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November 29, 2005
Ghrelin and Obestatin
There's a peptide hormone called ghrelin that a lot of pharmaceutical companies have worked on in the last few years. It's a good target for obesity (and perhaps diabetes, too), since it's involved in appetite signaling between the stomach and the central nervous system. It's also involved in growth hormone signaling, too, though, so the situation is complicated.
And it just got more so. It turns out, according to a paper in the November 11 issue of Science. A group at Stanford has discovered that the same percursor protein that's carved up to produce ghrelin is also used to produce another peptide hormone that they've called obestatin. That one has its own receptor, and its own signaling network, and it appears to do the exact oppositeof what ghrelin does. Injections of ghrelin stimulate feeding in mice, and injections of obestatin inhibit it, for example. Similarly, ghrelin increases gastric emptying, and obestatin slows it down. (One place where the two peptides don't match up is their effects on growth hormone secretion - obestatin doesn't seem to do anything to the growth hormone axis at all).
So now we know more about the regulation of appetite than we used to, although researchers in that field probably thought it was complicated enough already, thanks very much. What I find particularly interesting about this discovery is how these two opposing hormones are cut from the same larger protein. That means that they both come from the same gene, you know. Which shows you just how far a pure genome-driven approach to drug discovery will get you: not far enough. You'd never know about ghrelin from just reading off human genes, because it's produced after the orginal protein is transcribed. And you'd never know that the same protein is the source for another hormone that negates ghrelin, either. All that complexity is downstream of the DNA. (Update: see the comments for some dissenting voices on this issue).
We already knew that general principle, of course. As soon as the estimates of the total number of human genes starting coming in, it was clear that they were way too low to explain the number of different proteins that we already knew about. But examples like this one just rub it in. . .
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