SODIUM-DEPENDENT COTRANSPORTED ANALOGS OF GLUCOSE STIMULATE ORNITHINE DECARBOXYLASE MESSENGER-RNA EXPRESSION IN LLC-PK(1) CELLS

Non-metabolizable analogues of glucose, including 1-O-methyl alpha-D-glucopyranoside (alphaMDG), that are co-transported with Na- increase the specific activity of ornithine decarboxylase (ODC) in LLC-PK1 cells [Lundgren and Vacca (1990) Am. J. Physiol. 259, C647-C653]. The present study examines the effect of alphaMDG on LLC-PK1-cell ODC mRNA expression. The relative concentration of ODC mRNA in cells incubated in Earle's balanced salts solution minus glucose (EBSS-G) plus 3 mM alphaMDG was 5-6-fold higher than the concentration of ODC mRNA in cells incubated in either EBSS-G alone or in EBSS-G plus 3mM 3-O-methyl-D-glucopyranose, a non-metabolizable analogue of glucose that is taken up by a passive carrier-mediated glucose transporter. Actinomycin D and cycloheximide completely blocked the increase in ODC activity induced by alphaMDG. Actinomycin D was also a potent inhibitor of ODC mRNA expression by alphaMDG. Cycloheximide had very little effect on the ability of this sugar to increase ODC mRNA. The relative concentration of ODC mRNA increased as a function of the incubation time in EBSS-G plus alphaMDG. The amount of ODC mRNA also increased as a function of the concentration of alphaMDG in EBSS-G. The addition of phlorizin (100 muM) to EBSS-G prevented alphaMDG from increasing ODC mRNA in LLC-PK, cells. Phlorizin did not prevent phorbol 12-myristate 13-acetate (PMA) from enhancing LLC-PK1-cell ODC mRNA expression. The positive effect of both alphaMDG and TPA on ODC mRNA expression was suppressed when cells were incubated in hypertonic EBSS-G. From these results it is suggested that the uptake of Na+-dependent co-transported sugars increase ODC activity by enhancing ODC gene transcription and that this process may be dependent on cell volume expansion.