Opposing effects of tumour necrosis factor alpha and hyperosmolarity on Na+/myo-inositol co-transporter mRNA levels and myo-inositol accumulation by 3T3-L1 adipocytes.
Tumour necrosis factor alpha (TNF-alpha) regulates the transport of myo-inositol in 3T3-L1 adipocytes. Treating 3T3-L1 adipocytes with TNF-alpha decreases Na+/myo-inositol co-transporter (SMIT) mRNA levels and myo-inositol accumulation in a concentration-and time-dependent manner. TNF-alpha decreases the V'max for high-affinity myo-inositol transport with little change in the K'm. Studies with actinomycin D suggest that RNA synthesis is required for the TNF-alpha-induced effect on SMIT mRNA levels. In contrast with the effect of TNF-alpha, hyperosmolarity increases SMIT mRNA levels and myo-inositol accumulation in 3T3-L1 adipocytes. Hyperosmolarity increases SMIT gene expression as evidenced by the inhibition of hyperosmotic induction of SMIT mRNA levels by actinomycin D, and of myo-inositol accumulation by actinomycin D and cycloheximide. TNF-alpha and osmotic stress have previously been shown to activate similar signal transduction pathways in mammalian cells. In 3T3-L1 adipocytes, both TNF-alpha and hyperosmolarity increase mitogen-activated protein kinase kinase pathway activity; however, with the possible exception of c-Jun N-terminal kinase, this pathway does not seem to regulate SMIT mRNA levels or myo-inositol accumulation. TNF-alpha activates nuclear factor kappaB (NF-kappaB) in 3T3-L1 adipocytes but, unlike the effect of TNF-alpha on cultured endothelial cells, NF-kappaB does not seem to contribute to the regulation by TNF-alpha of SMIT gene expression in 3T3-L1 adipocytes. Therefore other signal transduction pathways must be considered in the regulation by TNF-alpha of SMIT mRNA levels and activity. Thus TNF-alpha and hyperosmolarity have opposing effects on SMIT mRNA levels and activity in 3T3-L1 adipocytes. Because myo-inositol in the form of phosphoinositides is an important component of membranes and signal transduction pathways, the regulation of myo-inositol metabolism by TNF-alpha might represent another mechanism by which TNF-alpha regulates adipocyte function.
Yorek
M A
MA
Department of Internal Medicine, University of Iowa, Diabetes Endocrinology Research Center and Veterans Affairs Medical Center, Iowa City, IA 52246, USA. myorek@icva.gov
Dunlap
J A
JA
Lowe
W L
WL
D016475:3T3 Cells; D017667:Adipocytes / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D002352:Carrier Proteins / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D002518:Ceramides / Q000494:pharmacology; D003513:Cycloheximide / Q000494:pharmacology; D016207:Cytokines / Q000494:pharmacology; D003609:Dactinomycin / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology; D006133:Growth Substances / Q000494:pharmacology; D006360:Heat-Shock Proteins / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D007294:Inositol / Q000378:metabolism*; D007328:Insulin / Q000494:pharmacology; D008565:Membrane Proteins; D051379:Mice; D019950:Mitogen-Activated Protein Kinase 1 / Q000037:antagonists & inhibitors / Q000187:drug effects / Q000378:metabolism; D016328:NF-kappa B / Q000187:drug effects / Q000378:metabolism; D009994:Osmolar Concentration; D012333:RNA, Messenger; D013110:Sphingosine / Q000494:pharmacology; D027981:Symporters; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*
D002352:Carrier Proteins; D002518:Ceramides; D016207:Cytokines; D004791:Enzyme Inhibitors; D006133:Growth Substances; D006360:Heat-Shock Proteins; D007328:Insulin; D008565:Membrane Proteins; D016328:NF-kappa B; D012333:RNA, Messenger; D027981:Symporters; D014409:Tumor Necrosis Factor-alpha; C101954:N-caproylsphingosine; C073884:SLC5A3 protein, human; D003609:Dactinomycin; D007294:Inositol; D003513:Cycloheximide; D019950:Mitogen-Activated Protein Kinase 1; D005947:Glucose; D013110:Sphingosine