The proteasome is a construct in cells that shreds damaged, misfolded, or unwanted proteins, reducing them to component parts that can be reused. It is a part of the ubuiquitin-proteasome system: molecules to be destroyed are tagged with ubiquitin, and drawn into a proteasome for recycling. Greater proteasome activity is thought to be a good thing, improving cell function. This is of particular relevance to aging, as proteasomal function declines with age, contributing to faltering cell and tissue function, particularly in the long-lived cells of the nervous system.
While established drugs exist to inhibit activity of the proteasome, useful in cancer therapies in which cell death is a goal, improving proteasomal activity is less well explored. The one approach shown to work well to date is to increase expression of some of the individual rate-limiting proteins that make up proteasomal structure. This has been shown to extend life in short-lived laboratory species, an enhancement therapy that partially compensates for the progressive loss of proteasomal function with age. Researchers here outline their discovery of a different methodology, one that may be more amenable to the production of a drug capable of upregulating proteasome function.
As the central protease of the ubiquitin-proteasome pathway, the proteasome has long been considered an attractive target for drugs potentially affecting multiple aspects of cell physiology. Indeed, small molecules targeting the proteasome have entered the clinic with great success. However, their scope at present is very limited: all proteasome-modifying compounds currently approved or clinically tested as drugs are competitive inhibitors and all are used to treat advanced blood cancers. Here we turn to the opposite side of pharmacological intervention into the proteasome: augmentation of catalytic activity. Since dysfunction of proteasome-mediated controlled protein degradation is a hallmark of both cellular aging and neurodegenerative diseases, enhancement of the enzyme's activity should be considered an attractive therapeutic option.
The complex structure of the catalytic core 20S proteasome (the "core particle") presents fascinating options for allostery-based augmentation. The peptidase responsible for post-hydrophobic (chymotrypsin-like, ChT-L) cleavages is considered a rate-limiting "workhorse" and is the major target for inhibitors and activators alike. Indeed, overexpression of a catalytic subunit harboring the active site of the ChT-L peptidase has been shown not only to extend lifespan but also to reduce age-related cognitive decline in animal models. However, reports on pharmacological augmentation are limited to in vitro and cell culture studies.
Here, we describe a series of short, modified peptides based on the basic domain of the viral Human Immunodeficiency Virus-1 (HIV-1) Tat protein. Among many intracellular effects, the HIV-1 Tat protein inhibits the core proteasome. In our previous studies, we noted that short peptide fragments of HIV-1 Tat displayed peculiar in vitro properties: they inhibited detergent-treated core particle but mildly activated the latent core. However, treatment with detergent, although convenient for in vitro assays, yields mildly denatured proteasome and, under these far-from-physiological conditions, likely with destroyed natural allosteric routes.
Therefore, we turned our attention to the activating properties of HIV-1 Tat protein-derived "TAT peptides". After observing a strong in vitro proteasome augmentation by modified HIV-1 Tat-derived peptides, we tested selected compounds in cell culture. In a separate study, we found that proteasome stimulation by TAT peptides partially prevented cognitive deficits and mortality in animal models of Alzheimer's disease. The very encouraging results included increased proteasome-mediated turnover of amyloid precursor protein (APP) and β-secretase (which cleaves APP to generate β-amyloid peptide), concomitant with lowered levels of β-amyloid, lowered mortality and protection against cognitive decline. We propose that the proteasome-stimulating TAT pharmacophore provides an attractive lead for future clinical use.