The mitogen-activated protein kinase (MAPK) cascade is one of the most ubiquitous signal transduction systems and is rapidly activated by various stimuli, such as cellular stress and death. The Caco-2 cell line is an in vitro model for colon cancer studies. We investigated the activation status of the ERK1/2, p38, JNK1/2, and ERK5 kinases and their respective upstream intracellular activators in Caco-2 cells induced to proliferate by 10% fetal bovine serum (FBS). The states of phosphorylation of the above MAPKs and their upstream kinases, MEK1/2, MKK3/6, MKK4, and MKK7, respectively, were studied by Western blot analysis. Phosphorylation was barely detectable before serum stimulation, and the stimulation of cell proliferation by the addition of FBS increased MEK1/2 and ERK1/2 phosphorylation 2 to 3 fold after 3 min. FBS stimulated p38 and MKK3/6 to the same extent within 2 min of treatment and JNK1/2 and its upstream kinases MKK4 and MKK7 5-fold (3 min). Addition of FBS also rapidly phosphorylated ERK5 (2 to 3.5-fold between 2 and 5 min) and the transcription factor CREB. Incubation of Caco-2 cells with FBS was followed by a rapid induction of c-Fos and c-Myc expression. Studies with ERK1/2 specific inhibitor PD98059, p38 MAPK inhibitor SB203580, or JNK inhibitor SP600125 showed that FBS regulates Caco-2 cell proliferation via the three MAPK pathways.Fil: Buzzi, Natalia Sol. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Nacional del Sur; ArgentinaFil: Colicheo Oyarzo, Fabiola Andrea. Universidad Nacional del Sur; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Boland, Ricardo Leopoldo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Sur; ArgentinaFil: Russo, Ana Josefa. Universidad Nacional del Sur; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina