The JNK cascade as a biochemical switch in mammalian cells: ultrasensitive and all-or-none responses.

JNK proteins are ubiquitously expressed, evolutionarily conserved MAP kinases that are involved in stress responses. Recently, it was shown that the JNK cascade in Xenopus oocytes exhibits sustained, all-or-none responses to graded, transient stimuli. Here, we have examined the character of the JNK cascade's response in mammalian cells. The steady-state responses of JNK to sorbitol and anisomycin were found to be highly ultrasensitive in HeLa cells, HEK 293 cells, and Jurkat T cells. The JNK responses were also reversible, not sustained, as was the case in oocytes. Jurkat cells activated their JNK in response to phorbol myristate acetate (PMA), and the response of the entire population of Jurkat cells was graded. However, analysis of subpopulations of the PMA-treated Jurkat cells revealed that the steady-state responses of both JNK and CD69, a T cell surface activation marker, were essentially all-or-none in character. These studies show that the JNK cascade commonly exhibits switch-like responses to a variety of stimuli.