Profiling proteasome activity in tissue with fluorescent probes

With proteasome inhibitors in use in the clinic for the treatment of multiple myeloma and with clinical trials in progress investigating the treatment of a variety of hematologic and solid malignancies, accurate methods that allow profiling of proteasome inhibitor specificity and efficacy in patients are in demand. Here, we describe the development, full biochemical validation, and comparison of fluorescent proteasome activity reporters that can be used to profile proteasome activities in living cells with high sensitivity. Seven of the synthesized probes tested label proteasomes in lysates, although the fluorescent dye used affects their specificity. Two differentially labeled probes tested are suitable for studying proteasome activity in living cells by gel-based assays, by confocal laser scanning microscopy, and by flow cytometry. We established methods using these fluorescent reporters to profile proteasome activity in different mouse tissues, carefully avoiding postlysis artifacts, and we show that proteasome subunit activity is regulated in an organ-specific manner. The techniques described here could be used to study in vivo pharmacological properties of proteasome inhibitors.

Costimulatory ligand CD70 is delivered to the immunological synapse by shared intracellular trafficking with MHC class II molecules

TNF family member CD70 is the ligand of CD27, a costimulatory receptor that shapes effector and memory T cell pools. Tight control of CD70 expression is required to prevent lethal immunodeficiency. By selective transcription, CD70 is largely confined to activated lymphocytes and dendritic cells (DC). We show here that, in addition, specific intracellular routing controls its plasma membrane deposition. In professional antigen-presenting cells, such as DC, CD70 is sorted to late endocytic vesicles, defined as MHC class II compartments (MIIC). In cells lacking the machinery for antigen presentation by MHC class II, CD70 travels by default to the plasma membrane. Introduction of class II transactivator sufficed to reroute CD70 to MIIC. Vesicular trafficking of CD70 and MHC class II is coordinately regulated by the microtubule-associated dynein motor complex. We show that when maturing DC make contact with T cells in a cognate fashion, newly synthesized CD70 is specifically delivered via MIIC to the immunological synapse. Therefore, we propose that routing of CD70 to MIIC serves to coordinate delivery of the T cell costimulatory signal in time and space with antigen recognition.

A Fluorescent Broad-Spectrum Proteasome Inhibitor for Labeling Proteasomes In Vitro and In Vivo

The proteasome is an essential evolutionary conserved protease involved in many regulatory systems. Here, we describe the synthesis and characterization of the activity-based, fluorescent, and cell-permeable inhibitor Bodipy TMR-Ahx(3)L(3)VS (MV151), which specifically targets all active subunits of the proteasome and immunoproteasome in living cells, allowing for rapid and sensitive in-gel detection. The inhibition profile of a panel of commonly used proteasome inhibitors could be readily determined by MV151 labeling. Administration of MV151 to mice allowed for in vivo labeling of proteasomes, which correlated with inhibition of proteasomal degradation in the affected tissues. This probe can be used for many applications ranging from clinical profiling of proteasome activity, to biochemical analysis of subunit specificity of inhibitors, and to cell biological analysis of the proteasome function and dynamics in living cells.

Radiation modulates the peptide repertoire, enhances MHC class I expression, and induces successful antitumor immunotherapy

Radiotherapy is one of the most successful cancer therapies. Here the effect of irradiation on antigen presentation by MHC class I molecules was studied. Cell surface expression of MHC class I molecules was increased for many days in a radiation dose-dependent manner as a consequence of three responses. Initially, enhanced degradation of existing proteins occurred which resulted in an increased intracellular peptide pool. Subsequently, enhanced translation due to activation of the mammalian target of rapamycin pathway resulted in increased peptide production, antigen presentation, as well as cytotoxic T lymphocyte recognition of irradiated cells. In addition, novel proteins were made in response to gamma-irradiation, resulting in new peptides presented by MHC class I molecules, which were recognized by cytotoxic T cells. We show that immunotherapy is successful in eradicating a murine colon adenocarcinoma only when preceded by radiotherapy of the tumor tissue. Our findings indicate that directed radiotherapy can improve the efficacy of tumor immunotherapy.

Statins Affect Cell-Surface Expression of Major Histocompatibility Complex Class II Molecules by Disrupting Cholesterol-Containing Microdomains

Statins, the main therapy for hypercholesterolemia, are currently considered as possible immunomodulatory agents. Statins inhibit the production of proinflammatory cytokines and reduce the expression of several immunoregulatory molecules, including major histocompatibility complex class II (MHC-II) molecules. In this study, we investigated the mechanism by which simvastatin reduces the membrane expression of MHC-II molecules on several human cell types. We demonstrate that the reduction of MHC-II membrane expression by simvastatin correlates with disruption of cholesterol-containing microdomains, which transport and concentrate MHC-II molecules to the cell surface. In addition, we demonstrate that statins reduce cell-surface expression of other immunoregulatory molecules, which include MHC-I, CD3, CD4, CD8, CD28, CD40, CD80, CD86, and CD54. Our observations indicate that the downregulation of MHC-II at the cell surface contributes to the immunomodulatory properties of statins and is achieved through disruption of cholesterol-containing microdomains, which are involved in their intracellular transport.