Alternative Splicing and Cleavage of GLUT8

ABSTRACT

The GLUT (SLC2) family of membrane-associated transporters are described as glucose transporters. However, this family is divided into three classes and, though the regulated transporter activity of class I proteins is becoming better understood, class III protein functions continue to be obscure. We have cataloged the relative expression and splicing of SLC2 mRNA isomers in tumors and normal tissues, with a focus on breast tumors and cell lines. mRNA for the class III protein GLUT8 is the predominant SLC2 species expressed alongside GLUT1 in many tissues, but GLUT8 mRNA exists mostly as an untranslated splice form in tumors. We confirm that GLUT8 is not presented at the cell surface and does not transport glucose directly. However, we reveal a lysosome-dependent reaction that cleaves the GLUT8 protein and releases the carboxy-terminal peptide to a separate vesicle population. Given the localization of GLUT8 at a major metabolic hub (the late endosomal/lysosomal interface) and its regulated cleavage reaction, we evaluated TXNIP-mediated hexosamine homeostasis and speculate that GLUT8 may function as a sensory component of this reaction.

KEYWORDS:

• GLUT8
• SLC2
• TXNIP
• alternative splicing
• breast cancer
• chromophobe renal cell carcinoma
• cleavage
• glucose transport
• late endosomal-lysosomal boundary
• metabolic sensor

SUPPLEMENTAL MATERIAL

Supplemental material is available online only.

ACKNOWLEDGMENTS

This paper is dedicated to our brilliant and gentle coauthor, Heidi Dvinge, who passed away suddenly and tragically on 30 September 2019.

Thanks go to Aussie Suzuki and Nate Sherer (McArdle Laboratory) for generous advice on the calibration of confocal microscopic images and the utilization of retroviral expression vectors. The results published here are, in part, based upon data generated by the TCGA Research Network: http://cancergenome.nih.gov. The Genotype-Tissue Expression (GTEx) project was supported by the Common Fund of the Office of the Director of the National Institutes of Health and by NCI, NHGRI, NHLBI, NIDA, NIMH, and NINDS. We appreciate the expert assistance of the Small Molecule Screening Facility (University of Wisconsin Carbone Cancer Center Support Grant; P30 CA014520).

J.A.M., I.K., and C.M.A. were supported by RO1 GM113142 and a Cancer Biology training award to J.A.M. (T32 CA009135). Antibody production was assisted by support from the UW SDRC grant P30 AR066524, funded by the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS).