Heat shock proteins (HSPs) are a family of proteins that are produced by cells in response to exposure to stressful conditions. They were first described in relation to heat shock, but are now known to also be expressed during other stresses including exposure to cold, UV light, and during wound healing or tissue remodeling. Many members of this group perform “chaperone” functions by stabilizing new proteins to ensure correct folding or by helping to refold proteins that were damaged by the cell stress. This increase in expression is transcriptionally regulated. The dramatic upregulation of the heat shock proteins is a key part of the heat shock response and is induced primarily by heat shock factor (HSF). HSPs also prevent muscle atrophy and much more.
Increased production of heat shock proteins (HSPs) promotes muscle growth and reduces protein degradation. Protein degradation occurs naturally during both muscle use and disuse. HSPs induced by heat help to both prevent and repair damaged proteins. HSPs are used by the cells to counteract potentially harmful stimuli. HSPs can prevent damage by scavenging free radicals and supporting cellular antioxidant capacities via their help in maintaining glutathione levels. HSPs also repair misfolded and damaged proteins so proper structure and function is maintained.
HSPs are most commonly activated as part of the body’s hormetic response to heat stress from physical activity or hyperthermia (heat exposure). In the brain, HSPs prevent and even reverse the accumulation of aggregates include amyloid beta plaques known to cause Alzheimer’s Disease. HSPs are found in virtually all living organisms, from bacteria to humans. Production of high levels of heat shock proteins can also be triggered by exposure to different kinds of environmental stress conditions, such as infection, inflammation, exercise, exposure of the cell to toxins (ethanol, arsenic, trace metals, and ultraviolet light, among many others), starvation, hypoxia (oxygen deprivation), nitrogen deficiency (in plants), or water deprivation. As a consequence, the heat shock proteins are also referred to as stress proteins and their upregulation is sometimes described more generally as part of the stress response.
The mechanism by which heat-shock (or other environmental stressors) activates the heat shock factor has been determined in bacteria. During heat stress outer membrane proteins (OMPs) do not fold and cannot insert correctly into the outer membrane. They accumulate in the periplasmic space. These OMP’s are detected by DegS, an inner membrane protease, that passes the signal through the membrane to the sigmaE transcription factor. However, some studies suggest that an increase in damaged or abnormal proteins brings HSPs into action. Some bacterial heat shock proteins are upregulated via a mechanism involving RNA thermometers such as the FourU thermometer, ROSE element and the Hsp90 cis-regulatory element. Several heat shock proteins function as intra-cellular chaperones for other proteins. They play an important role in protein-protein interactions such as folding and assisting in the establishment of proper protein conformation (shape) and prevention of unwanted protein aggregation (as shown below). By helping to stabilize partially unfolded proteins, HSPs aid in transporting proteins across membranes within the cell.
Some members of the HSP family are expressed at low to moderate levels in all organisms because of their essential role in protein maintenance. Heat-shock proteins also occur under non-stressful conditions, simply “monitoring” the cell’s proteins. Some examples of their role as “monitors” are that they carry old proteins to the cell’s “recycling bin” (proteasome) and they help newly synthesized proteins fold properly. These activities are part of a cell’s own repair system, called the “cellular stress response” or the “heat-shock response”.
APL (American Performance Labs) is a research group dedicated to the collection, analysis, and dissemination of published research and articles on the science of hyperthermia and the various applications, technologies and protocols for the use of hyperthermic conditioning.