Molecular biologists are rewriting the textbook explanation of steroid action. For 40 years, evidence has accumulated that some of the hormonally induced effects seemed too rapid for the classic model, in which steroids activate cytosolic receptors to modulate transcription. This evidence casts doubt on the so-called genomic pathway as the sole mode of steroid action. Increasing research now highlights the key role played by non-genomic pathways in hormone action.
The rewriting doesn't end there. Future textbook authors may need to include a virtual spider's web of modulatory proteins that seem to fine-tune clinical outcomes. They also might report a higher number of tissues recognized as steroid hormone sources. Researchers have, after all, isolated from the central nervous system the full array of enzymes needed for steroid synthesis. The finding could have implications for diseases as diverse as hypertension and depression.
Overall, new chapters on steroid action will describe a subtle, coordinated system. The transcriptional actions of steroids program the cell's long-term fate. Nongenomic actions continuously modulate this long-term program, allowing cells to adapt rapidly to environmental changes. "Transcriptional signaling of steroid hormones may be seen as the engine that drives the car, while nongenomic signaling may represent the steering wheel," says Tommaso Simoncini of the University of Pisa, Italy. "From an evolutionary point of view, it is easy to understand how a double regulation must have been advantageous."
In the old model, hormones released by the adrenal glands cross cell membranes and bind to cytosolic glucocorticoid and mineralocorticoid receptors. Accessory proteins, which ensure that the receptor's structure is conducive to corticosteroid binding, dissociate. The hormone-receptor complex migrates to the nucleus and interacts with specific regulatory elements to control transcription. A clinical response can take several hours.
The first hints of nontranscriptional actions emerged in 1963 when researchers found that vascular resistance changed within five minutes of injecting aldosterone, a mineralocorticoid.1 This response time is too quick for transcription. Aldosterone also rapidly changes sodium exchange in erythrocytes, which lack a nucleus; therefore, the effect could not be genomic. Since then, researchers have identified several nongenomic actions.
Steroids seem to exert some of these nongenomic actions, such as ion channel regulation, by acting on the cell membrane. Other effects, such as the direct regulation of protein and lipid kinase cascades, may arise in the cytosol. In some cases, these nongenomic signaling cascades also modify expression. "The field is still in its infancy, and we are just beginning to identify and classify actions," Simoncini says. Nevertheless, it's clear that these nongenomic actions allow steroid hormones to finely modulate dynamic cellular functions. "These effects may help the cells adapt and react to changes in the environment," he says.
AN INTRACELLULAR WEB A "spider's web" of related modulat-ory proteins further fine-tunes steroidal actions, comments Stoney Simons, of the National Institute of Diabetes and Digestive and Kidney Diseases. "Changing the concentration or activity of one protein can, via a series of coupled interactions, indirectly influence the properties of another," he says.
In a recent review, Simons underscores the modulatory role of coactivators and corepressors.2 For example, a coactivator protein called AIB1 binds to steroid receptors and promotes gene induction. Clinically, variable AIB1 expression in breast cancer cells may explain the partly different responses of tumors to circulating estrogens and antiestrogen therapies.
Moreover, until recently, researchers believed that thyroid and glucocorticoid receptors did not interact. But Simons' group found that changing thyroid receptor levels alters the glucocorticoid receptor's binding characteristics. Both receptors compete for limited amounts of the same coactivators and corepressors. "As more connections between the proteins of the spider's web are uncovered, we can expect additional nongenomic influences," Simons predicts. Modulator polymorphisms, he says, also could be "very important" in determining the final outcomes of endogenous and therapeutic steroids.
The spider's web is growing slowly. Researchers have managed to clone just two nongenomic receptors: the brassino-steroid receptor in plants and a progesterone receptor in sea trout cell membranes. "There is a big hunt for specific receptors that transmit nongenomic steroid action," comments Martin Wehling, University of Heidelberg, Germany.
MEMBRANOUS BEGINNINGS Some nongenomic actions, such as ion channel regulation, emerge when steroids act on the cell membrane. But Axel Alléra, University of Bonn, Germany, says he believes that genomic steroid action also starts at the plasma membrane. "There hasn't been a single cogent and convincing experiment in the history of research on steroid hormone action that supports the thesis of passive transmembrane diffusion," he says.
Alléra's research focuses on a plasma membrane protein, the steroid hormone recognition and effector complex (SHREC), that recognizes and actively imports steroids such as corticosterone, cortisol, and particular gestagens and estrogens into rat and human liver cells. In vitro and in vivo studies suggest SHREC mediates rapid, nongenomic responses to steroids. In addition, Alléra says that SHREC is "undoubtedly involved" in genomic steroid action, a process called membrane-initiated steroid signaling.
The ligand-SHREC interaction triggers signaling. Active importation of the ligand into the cell terminates the trigger events. The signal is transduced to glucocorticoid receptors, which is then transferred, not necessarily as a liganded complex, to the nucleus, Alléra says. So, SHREC may link nongenomic steroid responses to genomic effects. "We hope that our research on SHREC accelerates the slowly growing consensus in support of an integrated view of peptide and steroid hormone action," he concludes.
Courtesy of Tommaso Simoncini, University of Pisa
STEROIDS ON THE BRAIN Recent research also expands the number of tissues that seem to synthesize steroids. Traditionally, researchers believed that only the adrenal cortex possessed the full array of enzymes to synthesize corticosteroids from cholesterol. But Scott MacKenzie from the University of Glasgow, Scotland, demonstrated recently that the rat brain produces aldosterone and corticosterone (the rat equivalent of cortisol).3
Other groups found that the cardiovascular system also expresses at least some of the synthetic enzymes. "Circulating adrenal corticosteroids exert certain effects through actions on their receptors in the brain," he says. "We hypothesize that these locally generated corticosteroids might act in a paracrine manner to modulate a potentially diverse range of characteristics such as blood pressure, learning ability, the acquisition of memory, the degeneration of the brain in old age or under chronic stress, and the development of depressive behavior."
Nevertheless, fundamental differences have emerged between steroid synthesis in the brain and the adrenal gland. Corticosteroid synthesis involves converting deoxycorticosterone (DOC) to either aldosterone by aldosterone synthase or to corticosterone by 11b-hydroxylase. The enzymes are never expressed in the same adrenal zone and therefore do not compete for DOC. "Surprisingly, in the brain we always see the enzymes coexpressed in the same region and even within the same cell," MacKenzie says. "Therefore, in the brain, aldosterone synthase and 11b-hydroxylase must compete for DOC .... We have no idea why this should occur in the brain, or what the implications for steroid production might be."
Indeed, whether nonadrenal synthesis is physiologically significant remains an open question, especially given the low steroid levels produced locally, as MacKenzie admits. "Unless we can demonstrate a physiological effect of the locally produced corticosteroids, then it remains nothing more than a curiosity. There is still the possibility that this local system is nothing more than an evolutionary hangover that is now obsolete."
But MacKenzie says he believes a role will emerge. Several strands of evidence, he says, support a role for locally produced steroid. For example, the brain expresses all the enzymes required to convert cholesterol to corticosteroids, he explains. Furthermore, aldosterone synthase expression and 11b-hydroxylase closely correlate anatomically with the mineralocorticoid receptor. "So, high amounts of steroid production may not be required to exert a significant effect," he says. Finally, unpublished research shows that feeding animals low sodium diets increases expression of the aldosterone synthase gene in the hippocampus and cerebellum. "This response encouraged us to believe that we are dealing with an active system," MacKenzie says. "Nevertheless, conclusive evidence is yet to be published."
In the future, the nongenomic actions of steroids could become the target for new drugs. Aldosterone's nongenomic actions seem to contribute to cardiac fibrosis and arrhythmias. And spironolactone, an aldosterone antagonist, reduces mortality and morbidity in patients with severe heart failure. Heidelberg's Wehling speculates that a "super-antialdosterone" that blocks the genomic and nongenomic effects should be even more beneficial than spironolactone. "Given that aldosterone seems to be deleterious for the cardiovascular system, improved understanding of the mechanisms of action ... seems to be a very attractive aim," he says
A growing body of studies in cardiovascular medicine underscores the clinical and biological importance of steroidal nongenomic action. Last year, Simoncini's group reported that "extremely short exposure" to glucocorticoids reduced the area of myocardial infarction in a mouse model of induced heart ischemia by 30%.4 "This is absolutely fantastic," he concludes, "and fully highlights the biological potency arising from steroid hormones' nongenomic signaling."
Mark Greener (email@example.com) is a freelance writer in Cambridge, UK.
1. K. Klein et al., "Klinisch-experimentelle Untersuchungen über den Einflub von Aldosteron auf Hämodynamik und Gerinnung," Z Kreisl Forsch, 52:40-53, 1963.
2. S.S. Simons, "The importance of being varied in steroid receptor transactivation," Trends Pharmacol Sci, 24:253-9, May 2003.
3. E. Davies et al., "Extra-adrenal production of corticosteroids," Clin Exp Pharmacol Physiol, 30:437-45, July 2003.
4. A. Hafezi-Moghadam et al., "Acute cardiovascular protective effects of corticosteroids are mediated by non-transcriptional activation of endothelial nitric oxide synthase," Nat Med, 8:473-9, 2002.