Glutamine is an essential nutrient for cell integrity during acidotic states such as
shock, but the effect of extracellular pH on intestinal mucosal cell
glutamine uptake is poorly understood. The purpose of this in vitro study was to investigate the intracellular signaling pathways involved in controlling intestinal
glutamine transport during
acidosis. Lowering the pH in the cell culture medium resulted in an increase in
glutamine transport activity in a time- and pH-dependent fashion. Chronic
acidosis (pH 6.6 for 48 hours) resulted in a twofold increase in
glutamine transport activity (1.63+/-0.25 nmole/mg
protein/minute in
acidosis vs. 0.78+/-0.11 nmole/mg
protein/minute in control) and a threefold increase in
glutamine transport gene ATB(0)
messenger RNA levels. This
acidosis-induced increase in
glutamine transport activity was due to a stimulation of transporter maximal transport capacity (V(max) 13.6+/-0.73 nmole/mg
protein/minute in
acidosis vs. 6.3+/-0.46 nmole/mg
protein/minute in control) rather than a change in transporter affinity (K(m)=0.23+/-0.02 mmol/
L glutamine in
acidosis vs. 0.19+/-0.02 mmol/
L glutamine in control). This
acidosis-stimulated
glutamine transport activity was blocked by
actinomycin-D or
cycloheximide. Cellular
mitogen-activated protein kinase (MAPK) MEK1/2 and p42/44 levels were elevated in acidotic cells, and the
acidosis-induced
glutamine transport activity was blocked by the MAPK
MEK 1 inhibitor
PD 98059.
Acidosis stimulates
glutamine transport in Caco-2 cells via signaling pathways that lead to transcription of the
glutamine transporter gene and translation of functional transporters.
Mitogen-activated protein kinases are key intracellular regulators involved in this signal transduction cascade. An increased availability of
glutamine to cells subjected to redox stress may help in maintaining cellular integrity.