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February 21, 2014
Ces3 (Ces1) Inhibition As a Drug Target
Update: the nomenclature of these enzymes is messy - see the comments.
Here's another activity-based proteomics result that I've been meaning to link to - in this one, the Cravatt group strengthens the case for carboxylesterase 3 as a potential target for metabolic disease. From what I can see, that enzyme was first identified back in about 2004, one of who-knows-how-many others that have similar mechanisms and can hydrolyze who-knows-how-many esters and ester-like substrates. Picking your way through all those things from first principles would be a nightmare - thus the activity-based approach, where you look for interesting phenotypes and work backwards.
In this case, they were measuring adipocyte behavior, specifically differentiation and lipid accumulation. A preliminary screen suggested that there were a lot of serine hydrolase enzymes active in these cells, and a screen with around 150 structurally diverse carbamates gave several showing phenotypic changes. The next step in the process is to figure out what particular enzymes are responsible, which can be done by fluorescence labeling (since the carbamates are making covalent bonds in the enzyme active sites. They found my old friend hormone-sensitive lipase, as well they should, but there was another enzyme that wasn't so easy to identify.
One particular carbamate, the unlovely but useful WWL113, was reasonably selective for the enzyme of interest, which turned out to be the abovementioned carboxyesterase 3 (Ces3). The urea analog (which should be inactive) did indeed show no cellular readouts, and the carbamate itself was checked for other activities (such as whether it was a PPAR ligand). These established a strong connection between the inhibitor, the enzyme, and the phenotypic effects.
With that in hand, they went on to find a nicer-looking compound with even better selectivity, WWL229. (I have to say, going back to my radio-geek days in the 1970s and early 1980s, that I can't see the letters "WWL" without hearing Dixieland jazz, but that's probably not the effect the authors are looking for). Using an alkyne derivative of this compound as a probe, it appeared to label only the esterase of interest across the entire adipocyte proteome. Interestingly, though, it appears that WWL13 was more active in vivo (perhaps due to pharmacokinetic reasons?)
And those in vivo studies in mice showed that Ces3 inhibition had a number of beneficial effects on tissue and blood markers of metabolic syndrome - glucose tolerance, lipid profiles, etc. Histologically, the most striking effect was the clearance of adipose deposits from the liver (a beneficial effect indeed, and one that a number of drug companies are interested in). This recapitulates genetic modification studies in rodents targeting this enzyme, and shows that pharmacological inhibition could do the job. And while I'm willing to bet that the authors would rather have discovered a completely new enzyme target, this is solid work all by itself.
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