Basolateral 3-O-methylglucose transport by cultured kidney (LLC-PK1) epithelial cells.

In previous work we demonstrated the similarity of basolateral sugar transport of LLC-PK1 renal epithelia to basolateral kidney sugar transport using 2-deoxy-D-glucose as a substrate. In this study we first examine a central limitation to use of 2-deoxyglucose for basolateral sugar transport study in LLC-PK1 epithelia, namely, a shift of the rate-limiting step in uptake from transport to phosphorylation. Use of 3-O-methylglucose avoids this complication because it is not phosphorylated. However, use of 3-O-methylglucose requires much shorter incubation periods to examine linear rates of uptake (steady state is reached by 60 s at 22 degrees C for 0.1 mM 3-O-methylglucose). As was true for 2-deoxyglucose, apical uptake of 3-O-methylglucose was only a fraction of total uptake. Basolateral uptake was characteristically more sensitive to phloretin and cytochalasin B inhibition, relative to phlorizin. Inhibition studies indicate a requirement for a free hydroxyl on C-1 carbon of the pyranose ring, as is characteristic for renal basolateral sugar transport. Kinetic analysis indicates a single transport system with a Km of 10.9 mM and Vmax of 17.2 pmol.micrograms DNA-1.15 s-1. Subconfluent, undifferentiated LLC-PK1 cells show a similar Km (12.7 mM) but a ninefold higher Vmax (166.2 pmol.micrograms DNA-1.15 s-1). Stimulation of 3-O-methylglucose transport rate in confluent cultures by phorbol ester is relatively small (less than 100%) compared with effects on other somatic cells. The uptake rate of 3-O-methylglucose is not affected by glucose starvation, but subsequent refeeding with glucose-containing medium does significantly stimulate uptake.

Isolation of mutant renal (LLC-PK1) epithelia defective in basolateral, Na(+)-independent glucose transport.

To obtain mutant renal epithelia defective in the uptake of 2-deoxyglucose (Na(+)-independent glucose transport), LLC-PK1 renal epithelial cells were subjected to a combination of DNA alkylation, tritium-suicide with 2-[3H]deoxyglucose, and replica plating. One of the mutant sublines obtained, LLC-PK1M-7A, possesses only 40% of the 2-deoxyglucose uptake rate of the parent line, LLC-PK1M, and this defect is stable over at least 30 population doublings. Initial rate of 3-O-methylglucose transport into these mutant cells is only 20% of the parental rate, and efflux of 3-O-methylglucose from the mutant cells is correspondingly low. Glucose metabolism in the mutant cells does not appear to be altered, nor is free glucose accumulated in the cells against a concentration gradient. The uptake rate of L-leucine is the same in both mutant and parent, whereas the (Na(+)-dependent) uptake of alpha-methyl-D-glucoside and methylaminoisobutyric acid is greater in the mutant than the parent. The population doubling times of LLC-PK1M and LLC-PK1M-7A cultures are similar. LLC-PK1M-7A cell morphology is similar to LLC-PK1M when cultures are subconfluent, but on reaching confluence, LLC-PK1M-7A appear larger than LLC-PK1M cells. Their measured cell volume is then 150% of the volume of the parent cells. Future studies of the LLC-PK1M-7A mutant, and acquisition of additional mutant sublines, should elucidate the roles of transepithelial glucose transport in proximal tubular cell physiology.

Na+-independent sugar transport by cultured renal (LLC-PK1) epithelial cells.

The LLC-PK1 cell line has been well characterized concerning its proximal tubule-like Na+-dependent active sugar transporter in the apical membrane. In this study, we investigated the uptake of the glucose analogue, 2-deoxy-D-glucose (2DOG), a paradigm substrate for the facilitated diffusion, Na+-independent sugar transporter in the renal basolateral membrane. The uptake of 0.1 mM 2-[14C]DOG by confluent LLC-PK1 cell sheets at 25 degrees C is linear at least to 10 min, at which time greater than 90% of intracellular radioactivity is 2DOG phosphate. The uptake of this analogue by LLC-PK1 cells is Na+ independent, and the transporter appears to be localized to the basolateral cell membrane. Phlorizin is a much less effective inhibitor than its aglycon, phloretin. Cytochalasin B is also an effective inhibitor, but it causes morphological changes in the cells at concentrations required to inhibit transport. Specificity studies indicate that this transport system requires a hexose with a free hydroxyl at C-1, and that the hydroxyls at C-3 and C-4 be preferably in the equatorial position. Glucose starvation causes an increased rate of 2DOG uptake. Subconfluent (cycling) cultures of LLC-PK1 cells have a threefold greater rate of 2DOG uptake than that seen in confluent (noncycling) LLC-PK1 cells.