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 lines. Gln was additive to epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I) in stimulating DNA synthesis, as assessed by [3H]thymidine incorporation. Extracellular signal-regulated kinases (ERKs) p42mapk and p44mapk and Jun nuclear kinases (JNKs) phosphorylate 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 both ERKs and JNKs, resulting in a fourfold increase in AP-1-dependent gene transcription. Gln was required for EGF signaling through ERKs. Maximal stimulation of proliferation required approximately 2.5 mM Gln. c-Jun mRNA levels responded to Gln in "Gln-starved" porcine IPEC-J2 cells and in rat IEC-6 cells. Although Gln metabolism is required for the proliferative response, several Gln by-products did not stimulate [3H]thymidine incorporation, with the exception of arginine. Gln may be a unique nutrient for enterocytes, capable of dual signaling and augmenting the effects of growth factors that govern cellular proliferation and repair.