Cytokines, Stress, and Growth Modulation
The relationship between cytokines, stress, and growth is complex. Stress can be a perception, cognitive or more biochemically subtle, that something in the internal or external world is awry; that something is acting as a pressure to change or to threaten the stability and well-being of the perceiver. Responses to stress can be thought of as attempts by the body to maintain homeostatic equilibrium. Cytokines participate in the response to stress and serve several functions—some beneficial and others, however, potentially harmful. Cytokines, as a group, respond to challenges from infective diseases, the environment, and behavior. Responses in the brain and immune system are expressed as effector signals in the form of regulatory hormones from the hypothalamus and pituitary and as cytokines from the immune system. Following “recognition” of the challenge, and the elaboration of effector signals, the body must appropriately coordinate responses for individual and specific tissue needs. How does stress manifest itself in that across the total spectrum of stress levels we can experience a continuum of tissue responses that range from enhanced and anabolic to stagnant and catabolic? How does stress impact growth in the context of this 262challenge? Some growth processes are slowed or halted so that nutrients can be utilized efficiently in the homeostatic process. This can mean that some tissues normally accustomed to anabolic accretion processes will change roles and become nutrient donors during stress. This integration of nutrition with the endocrine and cytokine responses to stress is an essential component of the complex growth response to stress discussed in the rest of this chapter.