J. Biol. Chem., Vol. 266, Issue 16, 10400-10405, Jun, 1991
Induction of glycinebetaine uptake into Xenopus oocytes by injection of poly(A)+ RNA from renal cells exposed to high extracellular NaCl
RB Robey, HM Kwon, JS Handler, A Garcia-Perez and MB Burg
Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
Madin-Darby canine kidney (MDCK) cells accumulate glycinebetaine via
Na(+)-dependent transport in response to hypertonic stress. When
extracellular tonicity is increased by the addition of NaCl, Vmax for
glycinebetaine transport increases without an associated change in Km,
consistent with an increase in the number of functioning transporters. To
test whether increased transport activity results from increased gene
expression, we injected poly(A)+ RNA (mRNA) from MDCK cells into Xenopus
oocytes and assayed for glycinebetaine uptake in ovo. RNA- induced
Na(+)-dependent uptake is observed in oocytes injected with mRNA from cells
exposed to high extracellular NaCl, but not in oocytes injected with either
water or mRNA from cells maintained in isotonic medium. Unfractionated mRNA
induces glycinebetaine uptake in ovo at a rate which is approximately
3-fold higher than in water-injected controls. Size-fractionated mRNA
(median size 2.8 kilobases) induces uptake at a rate which is approximately
7-fold higher than controls. Such RNA-induced transport activity in ovo is
consistent with heterologous expression of Na(+)/glucinebetaine
cotransporters encoded by renal mRNA. Increased transporter mRNA in cells
exposed to hypertonicity probably underlies the pattern of expression
observed in ovo. This can account for the observed rise in MDCK cell
glycinebetaine transport during hypertonic stress.