Regulation of Na+/Glucose Cotransporter Expression by Protein Kinases in Oocytes

Abstract

Cotransporters are proteins responsible for the accumulation of nutrients, neurotransmitters, and drugs in cells. As forskolin has been shown to stimulate intestinal Na+/glucose cotransport, we have used electrophysiological techniques to examine the role of protein kinases in regulating Na+/glucose cotransporters, SGLT1, expressed in Xenopus laevis oocytes. We monitored SGLT1 kinetics, the number of SGLT1 cotransporters in the plasma membrane, and plasma membrane area before and after activation of protein kinases. 8-Bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) and sn-1, 2-dioctanoylglycerol (DOG) were used as membrane permeable activators of protein kinases A (PKA) and C (PKC), respectively. In oocytes expressing rabbit SGLT1 8-Br-cAMP increased by 28 +/- 4% (n = 10), and DOG decreased by 51 +/- 5% (n = 13) the maximum rate of Na+/glucose cotransport. These reversible changes in the maximum transport rate occurred within minutes, and were accompanied by proportional changes in the number of cotransporters in the membrane and area of the plasma membrane. This suggests that protein kinases regulate rabbit SGLT1 activity by controlling the distribution of transporters between intracellular compartments and the plasma membrane, and that this occurs by exo- and endocytosis. Similar increases in maximum transport were obtained with activation of PKA in oocytes expressing rabbit, human, and rat SGLT1 isoforms, but with activation of PKC the response was isoform-dependent. PKC activation decreased the maximum rate of transport by rabbit and rat SGLT1, but increased transport by human SGLT1. We conclude that: (i) the regulation of SGLT1 expression in oocytes by protein kinases occurs mainly by regulated endo- and exocytosis; (ii) it is independent of consensus phosphorylation sites in the transporter; and (iii) the effect of a given kinase depends upon the actual sequence of the cotransporter expressed. These considerations may also apply to the regulation of other cotransporters by protein kinases in oocytes, cells, and tissues.

Molecular Analysis of the SGLT2 Gene in Patients with Renal Glucosuria

“…Whether additional posttranslational modification is required before SGLT2 expresses in oocytes is uncertain. An important role of protein kinases for sodium-glucose co-transporters has been demonstrated for SGLT1, and it has been suggested that despite that SGLT show high sequence homology, minor differences in amino acid sequence may dramatically change the regulation of trafficking by protein kinases (36). The ultimate reason for poor expression of SGLT2 in heterologous systems, however, has remained unclear.…”

The Plasma Membrane-associated Protein RS1 Decreases Transcription of the Transporter SGLT1 in Confluent LLC-PK1 Cells

“…The amount of the Na ϩ -D-glucose co-transporter molecules in the plasma membrane may be changed within minutes via endo-and exocytosis (14). In contrast, the transcriptional regulation of SGLT1 is known to occur more slowly.…”

“…Regulation of SGLT1 has been studied in small intestine (7,(11)(12)(13), in Xenopus oocytes (14), and in the renal epithelial cell line LLC-PK 1 which is derived from porcine kidney proximal tubule cells (15)(16)(17)(18)(19)(20)(21)(22). One of the factors that affect the expression of SGLT1 in small intestine and LLC-PK 1 cells is extracellular D-glucose concentration (7,17,18).…”

Localization of brain endothelial luminal and abluminal transporters with immunogold electron microscopy

“…including cytosol to membrane recycling in oocytes, 66 intestinal GLUT2 recruitment, 67 and translocation of GLUT1 in retinal capillary membranes 68,69 and mesangial cells. 70 PKC also regulates translocation of GLUT1 71 and GLUT4 in muscle 72,73 and fat cells.…”