Development of Na+-dependent hexose transport in cultured renal epithelial cells (LLC-PK1)

A fluorescence method for measurement of glucose transport in kidney cells

BACKGROUND

Diabetes may alter renal glucose reabsorption by sodium (Na(+))-dependent glucose transporters (SGLTs). Radiolabeled substrates are commonly used for in vitro measurements of SGLT activity in kidney cells. We optimized a method to measure glucose uptake using a fluorescent substrate, 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose (2-NBDG).

METHODS

Uptake buffers for 2-NBDG were the same as for (14)C-labeled α-methyl-d-glucopyranoside ([(14)C]AMG). Cell lysis buffer was optimized for fluorescence of 2-NBDG and Hoechst DNA stain. Uptake was performed on cultures of primary mouse kidney cells (PMKCs), the LLC-PK(1) proximal tubule cell line, or COS-7 cells transiently overexpressing mouse SGLT1 or SGLT2 by incubating cells at 37°C in buffer containing 50-200 μM 2-NBDG. Microscopy was performed to visualize uptake in intact cells, while a fluorescence microplate reader was used to measure intracellular concentration of 2-NBDG ([2-NBDG](i)) in cell homogenates.

RESULTS

Fluorescent cells were observed in cultures of PMKCs and LLC-PK(1) cells exposed to 2-NBDG in the presence or absence of Na(+). In LLC-PK(1) cells, 2-NBDG transport in the presence of Na(+) had a maximum rate of 0.05 nmol/min/μg of DNA. In these cells, Na(+)-independent uptake of 2-NBDG was blocked with the GLUT inhibitor, cytochalasin B. The Na(+)-dependent uptake of 2-NBDG decreased in response to co-exposure to the SGLT substrate, AMG, and it could be blocked with the SGLT inhibitor, phlorizin. Immunocytochemistry showed overexpression of SGLT1 and SGLT2 in COS-7 cells, in which, in the presence of Na(+), [2-NBDG](i) was fivefold higher than in controls.

CONCLUSION

Glucose transport in cultured kidney cells can be measured with the fluorescence method described in this study.

Na(+)-coupled alanine transport in LLC-PK1 cells

Transport of alanine (Ala) was characterized in LLC-PK1 renal epithelial cells. Transport capability for Ala falls by 75% in postconfluent cultures, while Na(+)-coupled alpha-methylglucoside (AMG) transport rises more than fourfold during the same interval. The kinetics of Ala transport were characterized in ATP-depleted cells to allow experimental imposition of changes in Na+ gradient and control of membrane potential across the plasma membrane. At 100 microM Ala and 135 mM Na+, > 98% of the unidirectional Ala influx is dependent on the presence of Na+ in cells from postconfluent cultures. Li+ is only 1% as effective as Na+, and other monovalent cations are ineffective in supporting Ala uptake. alpha-(Methylamino)isobutyric acid (MeAIB; 5 mM) causes only a small inhibition (approximately 10%) of 100 microM Ala influx. The low selectivity for Li+; low sensitivity to competition by MeAIB or aminoisobutyric acid; pronounced inhibition by serine, homoserine, cysteine, homocysteine and threonine; moderate inhibition by valine, isoleucine, proline and histidine; and lack of inhibition by lysine, arginine, and aspartate are more consistent with those characteristics reported for entry via the ASC amino acid transport system rather than those associated with the A system. Alanine influx exhibits a hyperbolic relationship with increasing Ala or Na+ concentration. Kinetic analysis suggests a single transport pathway with a Michaelis constant (Km) for alanine of 380 microM (when Na+ is 135 mM), apparent Km for Na+ of 32 mM (with 100 microM Ala), and a maximum velocity of 7 nmol.min-1.mg cell protein-1. An interior-negative diffusion potential induces a similar enhancement of [14C]alanine or [14C]tetraphenylphosphonium influx (approximately 40%). In contrast, AMG influx is enhanced by a factor of 2.2 under the same conditions. AMG uptake also shows a sigmoidal relationship with Na+ concentration. Hill coefficients are 1.56 for AMG and 1.0 for alanine. Direct measurement of Na(+)-Ala coupling stoichiometry yields a value of 1.01 +/- 0.07. Under the same conditions, Na(+)-AMG coupling stoichiometry is 2.1 +/- 0.25. The difference in coupling stoichiometries provides an explanation for differences in intensity of interaction between Na(+)-coupled transport systems for sugars and amino acids.

Whole cell recording of sugar-induced currents in LLC-PK1 cells

Gigaohm-seal whole cell recording techniques were used to monitor function of the Na(+)-coupled sugar transport system in LLC-PK1 cells. The currents coupled to sugar transport were identified as those that are induced by the presence of 10 mM alpha-methylglucoside (AMG) in either the extracellular or intracellular compartment and were inhibited by addition of 320-800 microM phlorizin to the extracellular bathing medium. The sugar-induced currents are small, 15-20 pA, but of the expected magnitude as determined from the known kinetic parameters for Na(+)-coupled sugar transport in LLC-PK1 cells. The phlorizin-sensitive currents are Na+ dependent and can be studied under conditions in which the net Na+ and sugar flux (and consequently the Na+ electrical current) is in either the inward or outward direction. The reversal potential of the sugar-induced currents measured under conditions with high Na+ and AMG concentrations inside the cell is close to values predicted from thermodynamic principles, assuming a coupling stoichiometry of 2 Na+: 1 sugar for the transport system. The reversal potential of the sugar-induced currents with high extracellular Na+ and AMG is not equal to the predicted value, but it is of the polarity expected for inward-imposed solute gradients. Reasons for the observed discrepancy between observed and calculated values are discussed.

Inhibition by amiloride analogues of Na+-dependent hexose uptake in LLC-PK1/Cl4 cells

Amiloride and four analogues of amiloride were shown to inhibit Na+-dependent, phlorizin-sensitive hexose uptake by a clone of pig kidney cells, LLC-PK1/Cl4. The analogues tested were: 5-(N-ethyl-N-isopropyl)amiloride (EIPA), 5-(N-methyl-N-isobutyl)amiloride (MIBA), 3',4'-dichlorobenzamil, and phenamil. The transport substrate was the nonmetabolizable glucose analogue alpha-methyl-D-glucoside. Blockade of Na+-K+ transport at the basolateral membranes or removal of divalent cations from the assay medium had little effect on the initial rate of hexose uptake, whereas MIBA remained an effective inhibitor under both conditions. The inhibitions by EIPA of Na+-H+ exchange and hexose-dependent Na+ uptake could be distinguished by appropriate choice of concentrations of the inhibitor. Hexose transport inhibition does not appear to be secondary to other known effects of the amilorides. Inhibition by all analogues is enhanced when they are tested in low (2 mM) Na+ medium, where they show half-maximal inhibition in the range of 100-300 microM. More detailed kinetic analysis of inhibition by EIPA shows it to be competitive with Na+ with a Ki of 73-107 microM. It is concluded that the amilorides are acting directly on the hexose transporter.