The ubiquitin-proteasome system is the major pathway for intracellular protein degradation and is also deeply involved in the regulation of most basic cellular processes. Its proteolytic core, the 20S proteasome, has found to be attached also to the cell plasma membrane and certain observations are interpreted as to suggest that they may be released into the extracellular medium, e.g. in the alveolar lining fluid, epididymal fluid and possibly during the acrosome reaction. Proteasomes have also been detected in normal human blood plasma and designated circulating proteasomes; these have a comparatively low specific activity, a distinct pattern of subtypes and their exact origin is still enigmatic. In patients suffering from autoimmune diseases, malignant myeloproliferative syndromes, multiple myeloma, acute and chronic lymphatic leukaemia, solid tumour, sepsis or trauma, respectively, the concentration of circulating proteasomes has been found to be elevated, to correlate with the disease state and has even prognostic significance. Similarly, ubiquitin has been discovered as a normal component of human blood and seminal plasma and in ovarian follicular fluid. Increased concentrations were measured in diverse pathological situations, not only in blood plasma but also in cerebrospinal fluid, where it may have neuroprotective effects. As defective spermatozoa are covered with ubiquitin in the epididymal fluid, extracellular ubiquitination is proposed to be a mechanism for quality control in spermatogenesis. Growing evidence exists also for a participation of extracellular proteasomes and ubiquitin in the fertilization process.

The intrapulmonary renin-angiotensin system via tissue concentration of angiotensin II or bradykinin may have multiple effects on pulmonary pathophysiology. Therefore, it was investigated whether the presence of the D allele of the angiotensinconverting enzyme (ACE) insertion/deletion (I/D) polymorphism or the A allele of angiotensinogen (AGT) promoter polymorphism (-6)A/G are independent risk factors for 30-day survival in acute respiratory distress syndrome (ARDS) patients.In a prospective study, adults (Germans of Caucasian ethnicity) with ARDS (n584) were recruited from the current authors' intensive care unit and genotyped for the ACE I/D and the AGT (-6)A/G polymorphisms, as were 200 healthy Caucasian controls.Mortality was increased in the ACE DD genotype compared with the I allele, and the ACE I/D polymorphism was an independent prognostic factor for 30-day survival. Patients with a homozygous DD genotype were at highest risk for death (hazard ratio 5.7; 95% confidence interval 1.7-19.2) compared with the II genotype. In contrast, the AGT (-6)A/G polymorphism was neither associated with an increased risk for development of ARDS nor with outcome.In patients with acute respiratory distress syndrome, the angiotensin-converting enzyme insertion/deletion polymorphism but not the angiotensinogen (-6)A/G promoter polymorphism is an independent risk factor with a pronounced effect on 30-day survival.

The 20S proteasome is almost exclusively localized within cells. High levels of extracellular proteasomes are also found circulating in the blood plasma of patients suffering from a variety of inflammatory, autoimmune and neoplastic diseases. However, the origin of these proteasomes remained enigmatic. Since the proteome of microparticles, small membrane enclosed vesicles released from cells, was shown to contain proteasomal subunits, we studied whether intact proteasomes are actively released into the extracellular space. Using human primary T lymphocytes stimulated with CaCl2 and the calcium ionophore A23187 to induce membrane blebbing we demonstrate that microparticles contain proteolytically active 20S proteasomes as well as the proteasome activator PA28 and subunits of the 19S proteasome regulator. Furthermore, our experiments reveal that incubation of in vitro generated T lymphocyte-microparticles with sphingomyelinase results in the hydrolysis of the microparticle membranes and subsequent release of proteasomes from the vesicles. Thus, we here show for the first time that functional proteasomes can be exported from activated immune cells by way of microparticles, the dissolution of which may finally lead to the generation of extracellular proteasomes.

For the first time, we identified extracellular, biologically active 20S proteasome in the alveolar space of patients with ARDS in concentrations much higher than in normal subjects or in those with ALI.