L-GLUTAMINE STIMULATES INTESTINAL-CELL PROLIFERATION AND ACTIVATES MITOGEN-ACTIVATED PROTEIN-KINASES

We studied the mechanisms by which L-glutamine (Gln), a major fuel for enterocytes, signals proliferation in intestinal epithelial cell line s. Gin was additive to epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I) in stimulating DNA synthesis, as assessed by [ H-3]thymidine incorporation. Extracellular signal-regulated kinases (E RKs) p42(mapk) and p44(mapk) and Jun nuclear kinases (JNKs) phosphoryl ate and activate nuclear transcription factors. Proteins of the c-Jun, ATF-2, and c-Fos families aggregate to form DNA-binding homodimers or heterodimers called activating protein 1 (AP-1). In vitro assays and functional assays of phosphorylation demonstrated that Gln activates b oth ERKs and JNKs, resulting in a fourfold increase in AP-1-dependent gene transcription. Gln was required for EGF signaling through ERKs. M aximal stimulation of proliferation required similar to 2.5 mM Gln. c- Jun mRNA levels responded to Gln in ''Gln-starved'' porcine IPEC-J2 ce lls and in rat IEC-6 cells. Although Gln metabolism is required for th e proliferative response, several Gln by-products did not stimulate [H -3]thymidine incorporation, with the exception of arginine. Gln may be a unique nutrient for enterocytes, capable of dual signaling and augm enting the effects of growth factors that govern cellular proliferatio n and repair.