Degradation of individual intracellular proteins analyzed by two-dimensional gel electrophoresis and computerized video densitometry

Concomitant determination of absolute values of cellular protein amounts, synthesis rates, and turnover rates by quantitative proteome profiling

Two-dimensional gel electrophoresis of protein fractions isolated from (35)S-radiolabeled cells provides qualitative information on intracellular amounts, (35)S incorporation rates, protein modifications, and subcellular localizations of up to thousands of individual proteins. In this study we extended proteome profiling to provide quantitative data on synthesis rates of individual proteins. We combined fluorescence detection of radiolabeled proteins with SYPRO ruby(TM) staining and subsequent autoradiography of the same gels, thereby quantifying protein amounts and (35)S incorporation. To calibrate calculation of absolute synthesis rates, we determined the amount and autoradiograph intensity of radiolabeled haptoglobin secreted by interleukin-6 pretreated HepG2 cells. This allowed us to obtain a standard calibration value for (35)S incorporation per autoradiograph intensity unit. This value was used to measure protein synthesis rates during time course experiments of heat-shocked U937 cells. We measured the increasing amounts of hsp70 and calculated it by integration of the determined hsp70 synthesis rates over time. Similar results were obtained by both methods, validating our standardization procedure. Based on the assumption that the synthesis rate of proteins in a steady state of cell metabolism would essentially compensate protein degradation, we calculated biological half-lives of proteins from protein amounts and synthesis rates determined from two-dimensional gels. Calculated protein half-lives were found close to those determined by pulse-chase experiments, thus validating this new method. In conclusion, we devised a method to assess quantitative proteome profiles covering determination of individual amounts, synthesis, and turnover rates of proteins.

Receptor-mediated endocytosis is sensitive to antibodies against the uncoating ATPase (hsc70)

We have investigated the functional role of the coated vesicle-uncoating ATPase (UA), a cognate heat shock protein (hsc70), in receptor-mediated endocytosis. A monoclonal antibody against bovine brain UA/hsc70 was generated that recognizes a 26 kDa proteolytic fragment harbouring the putative clathrin-binding site. In vitro, this antibody blocked the UA/hsc70-mediated release of clathrin from isolated coated vesicles (CVs). Upon microinjection into tissue culture cells, it specifically inhibited the heat shock-induced nuclear migration of UA/hsc70. This antibody also interfered with endocytosis of ligand-receptor complexes in injected cells. Two different systems were studied: the uptake of aggregated human IgG by BHK cells transfected with a human Fc receptor (FcRII), and the internalization of LDL by human fibroblasts. Injection of the monoclonal antibody in concentrations yielding approximately equal molar ratios of antibody to enzyme resulted in a reduction of endocytosis to 20–30% of control values, as seen by conventional light and confocal laser scanning microscopy, and by electron microscopy. In the transfected BHK cells, the endocytosed ligand remained associated with the labeling for clathrin and was not delivered to the endosomal compartment within the period expected from control serum- or non-injected cells. Thin sections revealed an accumulation of coated structures in the antibody-injected cells as compared to controls. Thus, our data show that UA is essential for normal receptor-mediated endocytosis, and is presumably involved in the uncoating of CVs preceding their fusion with endosomes.

Heparins modulate extracellular matrix and protein synthesis of cultured rat mesangial cells

Heparins blunt the development of glomerulosclerosis in several disease models in the rat and this protective effect may be related to suppression of glomerular cell proliferation. In this study the direct effect of heparins on another key event in glomerulosclerosis, extracellular matrix (ECM) deposition, was examined. Standard heparin (hep) and non-anticoagulant N-desulfated acetylated heparin (DSA-hep) significantly reduced the fibronectin content in the conditioned media of subconfluent, confluent, and supraconfluent rat glomerular mesangial cells (MCs) in culture, as assessed by a sandwich ELISA technique. Both heparins significantly increased the amount of cell-associated fibronectin in sparse and subconfluent MCs. DSA-hep, but not hep, increased the fibronectin content of ECM formed by confluent and supraconfluent MCs. Using 3H-proline pulse-labeling, Hep and DSA-hep were found to significantly decrease cell-associated collagen in subconfluent but not in confluent MCs. No effects were seen on newly synthesized collagen secreted into the culture medium. Neither hep nor DSA-hep affected total protein synthesis, studied by metabolic labeling with 35S-methionine. High resolution 2-D electrophoresis (molecular weight range, 120 to 10 Kd; isoelectric interval, 5.0 to 7.0) revealed one particular intracellular protein (molecular weight 54 Kd, pI 5.91) which was consistently overexpressed in hep. Both heparins affected an identical set of another 19 different intracellular MC proteins (over-/underexpression or shift to higher molecular weights). In conclusion, the present data demonstrate the profound direct metabolic effects of hep and DSA-hep.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations of protein degradation and 2-D protein pattern in muscle cells of MDX and DMD origin

Intracellular protein turnover of MDX, DMD and normal muscle was determined by [35S]methionine pulse-chase experiments and subsequent high resolution 2-D gel electrophoresis. In MDX myotubes intracellular degradation of short-lived and long-lived proteins was markedly increased by a factor of 1,4-2,1. In wildtype the rate of degradation of short-lived proteins was approximately 2.6%/h, whereas in MDX these proteins were degraded by 5.7%/h. Long-lived proteins were degraded in wildtype at a rate of 1.8%/h, and in MDX at a rate of 2.5%/h. Furthermore, we have described a 51.000 Da protein with an IEP of 5.1 (p51/5.1), whose net content is highly and specifically reduced in cultured MDX and DMD muscle cells as well as in isolated MDX muscle fibers. Treatment with calcium-channel blockers Dantrolene and Verapamil inhibited the degradation of p51/5.1 in MDX myotubes by more than 90% in contrast to controls.

Vasoactive agents affect growth and protein synthesis of cultured rat mesangial cells

Mesangial cell (MC) proliferation and extracellular matrix (ECM) formation are hallmarks of chronic glomerular disease. The present in vitro study examined the effects of the vasoactive agents angiotensin II (Ang II), arginine vasopressin (AVP), and serotonin (5-HT) on growth and protein biosynthesis of cultured rat MCs after 72 hours of incubation. AVP and 5-HT (10(-6) M) significantly increased DNA synthesis and growth of quiescent subconfluent MCs to levels of 25 and 45%, respectively, of the optimal stimulatory effect of 10% fetal calf serum (FCS) (both P less than 0.001). The mitogenic effect of Ang II was 10% of the 10% FCS effect (P less than 0.01). ECM production was studied by ELISA assay for fibronectin (FN) secreted into the culture medium (SeFN) and cell-associated FN, that is, intra- and pericellular FN (CaFN). In all incubations, highly significant negative linear relationships were found between the numbers of MCs per well and quantities of both SeFN and CaFN after normalization of the data by logarithmic transformation (SeFN: r values greater than -0.9705; CaFN: r greater than -0.9620; P less than 0.001). Thus, increasing cell densities progressively suppressed ECM formation by MCs. The ECM production was found to be independent of growth activity. AVP significantly increased SeFN (P less than 0.05) and decreased CaFN (P less than 0.001) in subconfluent cultures; Ang II and 5-HT had no effect. Metabolic labeling with 35S-methionine (18 hr, 200 microCi/ml medium) and 2-D electrophoresis of MC lysates resulted in resolution of greater than 500 different radiolabeled intracellular proteins in molecular weight from 110 to 20 Kd over an isoelectric interval of 5.0 to 7.0.(ABSTRACT TRUNCATED AT 250 WORDS)

The progression of renal diseases: on the pathogenesis of renal interstitial fibrosis

Renal interstitial fibrosis (RIF) frequently occurs in inflammatory and non-inflammatory kidney diseases and is associated with a decline in renal excretory function. Fibroblasts which occupy the renal interstitium are involved mainly in the formation of RIF not only by the production of extracellular matrix, but also by regulatory processes. They respond to a variety of cytokines released by different cell types. To investigate mechanisms leading to RIF, immunohistochemical analysis and cell cultures of renal biopsies in various renal diseases have been performed. T lymphocytes are the major cells infiltrating the renal interstitium, and their number correlates with the impairment of renal function. In most forms of glomerulonephritis accompanied by interstitial inflammation, an abnormal expression of HLA-DQ/-DP molecules, frequently associated with an aberrant expression of the intercellular adhesion molecule 1 (ICAM-1), was observed on proximal tubular epithelial cells, indicating that these cells may play a role in antigen presentation. The cell biological experiments revealed the presence of the three mitotic fibroblast types (MFI-MFIII) and the three postmitotic types (PMFIV-PMFVI) in the cell culture. The number of fibroblasts in primary and passage-1 culture was increased seven-fold in cultures derived from kidneys with RIF (FKIF cells) in comparison to normal kidneys (NKF cells). FKIF cells show hyperproliferative growth and synthesize an increased amount of total collagen, especially types III and V. These cells express a protein, named "FIBROSIN", which seems to be specific for FKIF cells. Further extended cell biological analyses are currently being performed to investigate interactions of tubular cells, lymphocytes, macrophages, and fibroblasts in order to shed more light on the pathomechanisms involved in fibrogenesis leading to renal interstitial fibrosis.