Apoptosis in skeletal muscle myotubes is induced by ceramides and is positively related to insulin resistance.
Fatty acid-induced apoptosis occurs in pancreatic beta-cells and contributes to the metabolic syndrome. Skeletal muscle insulin resistance is mediated by fatty acid oversupply, which also contributes to the metabolic syndrome. Therefore, we examined whether fatty acids induce apoptosis in skeletal muscle myotubes, the proapoptotic signaling involved, and the effects on insulin sensitivity. Exposure of L6 myotubes to palmitate induced apoptosis, as demonstrated by increased caspase-3 activation, phosphatidylserine exposure on the plasma membrane, and terminal deoxynucleotide transferase dUTP nick end labeling and DNA laddering, both markers of DNA fragmentation. Ceramide content was concomitantly increased, indicating a potential role for ceramides in palmitate-induced apoptosis. Supporting this notion, reducing stearoyl-CoA desaturase-1 (SCD-1) protein content with short interfering RNA resulted in ceramide accumulation and was associated with increased apoptosis in the absence of palmitate. Furthermore, the membrane-permeable C(2)-ceramide enhanced apoptosis in myotubes, whereas the ceramide synthase inhibitor, fumonisin B(1), abrogated the proapoptotic effects of palmitate. Insulin-stimulated glucose uptake was inhibited by palmitate treatment, whereas the addition of effector caspase inhibitors [Ac-DEVD-aldehyde (DEVD-CHO), Z-DQMD-FMK] independently restored >80% of the insulin-stimulated glucose uptake. These effects were observed independently from changes in the protein content of insulin signaling proteins, suggesting that proteosomal degradation is not involved in this process. We conclude that lipoapoptosis occurs in skeletal muscle myotubes, at least partially via de novo ceramide accumulation, and that inhibiting downstream apoptotic signaling improves glucose uptake in vitro.
Turpin
Sarah M
SM
St. Vincent's Institute of Medical Research, Fitzroy, Victoria 3064, Australia.
Lancaster
Graeme I
GI
Darby
Ian
I
Febbraio
Mark A
MA
Watt
Matthew J
MJ
D000214:Acyl Coenzyme A / Q000378:metabolism; D000818:Animals; D000963:Antimetabolites / Q000378:metabolism; D017209:Apoptosis / Q000187:drug effects*; D015153:Blotting, Western; D053148:Caspase 3 / Q000378:metabolism; D002518:Ceramides / Q000494:pharmacology*; D053938:DNA Fragmentation / Q000187:drug effects; D003847:Deoxyglucose / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005227:Fatty Acids / Q000494:pharmacology; D005434:Flow Cytometry; D005947:Glucose / Q000378:metabolism; D007004:Hypoglycemic Agents / Q000494:pharmacology; D007328:Insulin / Q000494:pharmacology; D007333:Insulin Resistance / Q000502:physiology*; D008856:Microscopy, Fluorescence; D018485:Muscle Fibers, Skeletal / Q000187:drug effects*; D018482:Muscle, Skeletal / Q000166:cytology / Q000187:drug effects* / Q000648:ultrastructure; D010171:Palmitoyl Coenzyme A / Q000378:metabolism; D034741:RNA, Small Interfering / Q000494:pharmacology; D051381:Rats; D015398:Signal Transduction / Q000187:drug effects
D000214:Acyl Coenzyme A; D000963:Antimetabolites; D002518:Ceramides; D005227:Fatty Acids; D007004:Hypoglycemic Agents; D007328:Insulin; D034741:RNA, Small Interfering; D010171:Palmitoyl Coenzyme A; C017725:stearoyl-coenzyme A; D003847:Deoxyglucose; D053148:Caspase 3; D005947:Glucose