title: Subcellular localization of hexokinases I and II directs the metabolic fate of glucose.
abstract: BACKGROUND
  The first step in glucose metabolism is conversion of glucose to glucose 6-phosphate (G-6-P) by hexokinases (HKs), a family with 4 isoforms. The two most common isoforms, HKI and HKII, have overlapping tissue expression, but different subcellular distributions, with HKI associated mainly with mitochondria and HKII associated with both mitochondrial and cytoplasmic compartments. Here we tested the hypothesis that these different subcellular distributions are associated with different metabolic roles, with mitochondrially-bound HK's channeling G-6-P towards glycolysis (catabolic use), and cytoplasmic HKII regulating glycogen formation (anabolic use).
  
  
  RESULTS
  To study subcellular translocation of HKs in living cells, we expressed HKI and HKII linked to YFP in CHO cells. We concomitantly recorded the effects on glucose handling using the FRET based intracellular glucose biosensor, FLIPglu-600 mM, and glycogen formation using a glycogen-associated protein, PTG, tagged with GFP. Our results demonstrate that HKI remains strongly bound to mitochondria, whereas HKII translocates between mitochondria and the cytosol in response to glucose, G-6-P and Akt, but not ATP. Metabolic measurements suggest that HKI exclusively promotes glycolysis, whereas HKII has a more complex role, promoting glycolysis when bound to mitochondria and glycogen synthesis when located in the cytosol. Glycogen breakdown upon glucose removal leads to HKII inhibition and dissociation from mitochondria, probably mediated by increases in glycogen-derived G-6-P.
  
  
  CONCLUSIONS
  These findings show that the catabolic versus anabolic fate of glucose is dynamically regulated by extracellular glucose via signaling molecules such as intracellular glucose, G-6-P and Akt through regulation and subcellular translocation of HKII. In contrast, HKI, which activity and regulation is much less sensitive to these factors, is mainly committed to glycolysis. This may be an important mechanism by which HK's allow cells to adapt to changing metabolic conditions to maintain energy balance and avoid injury.
authors.affiliation: UCLA Cardiovascular Research Laboratory, Department of Medicine (Cardiology), David Geffen School of Medicine at University of California Los Angeles, Los Angeles, California, United States of America.
authors.forename: Scott
authors.initials: S
authors.lastname: John
authors.forename: James N
authors.initials: JN
authors.lastname: Weiss
authors.forename: Bernard
authors.initials: B
authors.lastname: Ribalet
mesh_terms: D000818:Animals; D016466:CHO Cells; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D002463:Cell Membrane Permeability / Q000187:drug effects; D006224:Cricetinae; D003412:Cricetulus; D003600:Cytosol / Q000187:drug effects / Q000201:enzymology; D005947:Glucose / Q000378:metabolism / Q000494:pharmacology; D051272:Glucose Transporter Type 1 / Q000378:metabolism; D019298:Glucose-6-Phosphate / Q000378:metabolism; D006003:Glycogen / Q000378:metabolism; D006593:Hexokinase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008928:Mitochondria / Q000187:drug effects / Q000201:enzymology; D011485:Protein Binding / Q000187:drug effects; D021381:Protein Transport / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051381:Rats; D013347:Subcellular Fractions / Q000187:drug effects / Q000201:enzymology; D013997:Time Factors; D014162:Transfection
chemical_list: D051272:Glucose Transporter Type 1; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D019298:Glucose-6-Phosphate; D006003:Glycogen; D006593:Hexokinase; D051057:Proto-Oncogene Proteins c-akt; D005947:Glucose