Abstract
The proteasome is an intracellular complex that degrades damaged or unfolded proteins and participates in the regulation of several processes. The immunoproteasome is a specialized form that is expressed in response to proinflammatory signals and is particularly abundant in immune cells. In a previous work, we found an anti-inflammatory effect in a diterpenoid extracted from the octocoral Pseudopterogorgia acerosa, here called compound 1. This compound prevented the degradation of inhibitor κB α (IκBα) and the subsequent activation of nuclear factor κB (NFκB), suggesting that this effect might be due to inhibition of the ubiquitin-proteasome system. Here we show that compound 1 inhibits the proteasomal chymotrypsin-like activity (CTL) of murine macrophages in the presence of lipopolysaccharide (LPS) but not in its absence. This effect might be due to the capacity of this compound to inhibit the activity of purified immunoproteasome. The compound inhibits the cell surface expression of major histocompatibility complex (MHC)-I molecules and the production of proinflammatory cytokines induced by LPS in vitro and in vivo, respectively. Molecular docking simulations predicted that compound 1 selectively binds to the catalytic site of immunoproteasome subunits β1i and β5i, which are responsible for the CTL activity. Taken together these findings suggest that the compound could be a selective inhibitor of the immunoproteasome, and hence could pave the way for its future evaluation as a candidate for the treatment of inflammatory disorders and autoimmune diseases.
Highlights
The proteasome is an enzymatic complex found in the nucleus and cytoplasm of eukaryotic cells, archaea and certain bacteria
We have previously demonstrated that compound 1 (Figure 1a) inhibits the degradation of inhibitor κB α (IκBα), leading to the prevention of nuclear factor κB (NFκB) activation and the subsequent transcription of genes encoding pro-inflammatory mediators [16]
The compound has no apparent effect on the chymotrypsin-like activity (CTL) activity in the absence of stimulus with LPS, it significantly inhibits caspase-like activity (Figure S5)
Summary
The proteasome is an enzymatic complex found in the nucleus and cytoplasm of eukaryotic cells, archaea and certain bacteria. This complex is responsible for the degradation of intracellular proteins that are damaged or misfolded. It works in collaboration with the ubiquitin system, which tags proteins for proteasome processing. The proteasome is composed of two types of domains: a core particle and one or two regulatory domains. The core particle is formed by four stacking rings, each of them consisting of seven α or β subunits. Central rings have three catalytic subunits, namely β1, β2 and β5, which have caspase-like, trypsin-like and chymotrypsin-like (CTL) activity, respectively.
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