[Effect of ASCT2 gene knock-down by shRNA on biological behaviors of colorectal cancer cells]

OBJECTIVE

To investigate the effect of ASCT2 gene (glutamine transporter) knock-down by shRNA on biological behaviors of colorectal cancer cells.

METHODS

shRNA was transfected into colorectal cancer cells Lovo and SW480 to knockdown ASCT2 mediated by Lipofectamine 2000. Reverse transcription-PCR and Western blot were used to examine the mRNA and protein expression of ASCT2. MTT and transwell assay were used to determine the proliferation and invasiveness of Lovo and SW480 cells. Radioactive-tracer was used to detect the uptake of glutamine.

RESULTS

ASCT2 mRNA and protein levels were significantly down-regulated by shRNA in Lovo and SW480 cells(P<0.01). MTT and transwell assays showed that ASCT2 knock-down could significantly inhibit the proliferation of Lovo and SW480 cells (A490) and decrease the number of invasive Lovo and SW480 cells from the membrane (both P<0.01). The number of membrane Lovo cells in shASCT group and control group was 46.3±5.9 and 197.7±9.1, respectively while the number of membrane SW480 cells in shASCT group and control group was 29.7±3.8 and 139.0±9.5, respectively. Radioactive-tracer showed that shASCT2 transfection could significantly reduce the uptake of glutamine, with an inhibition rate of 79.15% in Lovo and 67.22% in SW480 cells (both P<0.01).

CONCLUSIONS

ASCT2 plays an oncogenic role in colonic cancer, and its promotion mechanism may be associated with glutamine metabolism. ASCT2 may be a novel therapeutic target of colonic cancer.

Regulation of L-alanine transport systems A and ASC by cyclic AMP and calcium in a reptilian duodenal model

The regulation of neutral amino acid transport by cyclic AMP (cAMP) and calcium across the isolated duodenum of the lizard Gallotia galloti has been studied under short-circuit conditions. Active L-alanine transport was stimulated by forskolin, theophylline and dibutyryl cyclic AMP (db-cAMP). All these agents increased transmural potential difference (PD) and short-circuit current (I(sc)) in a manner consistent with the activation of a chloride secretory pathway. Both forskolin and theophylline increased intracellular cAMP levels in the lizard duodenal mucosa. Addition of calcium ionophore A23187 rapidly reduced mucosa-to-serosa L-alanine fluxes and diminished net L-alanine transport. Despite the reduction of alanine fluxes by A23187, transepithelial PD and I(sc) values were increased by the ionophore. Analyses of the responses of isolated transport pathways indicated that the Na(+)-independent L-alanine transport system was unaffected by db-cAMP or calcium ionophore. By contrast, Na(+)-dependent transport activities were profoundly modified by these agents. Thus, while system A [alpha-methylamino-isobutiric acid (MeAIB)-transporting pathway] was stimulated by increased calcium, system ASC activity was nearly abolished. Calcium ionophore also potentiated the electrogenic response of system A. Forskolin strongly stimulated system ASC activity but left system A activity unchanged. Activation of system ASC by forskolin was clearly electroneutral, as pre-incubation of the tissues with the chloride channel blocker diphenylamine-2-carboxilic acid (DPC) completely prevented forskolin-induced transepithelial electrical responses. It is concluded that intracellular messengers cAMP and calcium oppositely modulate active Na(+)-dependent (L)-alanine transport in the lizard intestine. The different sensitivity exhibited by individual transport pathways may well account for the changes observed in overall alanine transport.

Glutamine transport in C6 glioma cells: substrate specificity and modulation in a glutamine deprived culture medium

A previous study has shown that glutamine (Gln) uptake in C6 cells grown in a standard medium containing 2 mM Gln, is predominantly mediated by a sodium-dependent system that is inhibited by ASC system substrates alanine (Ala), serine (Ser), cysteine (Cys) and threonine (Thr), shows pH sensitivity and partial tolerance to substitution of Na+ by Li+, features compatible with system ASCT2 that is strongly expressed in cultured astrocytes. The uptake was not inhibited by the model system A substrate alpha-(methylamino)isobutyric acid (MeAiB), and glycine (Gly) or proline (Pro), indicating that the substrate-regulated system A as defined by routine criteria is relatively inactive in these cells (Dolinska et al., 2000). In this study we compared the uptake of radiolabeled Gln and a model ASC substrate -Thr in cells grown to the same density in Gln-containing and Gln-deprived media. Cells grown in the absence of Gln showed a reduced activity of system ASC-mediated Gln uptake, and the system lost tolerance for Li+ and became somewhat more resistant to lowering pH of the medium. In contrast to cultured astrocytes deprived of Gln, the overall Gln uptake activity in C6 cells adapted to grow in a medium without Gln was lower than in cells grown in a Gln containing medium, and the uptake by system A remained inactive. C6 cells cultured both in the presence and absence of Gln expressed ASCT2 mRNA, indicating that system ASCT2-mediated Gln uptake is modulated at a posttranscriptional level. In contrast to Gln uptake, Thr uptake was more active in cells cultured in the absence of Gln and showed neither pH dependence nor lithium tolerance in either medium, which is typical of an uptake mediated by the widespread ASCT1 isoform of system ASC. In C6 cells grown in the presence or absence of Gln alike, approximately 20% of the sodium-dependent Gln uptake was resistant to MeAiB+Thr, indicating contribution of system N. The N system-mediated uptake in C6 cells grown in the absence, but not in the presence of Gln was not inhibited by glutamate (Glu) that conforms to the characteristics of the glial N system variant, SN1.