By David N. Leff
A scientist, now in Texas, is the discoverer of a "CAR" that runs in reverse. When a key is inserted into the CAR-beta receptor, CAR switches off the target genes.
"The major point," explained molecular biologist David Moore, a professor of cell biology at Baylor College of Medicine, in Houston, "is that CAR's mechanism of action is the opposite of conventional receptor activity. This orphan molecule, which we named CAR, standing for 'constitutively acting receptor,' behaves like a conventional receptor running in reverse — like a car running backwards."
When Moore discovered CAR-beta several years ago, while he was at Massachusetts General Hospital, in Boston, he saw that it was a so-called 'orphan' receptor — a docking protein inside the cell's nucleus, waiting for its ligands — ships that never came in.
"We were interested in identifying as many orphan receptors as we could," Moore recalled, "so we initiated a program to clone them using oligonucleotide probes. CAR-beta was one of the first we got. It is one of only two new steroid receptors discovered in the last three decades."
Moore is senior author of a paper in today's Nature, dated Oct. 8, 1998. Its title is, "Androstane metabolites bind to and deactivate the nuclear receptor CAR-beta." By a deft acronymic twist, the middle letter of CAR turned into "androstane."
The article's lead author is molecular biologist Barry Forman, now at City of Hope National Medical Center, in Duarte, Calif.
"Forman," Moore recounted, "when a post-doctoral fellow at the Salk Institute [in La Jolla, Calif.] initiated a screen, trying to identify new orphan ligands. And he had these two androstane steroids in his pool. When he tried to hook them up to receptors, he found — to both of our surprise — that instead of making CAR more active, they actually shut it off.
"We knew from when we had first started characterizing CAR a number of years ago that this receptor behaved as a constitutive activator. That's to say, it was always 'on' in the absence of any added ligands or hormones.
"Androstanes," Moore told BioWorld Today, "are naturally occurring mammalian steroids — endocrine metabolites. They're present at low levels in normal human blood, but their biological roles have been a mystery until now. We still don't know what they might be doing specifically, outside of the fact that they can turn off the receptor CAR-beta."
The two androstanes in particular, androstenol and androstanol that turn off CAR's ignition, act as inverse agonists. "It's really a quite new mechanism," Moore pointed out, "in the context of the way these receptors work. The opposite to the conventional paradigm, where an agonist activates the receptor. In our case, it deactivates it instead."
Moore surmised that this mechanism "is likely to be shared by other members of the ligand-regulated super-family of transcription factors." He added, "Maybe the peculiar exception may actually be a more common mechanism of action."
Baseball Hero's Supplement Linked To CAR-beta Receptor
As for what these androstane metabolites actually do, he observed, scientists "still have a long way to go to figure that out specifically. But we certainly believe that since all of the other known steroid receptors have quite significant medical importance, it's certainly possible that this one will as well, though it's premature to say right now what that might be."
Forman told BioWorld Today that Mark McGwire, of current baseball fame, "announced that he had been taking androstenedione all year — presumably to boost his batting performance. Androstenedione," Forman pointed out, "is a chemical cousin of androstenol, the just-discovered CAR ligand. I'm not sure anyone knows what androstenedione really does. We're interested in finding the ligand, and using that as a tool to learn what the new physiology is, what diseases these hormones might be involved in."
"We know," Moore continued, "that the CAR receptors bind to DNA sites that are regulatory elements in particular genes. They are essentially the same as those recognized by the retinoid receptors. So, we believe that the CAR protein may actually overlap with retinoid receptors in terms of regulating target gene expression.
"In that context," Moore pointed out, "retinoids potentially regulate a variety of metabolic genes in the liver — where we know CAR is expressed — including things involved in lipoprotein metabolism. So, it could have quite a significant effect on response to nutrition, for example."
Putative, Prospective Therapeutic Potential
Forman ventured: "Much like the discovery of other hormones, such as insulin and estrogen decades ago, we hope that the discovery of this new hormone, and its novel mode of action, will lead to the production of new drugs to treat, or perhaps prevent, diseases such as cancer, atherosclerosis and diabetes."
In this vein, Moore observed that his ongoing research at Baylor "is looking in two directions. One of them is biochemical, or pharmacological, in that we're trying to see if we can identify additional compounds that behave in the same way that androstenol — one of those two androstane ligands — does. [We're] trying to identify the range of natural, and potentially synthetic, ligands for CAR.
"The point is," he pointed out, "that CAR is now one of several orphan receptors that have been linked up with new ligands. And we certainly believe that its brethren that remain to be hooked up represent quite an important potential resource for identifying pharmacologically important agents."
Moore's primary focus at present is to trying to generate a mouse knockout model lacking the murine CAR-beta gene. "We're well along toward that," he concluded, "though we don't have the animals yet." *