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 although 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 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.

Evolving Concepts of Glycogen-Selective Autophagy

“…There has been some indication that the Glut8 transporter may be involved in lysosomal glucose release, but conflicting reports exist. Glut8 exhibits an endosomal/lysosomal targeting sequence (61,62), and in Glut8-stably expressing CHO and MB231 cells, Glut8 colocalized with the lysosomal protein Lamp1 and was detected in lysosomal membranes (61,63). Furthermore, inhibition of the lysosome with chloroquine or bafilomycin resulted in an accumulation of Glut8 (63).…”

“…Other studies show that Glut8 is an insulin-sensitive plasma membrane transporter which participates in cellular glucose uptake in the heart and other tissues (65,66). Yet an in vitro study screening gainand loss-of-function of Glut8, showed that knockdown of Glut8 does not influence 2-deoxyglucose uptake (63). Thus whether Glut8 is operational as a lysosomal glucose transporter and has a role in glycophagy has not been fully elucidated.…”

Endometrial Glucose Transporters in Health and Disease

“…The late-endosomal/lysosomal limiting membrane is the cellular hub for various metabolic reactions (e.g., mTORC1 recruitment to the lysosomal membrane, modulation of mitochondrial homeostasis, regulation of protein, and lipid content of the cellular membrane and membranes of various vesicles). Therefore, besides transporting glucose, GLUT8 may also function as a sensor for various metabolites in cellular metabolic homeostasis ( Alexander et al, 2020 ). Some of the GLUT8 is cleaved into N-terminal and C-terminal peptides.…”

“…Of its 12 trans- membrane domains, the cleavage occurs at domain 10, which happens to be the only one that is 100% conserved across the species. The full-length and N-terminal cleaved proteins are retained at the endosomal/lysosomal boundary, whereas the carboxy domain accumulates in a different population of vesicles and is unlikely to perform transport ( Alexander et al, 2020 ). GLUT8 is one of several GLUT species that exists as several alternatively spliced mRNA variants, and there is few available data on its functionality.…”

“…GLUT8 is one of several GLUT species that exists as several alternatively spliced mRNA variants, and there is few available data on its functionality. Alexander et al (2020) concentrated on its expression in breast cancer and found that GLUT8 has three such variants: v1 (full-length), v2 (missing exon 9 and part of the C-terminus) and v3 (missing exon 2 and 3). In normal human tissue, by far the highest expressions of v1 and v3 mRNA variants are found in the testis, followed by liver and muscle, and only very little in the mammary gland, with the full-length version being predominant and v3 accounting for only about 5–10% of the total GLUT8 mRNA.…”