Pioglitazone promotes insulin-induced activation of phosphoinositide 3-kinase in 3T3-L1 adipocytes by inhibiting a negative control mechanism.
Activation of phosphoinositide 3-kinase (PI 3-kinase) is an early event in insulin signal transduction that is blocked completely in adipocytes from insulin-resistant KKAy mice. Treatment of KKAy mice with pioglitazone, an anti-diabetic thiazolidinedione, partially restores insulin-dependent changes in PI 3-kinase. The mechanism of this effect of pioglitazone was investigated using murine 3T3-L1 cells as an experimental model. Insulin and insulin-like growth factor I (IGF-I) each elicited rapid (within 2 min) and large (2-5-fold) increases in PI 3-kinase activity that could be immunoprecipitated using anti-phosphotyrosine (pY) antibodies. Maximal insulin-induced activity of PI 3-kinase in pY-immunoprecipitates was similar in 3T3-L1 adipocytes and mouse adipocytes, but the kinetics of activation differed. Insulin- and IGF-I-induced changes in PI 3-kinase were each half-maximal at 3-5 nM of hormone and were not additive. Increases in both insulin-induced and IGF-I-induced pY-immunoprecipitable PI 3-kinase activity were observed when 3T3-L1 fibroblasts became confluent and when they adopted the adipocyte phenotype. Pioglitazone (10 microM), administered either acutely or chronically to either 3T3-L1 adipocytes or 3T3-L1 fibroblasts, did not greatly alter the kinetics, magnitude or sensitivity of changes in PI 3-kinase elicited by either insulin or IGF-I. In contrast, the attenuation by isoproterenol of insulin-induced changes in PI 3-kinase was prevented in cells pretreated with pioglitazone. This effect of pioglitazone did not involve inhibition of isoproterenol-elicited accumulation of cyclic AMP. Pioglitazone also prevented attenuation of insulin induced changes in PI 3-kinase by cell penetrating analogs of cyclic AMP. Pioglitazone, therefore, has no direct effect on insulin-stimulated PI 3-kinase activity, but interferes with a cyclic AMP-dependent mechanism that normally antagonizes this action of insulin. These data support the proposition that the facilitation of insulin action by pioglitazone involves, at least in part, an inhibition of a negative control mechanism.
Sizer
K M
KM
Upjohn Laboratories, Kalamazoo, MI 49001.
Smith
C L
CL
Jacob
C S
CS
Swanson
M L
ML
Bleasdale
J E
JE
D019870:1-Phosphatidylinositol 4-Kinase; D015105:3',5'-Cyclic-AMP Phosphodiesterases / Q000378:metabolism; D016475:3T3 Cells; D017667:Adipocytes / Q000201:enzymology*; D000818:Animals; D002454:Cell Differentiation; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D007004:Hypoglycemic Agents; D007163:Immunosorbent Techniques; D007328:Insulin / Q000494:pharmacology*; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D007545:Isoproterenol / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism*; D019000:Phosphotyrosine; D000077205:Pioglitazone; D013844:Thiazoles / Q000494:pharmacology*; D045162:Thiazolidinediones; D014443:Tyrosine / Q000031:analogs & derivatives / Q000276:immunology
D007004:Hypoglycemic Agents; D007328:Insulin; D013844:Thiazoles; D045162:Thiazolidinediones; D019000:Phosphotyrosine; D014443:Tyrosine; D007334:Insulin-Like Growth Factor I; D000242:Cyclic AMP; D017853:Phosphotransferases (Alcohol Group Acceptor); D019870:1-Phosphatidylinositol 4-Kinase; D015105:3',5'-Cyclic-AMP Phosphodiesterases; D007545:Isoproterenol; D000077205:Pioglitazone