July 6, 1998 At least your cells know how to limit their stress.
Scientists at Northwestern University have learned how stress molecules in cells turn off their own production so they don't keep cells in a chronically stressed state even if adverse conditions persist. The finding, reported in the July issue of Genes & Development, identifies a new off-switch molecule in the control of the stress response and may have implications in the cell death that occurs in aging and in diseases from infections to heart disease and stroke.
Cells produce heat shock proteins, or HSPs, in response to stress caused by heat, poisons or signals from nerves or hormones. These heat shock proteins are sometimes called molecular chaperones, because their role is to protect and usher other protein molecules around in the cell. Even under non-stressed conditions, lower levels of chaperones are essential to protect newly made proteins from clumping together or being broken down before they can fold up into their final shape or be threaded through internal membranes to the cellular compartment where they are needed.
"Chaperones play an essential, normal role in regulating protein equilibrium, the balance between synthesis and degradation," said Richard I. Morimoto, John Evans Professor of Biology, who led the study. "If you expose the cell to stress -- whether heat stress, or oxygen stress as occurs in infection, immunity, myocardial stress or neurodegenerative disease -- the cell responds by making more chaperones to protect its vital proteins," he said.
But the level of chaperones is critical, and too much is also harmful to the cell. Even as they protect other proteins, chaperones straitjacket them and prevent them from carrying out their vital functions.
"If you make too much of the chaperones, the cell is never going to get back to its normal state," said Morimoto, who is also chair of biochemistry, molecular biology and cell biology and director of the Rice Institute for Biomedical Research. "Sustained exposure to stress causes cell death, so the level of chaperones is critical to the balance of cell survival and cell death."
The cell's need for relief accounts for an earlier finding: that the cell produces high levels of chaperones only briefly, even if stressful conditions persist. This self-limiting response suggested a feedback loop in which chaperones cause the shutdown of their own production.
Morimoto and his coworkers focused on a master protein called heat shock factor, which is known to spur the genes that encode chaperones to make more of them during stress. Earlier this year they reported test tube experiments showing that certain chaperone proteins can bind to and regulate heat shock factor, thus providing a molecular feedback loop to precisely control the level of chaperones made following stress.
Now they have found in the cell's nucleus another signal molecule they call HSBP-1, the first known "off-switch" regulator of chaperone production. HSBP-1 binds both to the chaperones and to heat shock factor and prevents the factor from activating genes to produce more chaperones.
"This is the first time we've seen a new regulatory molecule other than chaperones that controls the heat shock response," Morimoto said. The new molecule, he said, may explain the decline in cellular health that occurs with age.
"As cells age, the heat shock response doesn't function properly -- just when it needs to be most efficient. This molecule may be the reason why. It works together with the chaperones to sense what's going on and shuts off the response."
The heat shock response was first observed in fruit flies and later in yeast. Morimoto's group was first to clone a human heat shock gene in 1985, and since then the heat shock response has been regarded by scientists as universal. HSBP-1 exists in higher multicellular organisms.
"Many of our cells, especially nerve cells, divide rarely, if at all," Morimoto said. "The reason we have sophisticated, highly regulated systems has a lot to do with how long a single cell must live."
Morimoto next wants to compare the HSBP-1 from organisms of widely differing lifespans.
"If we can demonstrate how regulation of the heat shock response is defective in aged cells, this molecule could be a target for drug development," he said.
Other researchers on the paper are Sanjeev H. Satyal, Dayue Chen, Susan G. Fox and James M. Kramer of Northwestern Medical School.
The research was funded by the National Institutes of Health.
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