Proteasome

Types: 20S and 26S

Activities of 26S

chymotrypsin-like

trypsin-like

peptidylglutamyl-peptide hydrolase protease activity

Activators

Inhibitors

Curcumin

Most notably, curcumin (diferuloylmethane), a naturally-occurring polyphenolic compound extracted from turmeric root, has been shown to significantly inhibit UPS activity in both in vitro and in vivo animal studies. Moreover, it has been reported high doses of curcumin supplementation in humans are well tolerated without significant side effects (R6)

There have been multiple proposed mechanisms to explain the observed dysregulation in UPS activity. Curcumin has been reported to induce proteasomal malfunction through directly binding to the 20S subunit (likely due to curcumin’s carbonyl carbons susceptible to nucleophilic attack by Thr 1 within the β5 subunit of the proteasome) inhibiting chymotrypsin-like, trypsin-like, and peptidylglutamyl-peptide hydrolase protease activity, or indirectly by inhibiting DUB activity, inducing oxidative stress, increasing misfolded and/or oxidized proteins, or suppressing ubiquitin gene/protein expression. (R6)

Lithium Chloride

Several studies show that LiCl inhibits 26S:

LiCl alone, or in combination with all-trans-retinoic acid, increased cellular levels of ubiquitinated retinoic acid receptor α and markedly reduced chymotryptic-like activity of WEHI-3B D+ 20 S and 26 S proteasome enzymes. (R1)

LiCl interacts synergistically with all-trans-retinoic acid, promoting the terminal differentiation of WEHI-3B D(+) cells, a phenomenon partially due to the ability of the monovalent lithium cation to inhibit the proteasome-dependent degradation of retinoic acid receptor alpha protein. In this report, the 20S proteasome was purified from WEHI-3B D(+) cells and the effects of LiCl on chymotrypsin-like (Chtl) activity and peptidyl-glutamyl peptide hydrolyzing (PGPH) activity were determined. LiCl functions to inactivate both proteasomal activities in a time-dependent manner, without affecting non-proteasomal proteases. The half-lives for inactivation of Chtl and PGPH hydrolyzing activities were approximately 23 and 36min, respectively, at 10mM LiCl. … The findings suggest that the inactivation of Chtl and PGPH activities by LiCl occurs through a proteasomal conformational change. (R2)

However another study reports somewhat conflicting (or confusing) finding:

Lithium is also a potent inhibitor of glycogen synthase kinase-3β (GSK3β) activity, which is linked to Alzheimer’s disease (AD). In experiments with cultured HEK293T cells, we show here that GSK3β stabilizes synaptic acetylcholinesterase (AChE-S), a critical component of AD development.

Cells treated with lithium exhibited rapid proteasomal degradation of AChE-S. Furthermore treatment of the cells with MG132, an inhibitor of the 26S proteasome, prevented the destabilizing effect of lithium on AChE-S. (R3)

Another study shows that LiCl increases both content and activation of Glycogen Synthase:

Incubation of rat hepatocytes with LiCl resulted in an overall increase in the activity ratio of glycogen synthase (GS), concomitantly with a decrease in active GS kinase-3 levels. GS total activity was also increased in a dose- and time-dependent manner. This latter effect correlated with the amount of immunoreactive enzyme determined by immunoblotting. … Our results indicate that LiCl increases hepatocyte GS activity through increasing both the activation state of the enzyme and its cellular content. This latter increase is mediated through a modification of the proteasome-regulated proteolytic pathway of the enzyme. (R4)

MG132

MG132 is a potent, reversible, and cell-permeable 20S proteasome inhibitor and it is derived from a Chinese medicinal plant. … Our results showed that MG132 downregulated the expression of antiapoptotic proteins, including CDK2, CDK4, Bcl-xL, and Bcl-2, whereas it upregulated the expression of proapoptotic proteins, including p21, p27, p53, p-p53 (ser15, ser20, and ser46), cleaved forms of caspase-3, caspase-7, caspase-9, and PARP, and FOXO3 in U2OS cells.

These results demonstrated that MG132 activated apoptotic signaling pathways in U2OS cells.

Interestingly, MG132 downregulated the phosphorylation of Akt and Erk. (R7)

Manganese Chloride

In our research, manganese chloride exposure inhibited the activity of proteasome and induced oxidative stress. Both can be reversed by antioxidant agent N-acetylcysteine. N-acetylcysteine also inhibited the cytotoxicity induced by manganese chloride. In conclusion, our results imply that proteasome inhibition may be associated with manganese-induced cytotoxicity in dopaminergic neurons, which may be connected with oxidative damage. (R8)

The research performed shows that single and repeated Mn treatment of SN56 cholinergic neurons from BF induces P20S inhibition, increases Aβ and pTau protein levels, produces HSP90 and HSP70 proteins expression alteration, and oxidative stress generation, being the last two effects mediated by NRF2 pathway alteration. The increment of Aβ and pTau protein levels was mediated by HSPs and proteasome dysfunction. (R9)

Heme

The most significant signals specific to heme toxicity were consistent with oxidative stress and impaired protein degradation by the proteasome. This ultimately led to an activation of the response to unfolded proteins. These observations were explained mechanistically by demonstrating binding of heme to the proteasome that was linked to impaired proteasome function.

Oxidative heme reactions and proteasome inhibition could be differentiated as synergistic activities of the porphyrin. Based on the present data a novel model of cellular heme toxicity is proposed, whereby proteasome inhibition by heme sustains a cycle of oxidative stress, protein modification, accumulation of damaged proteins and cell death. (R10)

Health implications

Deactivation

Muscle growth defects

we report that the muscle-specific deletion of a crucial proteasomal gene, Rpt3 (also known as Psmc4), resulted in profound muscle growth defects and a decrease in force production in mice.

Specifically, developing muscles in conditional Rpt3-knockout animals showed dysregulated proteasomal activity. The autophagy pathway was upregulated, but the process of autophagosome formation was impaired. A microscopic analysis revealed the accumulation of basophilic inclusions and disorganization of the sarcomeres in young adult mice.

Our results suggest that appropriate proteasomal activity is important for muscle growth and for maintaining myofiber integrity in collaboration with autophagy pathways. The deletion of a component of the proteasome complex contributed to myofiber degeneration and weakness in muscle disorders that are characterized by the accumulation of abnormal inclusions. (R5)

Improves dystrophic phenotype

Interestingly, proteasome inhibition using MG-132 significantly improved the dystrophic phenotype (Carmignac et al., 2011). MG-132 also improves the dystrophic phenotype in a model of dystrophin deficiency (Bonuccelli et al., 2003; Winder et al., 2011).

Therefore, the upregulation of proteasomal proteolysis likely leads to a reduction in skeletal muscle mass, which is in contrast to animal models of proteasomal dysfunction or downregulation in brain neurons that leads to degeneration. (R5)