Insulin-like growth factor-2 activation of intestinal glutamine transport is mediated by mitogen-activated protein kinases

Insulin-like growth factor-2 (IGF-2) plays a pivotal role in regulating intestinal epithelial metabolism, growth, and proliferation, but its regulatory effects on mucosal cell amino acid transport have not been well studied. The purpose of this in vitro study was to investigate the regulatory mechanisms and intracellular signaling pathways involved in the regulation of IGF-2 on glutamine transport in cultured intestinal cells. Continuous incubation with IGF-2 stimulated glutamine transport activity in cultured IEC-6 cells in a dose- and time-dependent fashion. Prolonged incubation (up to 48 hours) resulted in a 50% increase in transport activity (0.81 ± 0.21 nmole/mg protein/min in IGF-2 cells vs. 0.57 ±0.15 nmole/ mg protein/min in control cells) and a threefold increase in glutamine transporter ATB0 mRNA levels. IGF-2 stimulated transport activity by increasing transport maximal capacity (Vmax 4.31 ± 0.36 nmole/ mg protein/min in IGF-2 cells vs. 2.51 ± 0.23 nmole/mg protein/min in control cells) without affecting the transport affinity (Km 0.31 ± 0.03 mmol/L glutamine in IGF-2 cells vs. 0.28 ± 0.03 mmol/L glutamine in control cells). This IGF-2-induced glutamine transport activity was attenuated by actinomycin-D or cycloheximide. The levels of mitogen-activated protein kinases p42/44, MEK1/2, and p38 as well as protein kinase C levels were elevated in IGF-2-treated cells and inhibitors of mitogen-activated protein kinase MEK1 (PD 98059), mitogen-activated protein kinase p38, and protein kinase C (chelerythrine chloride) individually attenuated the IGF-2-induced glutamine transport. These data suggest that IGF-2 stimulates intestinal glutamine uptake in cultured rat intestinal epithelial cells via a mechanism that involves transcription and translation of the transporter. Activation of mitogen-activated protein kinases and protein kinase C cascades are involved in the regulation. This increase in glutamine uptake may occur to support intestinal cell growth and proliferation.