang” of Ser/Thr Phosphorylation?

O-linked N-acetylglucosamine (O-GlcNAc) was discovered during studies using bovine milk galactosyltransferase to ‘map’ the surface topography on cells of the murine immune system (Torres and Hart, 1984). Later, O-GlcNAc was shown to reside almost exclusively in the nucleus and cytoplasm (Kearse and Hart, 1991b), and to be present in eukaryotes ranging from trypanosomes, yeast, plants, to man, as well as in viruses (Hart et al. 1989; Hart et al. 1994a; Greis and Hart, 1994). We now know that O-GlcNAc is an exceedingly abundant post-translational modification of specific serine/threonine residues of many important nuclear and cytoplasmic proteins (Haltiwanger et al.1992b). Figure 1 lists the O-GlcNAc-bearing proteins identified to date. O-GlcNAc is attached as a monosaccharide and is generally not further modified. The O-GlcNAc turn-over rates are typically many-times that of the polypeptide backbone on which it is found (Chou et al.1992a; Hart and Roquemore, unpublished). The saccharide is attached at sites similar to those also used by the ‘growth-factor’, proline-directed family of kinases (Roach, 1991; Taylor and Adams, 1992). O-GlcNAc-bearing proteins have a diverse range of functions, but are characterized by several common features: 1) They all are also phosphorylated. 2) They typically form specific and regulated multimeric associations with other polypeptides. 3) In several cases, O-GlcNAcylation and phosphorylation appear to be reciprocal events.

Initial validation of a novel protein biomarker panel for active pediatric lupus nephritis.

Lupus nephritis (LN) is among the main determinants of poor prognosis in systemic lupus erythematosus (SLE). The objective of this study was to 1) isolate and identify proteins contained in the LN urinary protein signature (PS) of children with SLE; 2) assess the usefulness of the PS proteins for detecting activity of LN over time. Using surface-enhanced or matrix-assisted laser desorption/ionization time of flight mass spectrometry, the proteins contained in the LN urinary PS were identified. They were transferrin (Tf), ceruloplasmin (Cp), α1-acid-glycoprotein (AGP), lipocalin-type prostaglandin-D synthetase (L-PGDS), albumin, and albumin-related fragments. Serial plasma and urine samples were analyzed using immunonephelometry or ELISA in 98 children with SLE (78% African American) and 30 controls with juvenile idiopathic arthritis. All urinary PS proteins were significantly higher with active vs. inactive LN or in patients without LN (all p < 0.005), and their combined area under the receiver operating characteristic curve was 0.85. As early as 3 mo before a clinical diagnosis of worsening LN, significant increases of urinary Tf, AGP (both p < 0.0001), and L-PGDS (p < 0.01) occurred, indicating that these PS proteins are biomarkers of LN activity and may help anticipate the future course of LN.

Proteomic analysis of rat soleus muscle undergoing hindlimb suspension‐induced atrophy and reweighting hypertrophy

A proteomic analysis was performed comparing normal rat soleus muscle to soleus muscle that had undergone either 0.5, 1, 2, 4, 7, 10 and 14 days of hindlimb suspension‐induced atrophy or hindlimb suspension‐induced atrophied soleus muscle that had undergone 1 hour, 8 hour, 1 day, 2 day, 4 day and 7 days of reweighting‐induced hypertrophy. Muscle mass measurements demonstrated continual loss of soleus mass occurred throughout the 21 days of hindlimb suspension; following reweighting, atrophied soleus muscle mass increased dramatically between 8 hours and 1 day post reweighting. Proteomic analysis of normal and atrophied soleus muscle demonstrated statistically significant changes in the relative levels of 29 soleus proteins. Reweighting following atrophy demonstrated statistically significant changes in the relative levels of 15 soleus proteins. Protein identification using mass spectrometry was attempted for all differentially regulated proteins from both atrophied and hypertrophied soleus muscle. Five differentially regulated proteins from the hindlimb suspended atrophied soleus muscle were identified while five proteins were identified in the reweighting‐induced hypertrophied soleus muscles. The identified proteins could be generally grouped together as metabolic proteins, chaperone proteins and contractile apparatus proteins. Together these data demonstrate that coordinated temporally regulated changes in the skeletal muscle proteome occur during disuse‐induced soleus muscle atrophy and reweighting hypertrophy.

Proteomic analysis of the atrophying rat soleus muscle following denervation

A proteomic analysis was performed comparing normal rat soleus muscle to denervated soleus muscle at 0.5, 1, 2, 4, 6, 8 and 10 days post denervation. Muscle mass measurements demonstrated that the times of major mass changes occurred between 2 and 4 days post denervation. Proteomic analysis of the denervated soleus muscle during the atrophy process demonstrated statistically significant (at the p < 0.01 level) changes in 73 soleus proteins, including coordinated changes in select groups of proteins. Sequence analysis of ten differentially regulated proteins identified metabolic proteins, chaperone and contractile apparatus proteins. Together these data indicate that coordinated temporally regulated changes in the proteome occur during denervation‐induced soleus muslce atrophy, including changes in muscle metabolism and contractile apparatus proteins.

MALDI-TOF MS as a label-free approach to rapid inhibitor screening

Mass Spectrometry (MS) has been widely reported for measuring the conversion of substrates to products for enzyme assays. These measurements are typically performed by time-consuming LC-MS to eliminate buffer salts that interfere with electrospray ionization MS. However, matrix-assisted laser desorption ionization, time-of-flight MS (MALDI-TOF MS) offers a label-free and direct readout of substrate and product, a fast sampling rate, and is tolerant of many buffer salts, reagents, and compounds that are typically found in enzyme reaction mixtures. In this report, a demonstration of how MALDI-TOF MS can be used to directly measure ratios of substrates and products to produce IC50 curves for rapid enzyme assays and compound screening is provided. Typical reproducibility parameters were <7% RSD—a value comparable to ESI MS quantitative assays and well within the acceptable limits for screening assays. The speed of the MALDI readout is currently about 10 s per sample, thus allowing for over 7500 samples/day. From a simplicity standpoint, the enzymatic reaction mixtures are prepared by liquid handling robots, the reactions are stopped by addition of a 10 times volume of acidic matrix solution, and the samples are simultaneously transferred to MALDI target plate for analysis. Importantly, the ratios of substrate to product are of sufficient reproducibility to eliminate the need for internal standards and, thus, minimize the cost and increasing the speed of assay development.

Akt and 14-3-3 control a PACS-2 homeostatic switch that integrates membrane traffic with TRAIL-induced apoptosis

TRAIL selectively kills diseased cells in vivo, spurring interest in this death ligand as a potential therapeutic. However, many cancer cells are resistant to TRAIL, suggesting the mechanism mediating TRAIL-induced apoptosis is complex. Here we identify PACS-2 as an essential TRAIL effector, required for killing tumor cells in vitro and virally infected hepatocytes in vivo. PACS-2 is phosphorylated at Ser437 in vivo, and pharmacologic and genetic studies demonstrate Akt is an in vivo Ser437 kinase. Akt cooperates with 14-3-3 to regulate the homeostatic and apoptotic properties of PACS-2 that mediate TRAIL action. Phosphorylated Ser437 binds 14-3-3 with high affinity, which represses PACS-2 apoptotic activity and is required for PACS-2 to mediate trafficking of membrane cargo. TRAIL triggers dephosphorylation of Ser437, reprogramming PACS-2 to promote apoptosis. Together, these studies identify the phosphorylation state of PACS-2 Ser437 as a molecular switch that integrates cellular homeostasis with TRAIL-induced apoptosis.