A bee sting, a nasty cut or burn, a viral infection, all put out urgent911-type calls for help. Those mayday distress signals go straight tothe command center of the body's immune defenses.
Staffing that system's traffic controls are a hundred or so moleculespoised to turn on the ever-ready T-cell-mediated and B-cell humorallegions, to repel or remove the offending immunogenic invaders.
These vast echelons of first-alert proteins are the cytokines andgrowth factors. Among the earliest-discovered and best known ofthem is, or rather are, the interferons (IFN) _ so-called because theyinterfere with the enemy's infectious or invasive attack.
IFN-alpha and beta both inhibit viral replication.
So does IFN-gamma, but it has many other missions in life as well,derived largely from its secretion by T helper cells.
Besides three interferons, the cytokine message dispatchers number amuch more numerous array of interleukins, numbered IL-1 throughIL-16, and still counting. Also high on the list of cytokines inbiotechnology-based drug development are tumor necrosis factor andtransforming growth factor.
Before any of these cytokine genes can get transcribed for activeduty, they have to have their card punched by a specific receptor.
And that's where matters stood until just a few years ago, whenscientists discovered the missing link between signal andtranscription.
"It was never clear," said molecular immunologist Michael Grusby,"how the binding of a growth factor or cytokine on the cell surfacereceptor actually transduced [forwarded] the signal to induce thetranscription of particular genes in that cell."
He added, "The things that bridge those two functions are recentlydiscovered proteins called STATs _ standing for `signal transducersand activators of transcription.'"
Grusby, who is in the cancer biology department at the HarvardSchool of Public Health in Boston, explained the way STATmolecules work: "As signal transducers, they actually dock tocytokine or growth factor receptors, and thus become activated. Thenthey translocate from cell surface to nucleus, and serve astranscription factors, inducing the DNA to transcribe new geneproteins."
New Protein On The Block
Since 1992, investigators have put six STATs on the map, numberedone through six. How this small clutch of molecules can ride herd onclose to 100 cytokines and growth factors is answered in the Aug. 9,1996, Science.
Its title: "Cooperative DNA binding and sequence-selectiverecognition conferred by the STAT-amino-terminal domain." Thereport's senior author is Timothy Hoey, a STAT-focused scientist atTularik Inc., of South San Francisco.
"The issue is this," Hoey explained to BioWorld Today: "How doesone cytokine turn on a particular subset of genes in a differentcytokine on a different subset? It was a mystery as to how they couldbind to promoter sequences of different genes.
"What we did in the Science paper," he continued, "was to identifynatural STAT binding sites near the IFN-gamma gene."
These sites are arranged in adjacent multiple copies. "They'repromoter elements," Hoey continued, "essentially a DNA sequence inthe flanking region of the gene, controlling whether it's going to beturned on or off. Their proximity to each other allows STAT proteinsto interact when they bind to the neighboring sequences."
Tularik's report in Science also added another data point to STAT'semerging image: "We showed," Hoey said, "that the cooperativebinding which is responsible for this selective recognition is mediatedby the amino-terminal domain of the protein. This domain isconserved among the STAT family, with six different members, andits function previous to this paper was not known."
Knockout Mice Encourage Drug Discovery
Hoey and Grusby are co-authors of a paper in Nature dated July 11,1996, which bears the title: "Impaired IL-12 responses and enhanceddevelopment of TH2 cells in STAT4-deficient mice."
Hoey observed that "IL-12 is a key regulator of the cell-basedimmune response. In our Nature paper, the STAT-minus knockoutmice showed that STAT4 is essential for IL-12 signaling."
He pointed out that "this indicates that STAT4 is not involved in anyother biological physiological functions. It's dedicated to thefunctioning of mature lymphocytes [T cells], and that's basicallygood news from the drug development point of view."
STAT4 would be a good drug target, Hoey continued, "becausemaking an inhibitor to it would specifically affect IL-12 signaling,which is involved in things like transplant rejection and certainautoimmune diseases. This is what you would want for developing anovel immunosuppressant, without affecting many other processes inthe body."
A related protein, STAT6 is involved in IL-4 signaling. "So thecompany's focus, he said, "is really these two proteins."
He pointed out that "These things were only recently discovered, andthe screens are just under way. We're still several years away fromknowing if we're going to have something clinically useful."
Tularik has applied for a patent on the human STAT4 gene. "Wewere the first to clone that," Hoey said.
"Also," he went on, "the STAT6 protein was discovered, purified andcloned by Steve McKnight's lab at Tularik. And of course, inhibitorsare patentable, so we're looking for inhibitors of STAT activation." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.