Hypothermic stress leads to activation of Ras-Erk signaling

The small GTPase Ras is converted to the active, GTP-bound state during exp osure of vertebrate cells to hypothermic stress. This activation occurs mor e rapidly than can be accounted for by spontaneous nucleotide exchange. Ras -guanyl nucleotide exchange factors and Pas GTPase-activating proteins have significant activity at 0 degrees C in vitro, leading to the hypothesis th at normal Ras regulators influence the relative amounts of Ras-GTP and Ras- GDP at low temperatures in vivo. When hypothermic cells are warmed to 37 de grees C, the Raf-Mek-Erk protein kinase cascade is activated. After prolong ed hypothermic stress, followed by warming to physiologic temperature, cult ured fibroblasts assume a rounded morphology, detach from the substratum, a nd die. All of these biologic responses are attenuated by pharmacologic inh ibition of Mek. Previously, it had been found that low temperature blocks a cute growth factor signaling to Erk. In the present study, we found that th is block occurs at the level of Raf activation. Temperature regulation of R as signaling could help animal cells respond appropriately to hypothermic s tress, and Ras-Erk signaling can be manipulated to improve the survival of cells in cold storage.