cAMP-independent synergistic effects of insulin and dexamethasone on fructose 2,6-bisphosphate metabolism in H4IIE cells.
Hormonal regulation of fructose 2,6-bisphosphate (Fru-2,6-P2) content was studied in H4IIE cells. These cells were found to be very sensitive to physiological concentrations of insulin. Addition of either insulin or dexamethasone alone increased Fru-2,6-P2 in a time- and dose-dependent manner, and the maximal effect of the hormones was seen at 1 h. Neither hormone had any measurable effect on cAMP levels. The effect of addition of both insulin and dexamethasone on Fru-2,6-P2 was synergistic. Insulin, but not dexamethasone, rapidly increased 6-phosphofructo-2-kinase (6PF-2-K) activity by causing dephosphorylation of the enzyme as judged by a decrease in the Km for fructose-6-phosphate. Addition of both hormones also resulted in a synergistic 10-fold increase in enzyme protein as measured by kinase activity and phosphoenzyme formation. Dexamethasone increased liver 6PF-2-K/Fru-2,6-P2 mRNA abundance by 10- to 12-fold as measured by a ribonuclease protection assay, and insulin increased it by only 4-fold. Effects were observed as early as 1 h after hormone addition, but addition of both hormones together showed no synergy. We conclude that the synergistic effects of insulin and dexamethasone on Fru-2,6-P2 content are mediated by a combination of stimulation of expression of the bifunctional enzyme gene by both hormones and insulin-induced modulation of the activation state of the bifunctional enzyme, both of which are mediated by cAMP-independent mechanisms.
Vargas
A M
AM
Department Bioquimica y Biologia Molecular, Facultad de Ciencias, Granada, Spain.
Sola
M M
MM
Lange
A J
AJ
Poveda
G
G
Pilkis
S J
SJ
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology*; D000818:Animals; D002460:Cell Line; D016895:Culture Media, Serum-Free; D000242:Cyclic AMP / Q000378:metabolism*; D003907:Dexamethasone / Q000494:pharmacology*; D004357:Drug Synergism; D005635:Fructosediphosphates / Q000378:metabolism*; D007328:Insulin / Q000494:pharmacology*; D007700:Kinetics; D008114:Liver Neoplasms, Experimental; D025481:Phosphofructokinase-2; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism; D051381:Rats; D013997:Time Factors; D014407:Tumor Cells, Cultured
D016895:Culture Media, Serum-Free; D005635:Fructosediphosphates; D007328:Insulin; D015124:8-Bromo Cyclic Adenosine Monophosphate; C027652:fructose 2,6-diphosphate; D003907:Dexamethasone; D000242:Cyclic AMP; D017853:Phosphotransferases (Alcohol Group Acceptor); D025481:Phosphofructokinase-2; D010744:Phosphoric Monoester Hydrolases