Interplay among cell’s nuclear receptors affects transcription
On the football field, the neophyte fan focuses on the receiver, whose daring catch and dash for the goal line captures the attention. However, focusing on that one player means the fan misses the contributions of the quarterback, the blockers and the other players who make that run happen.
The same is true in the cell, where too often a single finding about one protein, enzyme or receptor seems to solve a problem. In an online report in the journal Nucleic Acids Research, Dr. Michael Mancini, professor in the department of molecular and cellular biology at Baylor College of Medicine and his colleagues demonstrate how the interaction of two nuclear receptors--the glucocorticoid receptor and estrogen receptor alpha--modifies the transcription of DNA into RNA.
In the laboratory, graduate student and first author Mike Bolt used a unique model cell system comprised of a multicopy estrogen responsive reporter gene array and high throughput microscopy to determine how estrogen receptor alpha affects DNA transcription and what other cellular components are involved. This novel system enabled them to actually see and simultaneously count the steps that the estrogen receptor took in regulating DNA transcription.
"The experimental platform we developed over the last approximately six years has become ideal for measuring a wide range of biological activities at the same time, and it now can be performed at high throughput speeds," said Mancini, who is also director of the Integrated Microscopy Core at BCM and the co-director of the John S. Dunn Gulf Coast Consortia for Chemical Genomics.
He and his colleagues found that the glucocorticoid receptor could not affect transcription at the array without the presence of another--estrogen receptor alpha. Glucocorticoid receptor was recruited to the response element through two steroid receptor coactivators (SRC-2 andSRC-3) and a mediator component called MED14. (Steroid receptor coactivators are master regulators of cellular activities.)
"In this way, we identified a previously unknown mechanism through which DNA binding of one nuclear receptor can influence the binding of a different receptor (i.e., the glucocorticoid receptor) through use of coregulator intermediates that fine tunes transcriptional readout," said Bolt.
Cross-talk among molecules
Then, with additional guidance of Dr. Fabio Stossi, assistant professor of molecular and cellular biology at BCM, he and his colleagues further studied cultured breast cancer cells to validate this cross-talk among molecules.
"Often, glucocorticoids are administered to women with breast cancer together with chemotherapy in order to alleviate side effects," said Stossi. "Here, we are performing an increasingly high throughput basic science approach to push toward a more translationally relevant system."
"If you reduce your science to one hormone, one stimulus, it physiologically underrepresents what the cell sees. We have to see more biology at the same time to better appreciate how gene regulation works in a cellular context," said Mancini.
This finding may play a role in diseases such as Cushing’s syndrome in women, which results from exposure to high levels of cortisol and/or steroids.
"Could this interplay also occur in bone where estrogen receptor alpha and glucocorticoid receptors are both expressed?" said Stossi. "According to recent literature, high glucocorticoid receptor levels reflect a better prognosis in estrogen receptor-positive breast cancer. Could mechanisms like this be the reason?"
Future work along these lines may help to explain these issues, they said.
Others who took part in this research include Justin Y. Newberg of BCM, Arturo Orjalo and Hans E. Johansson of Biosearch Technologies Inc., in Novato, California.
Funding for this work comes from a National Institute of Environmental Health Sciences Grand Opportunity Award (1RC2ES018789); Keck Foundation pre-doctoral fellowship and imaging/automation resource support from the John S. Dunn Gulf Coast for Chemical Genomics; Dan L Duncan Cancer Center at BCM; Center for Reproductive Biology; the Digestive Disease Center (DK56338-06A2); Keck Center National Library of Medicine Training Program in Biomedical Informatics of the Gulf Coast Consortia National Library of Medicine (T15LM007093); and National Institutes of Health (K12 DK0083014).