Heat Stroke Up Regulates Interleukin-6 In Skeletal Muscle.

Abstract

Circulating interleukin 6 (IL-6) is elevated in whole body hyperthermia and may play a role in whole body thermotolerance. Previously, we have identified heat as a unique stimulus of IL-6 transcription in skeletal muscle (SM) in both cell culture and in vitro mouse muscle. PURPOSE: The objectives of this study were: 1) to determine if C2C12 secrete IL-6 protein in response to a heat stimulus, 2) to develop a relevant in vivo model to test the effects of hyperthermia sufficient to induce a heat stroke (HS) event on IL-6, TNF-α, and heat shock protein 72 (HSP72) transcription in SM, and 3) to test the effects IL-6 supplementation on SM IL-6, TNF-α, and HSP72 transcription during heat stroke. METHODS: C2C12 myoblasts were grown to confluence, exposed to 42°C for 1 hour (hr) and recovered at 37°C for 3 hr. Cellular supernatant was removed at three time points: before exposure to 42°C (BASE), immediately after exposure to 42°C for 1hr (HEAT), and after 3 hr of recovery at 37°C (REC). Cellular supernatant was analyzed for IL-6 protein using ELISA. For HS studies there were three groups: all mice were anesthetized. Mice in which heat stroke was induced (HSM) were exposed initially to 39.5°C and then environmental temperature was increased by 0.5°C every 30 mins until core temperature reached a 42.4°C maximum. Mice recovered for 30 min prior to tissue collection. The IL-6 injected mice (IL-6M) were given 0.5ug IL-6 (I.P.) for 2-4 hr prior to beginning the HS protocol. An identical procedure was followed for sham control (CON) mice, without heat exposure. RESULTS: Significant up regulation of IL-6 protein secretion was seen during the REC period following heat exposure (p<.05). Additionally, in our anesthetized HS model, increases in muscle IL-6 mRNA and HSP72 transcription (p<.05) were observed compared to the CON group. IL-6 treatment (IL-6M) resulted in a significant attenuation in the up regulation of HSP72 compared to HSM. CONCLUSION: These results further demonstrate that hyperthermia is a functional stimulus for IL-6 transcription and translation. Furthermore, IL-6 IP injections suppress the heat shock protein response of SM. We speculate that IL-6 produced by skeletal muscle is a significant source of circulating IL-6 seen following hyperthermia exposure in man and animals and may have a coordinating influence on the expression of other heat-stimulated proteins important in the overall response to hyperthermia.