Sodium-dependent co-transported analogues of glucose stimulate ornithine decarboxylase mRNA expression in LLC-PK1 cells

Abstract

Non-metabolizable analogues of glucose, including 1-O-methyl alpha-D-glucopyranoside (alpha MDG), 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 alpha MDG 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 alpha MDG was 5-6-fold higher than the concentration of ODC mRNA in cells incubated in either EBSS-G alone or in EBSS-G plus 3 mM 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 alpha MDG. Actinomycin D was also a potent inhibitor of ODC mRNA expression by alpha MDG. 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 alpha MDG. The amount of ODC mRNA also increased as a function of the concentration of alpha MDG in EBSS-G. The addition of phlorizin (100 microM) to EBSS-G prevented alpha MDG from increasing ODC mRNA in LLC-PK1 cells. Phlorizin did not prevent phorbol 12-myristate 13-acetate (PMA) from enhancing LLC-PK1-cell ODC mRNA expression. The positive effect of both alpha MDG 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 cotransported sugars increase ODC activity by enhancing ODC gene transcription and that this process may be dependent on cell volume expansion.