Malic enzyme is present in mouse islets and modulates insulin secretion.
OBJECTIVE
The pyruvate-malate shuttle is a metabolic cycle in pancreatic beta cells and is important for beta cell function. Cytosolic malic enzyme (ME) carries out an essential step in the shuttle by converting malate to pyruvate and generating NADPH. In rat islets the pyruvate-malate shuttle may regulate insulin secretion and it has been shown to play a critical role in adaptation to obesity and insulin resistance. However, ME has not been demonstrated in mouse islets and three reports indicate that mouse islets contain no ME activity. If mouse islets lack ME, rat and mouse islets must regulate insulin secretion by different mechanisms.


METHODS
We measured ME activity by a fluorometric enzymatic assay and Me mRNA by real-time PCR. ME activity was also measured in streptozotocin-treated mouse islets. FACS-purified beta cells were obtained from MIP-GFP mouse islets, agouti-L obese mouse islets and mouse beta cell line MIN-6. Insulin secretion and NADPH/NADP(+) ratios were measured in Me siRNA-treated beta cells.


RESULTS
ME activity and Me mRNA were present in C57BL/6 mouse islets. ME activity was reduced in streptozotocin-treated mouse islets. ME activity was also measurable in FACS-purified mouse beta cells. In addition, ME activity was significantly increased in obese agouti-L mouse islets and the mouse MIN-6 cell line. Me siRNA inhibited ME activity and reduced glucose-stimulated insulin secretion and also inhibited NADPH products.


CONCLUSIONS
Mouse islets contain ME, which plays a significant role in regulating insulin secretion.
Xu
J
J
Kosair Children's Hospital Research Institute, Department of Pediatrics, University of Louisville School of Medicine, Louisville, KY, USA.
Han
J
J
Long
Y S
YS
Lock
J
J
Weir
G C
GC
Epstein
P N
PN
Liu
Y Q
YQ
D000818:Animals; D002460:Cell Line; D002469:Cell Separation; D003921:Diabetes Mellitus, Experimental / Q000139:chemically induced / Q000201:enzymology; D015971:Gene Expression Regulation, Enzymologic; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism*; D008291:Malate Dehydrogenase / Q000235:genetics / Q000378:metabolism*; D008297:Male; D051379:Mice; D009249:NADP / Q000378:metabolism; D009765:Obesity / Q000201:enzymology; D034741:RNA, Small Interfering / Q000235:genetics; D013311:Streptozocin / Q000494:pharmacology
D007328:Insulin; D034741:RNA, Small Interfering; D009249:NADP; D013311:Streptozocin; D008291:Malate Dehydrogenase