Cellular stress response

Cellular stress response is a general term covering a wide range of molecular changes that cells undergo in response to environmental stressors, including extremes of temperature, exposure to toxins, and mechanical damage. The various processes involved in cellular stress responses serve the adaptive purpose of protecting a cell against unfavorable environmental conditions, both through short term mechanisms that minimize acute damage to the cell's overall integrity, and through longer term mechanisms which provide the cell a measure of resiliency against similar adverse conditions.^[1]

General characteristics

Cellular stress responses are primarily mediated through what are classified as stress proteins. Stress proteins often are further subdivided into two general categories: those that only are activated by stress, or those that are involved both in stress responses and in normal cellular functioning. The essential character of these stress proteins in promoting the survival of cells has contributed to them being remarkably well conserved across phyla, with nearly identical stress proteins being expressed in the simplest prokaryotic cells as well as the most complex eukaryotic ones.^[2]

Stress proteins can exhibit widely varied functions within a cell- both during normal life processes and in response to stress. For example, studies in Drosophila have indicated that when DNA encoding certain stress proteins exhibit mutation defects, the resulting cells have impaired or lost abilities such as normal mitotic division and proteasome-mediated protein degradation. As expected, such cells were also highly vulnerable to stress, and ceased to be viable at elevated temperature ranges.^[1]

Although stress response pathways are mediated in different ways depending on the stressor involved, cell type, etc., a general characteristic of many pathways – especially ones where heat is the principle stressor – is that they are initiated by the presence and detection of denatured proteins. Because conditions such as high temperatures often cause proteins to denature, this mechanism enables cells to determine when they are subject to high temperature without the need of specialized thermosensitive proteins.^[3] Indeed, if a cell under normal (meaning unstressed) conditions has denatured proteins artificially injected into it, it will trigger a stress response.

Response to heat

Cells subjected to heat shock. Cells in slide 'e' exhibit dysmorphic nuclei as a result of this exposure to stress, however 24 hours later cells largely recovered, as shown in slide 'f'.

The heat shock response involves a class of stress proteins called heat shock proteins. These can help defend a cell against damage by acting as 'chaperons' in protein folding so as to ensure proteins assume their necessary shape and do not become denatured and thus useless to the cell.^[4] This role is especially crucial since elevated temperature would, on its own, increase the concentrations of malformed proteins. Heat shock proteins can also participate in marking malformed proteins for degradation via ubiquitin tags.

Response to toxins

Many toxins end up activating similar stress proteins to heat or other stress-induced pathways because it is fairly common for some types of toxins to achieve their effects - at least in part - by denaturing vital cellular proteins. For example, many heavy metals can react with sulfhydryl groups stabilizing proteins, resulting in conformational changes, and other toxins that either directly or indirectly lead to the release of free radicals can generate misfolded proteins.^[2]

Applications

Early research has suggested that cells which are better able to synthesize stress proteins and do so at the appropriate time are better able to withstand damage caused by ischemia and reperfusion.^[5] In addition, many stress proteins overlap with immune proteins. These similarities have medical applications in terms of studying the structure and functions of both immune proteins and stress proteins, as well as the role each plays in combating disease.^[1]

References

1. Welch, William (May 1993). "How Cells Respond to Stress": 56–64.
2. "The Cell Stress Response". Simon Fraser University.
3. Milisav, Irina. "Cellular Stress Responses" (PDF). Advances in Regenerative Medicine.
4. Richter, Klaus; Martin Haslbeck; Johannes Buchner (22 October 2010). "The Heat Shock Response: Life on the Verge of Death". Molecular Cell. 40 (2). doi:10.1016/j.molcel.2010.10.006.
5. Majmunda, Amar; Waihay J. Wong; M. Celeste Simon (22 October 2010). "Hypoxia-Inducible Factors and the Response to Hypoxic Stress". Molecular Cell. 40 (2). doi:10.1016/j.molcel.2010.09.022. PMC 3143508. PMID 20965423.

Further reading