The molecular biology/chemical biology tools we have now are quite something, and have opened up whole areas of research that previously wouldn’t have been feasible. But as a chemist, I’m glad to say that there’s often still nothing like a small molecule. That’s one of the things I take away from this recent paper in Cell, a multicenter collaboration between Dana-Farber, Harvard, Yale, the Broad Institute, and Scripps-Florida. They’re looking at a powerful protein involved in metabolic pathways, PGC-1-alpha. It’s a coactivator protein with the PPAR nuclear receptors, and is a central player in mitochondrial activity, adaptation to cold temperatures, exercise, lipid and glucose handling and more.
You’d think that this would be a fine target for pharmaceutical intervention, and to a first approximation you’re right. Type II diabetes is the most obvious target, but it’s not easy to see how you’d get a drug to work. If you knock down the protein in a whole animal model, it manages to compensate, for the most part, but selective ablation in the liver (in otherwise diabetic animal models) has shown much more promising effects.
That’s because the liver is an inadvertent bad actor in Type II patients. It’s well known that the brain uses glucose as its fuel source, whereas other tissues can switch over to fatty acids if need be. This metabolic switch, which is activated during fasting, also tells the liver to start making glucose from scratch, for the brain to use. Insulin levels are generally believed to be the proximate signal for these events, but Type II patients are characterized by insulin resistance. Their tissues don’t respond to the (rather high) insulin levels they have, and that means (among other things) that their livers assume that they’re in a low-insulin fasting state, and continue cranking away on gluconeogenesis. The last thing a diabetic patient needs being dumped into their blood is a continuous supply of fresh glucose, but that’s just what’s going on.
Many are the mechanisms that have been tried over the years to shut down this inappropriate sugar source, and PGC-1a is on that list as well. But PGC has no small-molecule binding sites of its own – it’s not regulated in that fashion, and it does its work by a variety of protein-protein interactions. These are, of course, notoriously hard to target with small molecules. It’s not impossible, but if you have another potential mechanism, you should strongly consider it as an option. In this case, the possible way out is acetylation of PGC-1a. Multiple lysines on it are post-translationally modified in this way, and that’s an important regulatory pathway. Everyone’s old friend in this area, the sirtuin SIRT1, seems to be the deacetylation enzyme that activates PGC-1a in response to fasting (thus a big part of the connection of sirtuin inhibitors to diabetes therapy). If you could find a compound that keeps the protein acetylated, by one mechanism or another, you could have something interesting. This latest paper describes a screen for just such molecules:
Here, we designed and developed a cell-based high- throughput chemical screen using an AlphaLisa assay aimed at identifying chemical scaffolds that induce PGC-1a lysine acetylation. Subsequent secondary assays identified a subset of previously uncharacterized small molecules that were able to reduce glucose production in primary hepatocytes. As a proof of concept of the potential use of these compounds as anti-diabetic drugs, a single hit from our screen reduced fasting blood glucose, significantly increased hepatic insulin sensitivity, and improved glucose homeostasis ameliorating diabetes in dietary and genetic mouse models.
That hit is SR-18292, shown at right. The original hit was the analog with a 3-methyl on the indole ring, but that one (from the NIH libraries collection) wasn’t available in quantity, so the group synthesized the desmethyl shown, which seems equally active. (Several other close analogs with changes around the indole portion of the molecule were inactive, though). Interestingly, it retained that activity in cells even in the presence of sirtuin inhibitors and HDAC inhibitors, which suggests that it’s working independently of the deacetylation pathways.
As mentioned in that quote above, the compound has just the downstream effects, in cells and in whole animals, that you’d expect from a compound that is (somehow) increasing the amounts of acetylated PGC-1a versus its more active deacetylated form. But the exact mechanism still isn’t clear – tracking that down could itself lead to new sorts of diabetes targets, of course, since that could be screened on its own. This paper, then, is a good example of the sort of thing that a solid phenotypic screen can pull out for you – a useful small molecule that might (if everything goes well) lead to a therapy on its own, but at the very least a tool that lets you look into processes that you didn’t even know existed before. I will be glad to see what comes of both of these lines of research.
Update: the comments to this post raise a number of interesting questions about this compound – I’ve written to the authors for comment, and will report back with any news.