Protein RS1 (RSC1A1) Downregulates the Exocytotic Pathway of Glucose Transporter SGLT1 at Low Intracellular Glucose via Inhibition of Ornithine Decarboxylase

Abstract

Na-d-glucose cotransporter 1 (SGLT1) is rate-limiting for glucose absorption in the small intestine. Shortly after intake of glucose-rich food, SGLT1 abundance in the luminal membrane of the small intestine is increased. This upregulation occurs via glucose-induced acceleration of the release of SGLT1-containing vesicles from the trans-Golgi network (TGN), which is regulated by a domain of protein RS1 (RSC1A1) named RS1-Reg. Dependent on phosphorylation, RS1-Reg blocks release of vesicles containing SGLT1 or concentrative nucleoside transporter 1. The hypothesis has been raised that RS1-Reg binds to different receptor proteins at the TGN, which trigger release of vesicles with different transporters. To identify the presumed receptor proteins, two-hybrid screening was performed. Interaction with ornithine decarboxylase 1 (ODC1), the rate-limiting enzyme of polyamine synthesis, was observed and verified by immunoprecipitation. Binding of RS1-Reg mutants to ODC1 was characterized using surface plasmon resonance. Inhibition of ODC1 activity by RS1-Reg mutants and the ODC1 inhibitor difluoromethylornithine (DFMO) was measured in the absence and presence of glucose. In addition, short-term effects of DFMO, RS1-Reg mutants, the ODC1 product putrescine, and/or glucose on SGLT1 expressed in oocytes of Xenopus laevis were investigated. High-affinity binding of RS1-Reg to ODC1 was demonstrated, and evidence for a glucose binding site in ODC1 was provided. Binding of RS1-Reg to ODC1 inhibits the enzymatic activity at low intracellular glucose, which is blunted at high intracellular glucose. The data suggest that generation of putrescine by ODC1 at the TGN stimulates release of SGLT1-containing vesicles. This indicates a biomedically important role of ODC1 in regulation of glucose homeostasis.

Glucose transporters in the small intestine in health and disease

“…Similarly, in differentiated CaCo-2 cells, a model of human enterocytes, SGLT1, was localized to the BBM and intracellular vesicles [198]. The intracellular location of SGLT1/Sglt1 in enterocytes is consistent with data revealing a D-glucose-dependent regulation of the exocytotic pathway of SGLT1/Sglt1 in the small intestine of mouse and humans [66,341,408]. In rats, the BBM abundance and transport capacity of Sglt1 per unit length in the jejunum was higher compared to that in the duodenum and ileum [25,83,142,203].…”

“…3). Accordingly, D-glucose-dependent upregulation of SGLT1 in the plasma membrane is due to a glucose-induced acceleration of the exocytotic pathway of SGLT1 from the Golgi that is modulated by protein RS1 (RSC1A1) [66,407,408]. Human SGLT1 or YFP-SGLT1 was expressed in oocytes, and the post-translational active domain of RS1 (RS1-Reg) or peptide motifs of RS1-Reg were injected into the oocytes.…”

RS1 (Rsc1A1) deficiency limits cerebral SGLT1 expression and delays brain damage after experimental traumatic brain injury

“…RS1 regulates SGLT1 on transcriptional and post-translational levels. Post-translational regulation of SGLT1 was extensively investigated in vitro and was demonstrated in tissue of the small intestine (Veyhl et al 2006;Chintalapati et al 2016;Veyhl-Wichmann et al 2016), whereas transcriptional regulation was demonstrated in the renal cell line LLC-PK 1 (Korn et al 2001). In mice in which RS1 was removed (RS1 À/À mice), we observed that plasma membrane abundance of SGLT1 in small intestine was increased to a similar level as after glucose-induced post-translational up-regulation (Osswald et al 2005;Veyhl-Wichmann et al 2016).…”

“…The finding that SGLT1 mRNA abundance after TBI was decreased in RS1 À/À mice was not expected because RS1 has been described to down-regulate SGLT1 expression. Previous data showed that RS1 which was localized in the cytosol, at the plasma membrane, at the Golgi, and within the nucleus (Kroiss et al 2006), down-regulates SGLT1 on the transcriptional and post-translational levels (Korn et al 2001;Veyhl et al 2006;Chintalapati et al 2016;Veyhl-Wichmann et al 2016). In LLC-PK 1 cells it has been shown that confluence-dependent migration of RS1 into the nucleus is steered by a nuclear location signal (Filatova et al 2009) and that removal of RS1 up-regulates SGLT1 transcription (Korn et al 2001).…”

Saving the sweetness: renal glucose handling in health and disease

“…Interestingly, a mutation in MAP17 is also associated with familial renal glycosuria (see below), suggesting that MAP17 may be a physiological regulator of Sglt2 function (9). In a separate study, it was reported that RSC1A1 (RS1), a 67/68-kDa protein found in both the small intestine and all three segments of the kidney proximal tubule (24), serves as a regulator of Sglt1 (8,33,43,(57)(58)(59). RS1 localizes to the trans-Golgi network (TGN), plasma membrane, and nucleus in renal proximal tubule cells in culture (8,33,43,(57)(58)(59) Fig.…”