Conditional, tissue-specific expression of Q205L G alpha i2 in vivo mimics insulin action.

Deficiency of the G protein subunit G alpha i2 that is known to mediate the inhibitory control of adenylylcyclase impairs insulin action [11]. Using the promoter for the phosphoenolpyruvate carboxykinase gene, conditional tissue-specific expression of the constitutively active mutant form (Q205L) of G alpha i2 was achieved in mice harboring the transgene. Expression of Q205L G alpha i2 was detected in liver and adipose tissue of transgenic mice. Whereas the G alpha i2 deficient mice displayed blunted glucose tolerance, the Q205L G alpha i2 expressing mice displayed enhanced glucose tolerance. Hexose transport and the recruitment of GLUT4, but not GLUT1, transporters to the membrane were elevated in adipocytes from Q205L G alpha i2 expressing mice in the absence of insulin. Additionally, hepatic glycogen synthase was found to be activated in Q205L G alpha i2 expressing mice, in the absence of the administration of insulin. Serum insulin levels in transgenic mice fasted overnight were equivalent to those of their control littermates. These data demonstrate that much as G alpha i2 deficiency leads to insulin resistance, expression of Q205L constitutively active G alpha i2 mimics insulin action in vivo, reflecting a permissive role of G alpha i2 in signaling via this growth factor receptor tyrosine kinase linked pathway.

Induction of Galphaq-specific antisense RNA in vivo causes increased body mass and hyperadiposity.

Transgenic BDF-1 mice harboring an inducible, tissue-specific transgene for RNA antisense to Galphaq provide a model in which to study a loss-of-function mutant of Galphaq in vivo. Galphaq deficiency induced in liver and white adipose tissue at birth produced increased body mass and hyperadiposity within 5 weeks of birth that persisted throughout adult life. Galphaq-deficient adipocytes display reduced lipolytic responses, shown to reflect a newly discovered, alpha1-adrenergic regulation of lipolysis. This alpha1-adrenergic response via phosphoinositide hydrolysis and activation of protein kinase C is lacking in the Galphaq loss-of-function mutants in vivo and provides a basis for the increased fat accumulation.

Jun N-terminal kinase mediates activation of skeletal muscle glycogen synthase by insulin in vivo.

Mitogen-activated protein kinases (MAPKs) represent a conserved family of Ser/Thr protein kinases with central roles in intracellular signaling. Activation of three prominent members of the MAPK family, i.e. extracellular response kinases (ERK), jun N-terminal kinase (JNK), and p38, was defined in vivo in order to establish their role, if any, in the cardinal response of skeletal muscle to insulin, the activation of glycogen synthesis. Insulin was found to activate ERK, JNK, and p38 in skeletal muscle. The time courses for activation of the three MAPKs by insulin, however, are distinctly different. Activation of JNK occurs most rapidly, within seconds. Activation of p38 by insulin follows that of JNK, within minutes. Activation of ERK occurs last, 4 min after administration of insulin. The temporal relationship between the activation of ERK, JNK, p38 and the downstream elements p90(rsk) and PP-1 in vivo suggest that JNK, but neither ERK nor p38 MAPKs, mediates insulin activation of glycogen synthase in vivo. Activation of JNK by anisomycin in vivo mimics activation of glycogen synthase by insulin. Challenge by anisomycin and insulin, in combination, are not additive, suggesting a common mode of glycogen synthase activation. The p90(rsk) isoform rapidly activated by insulin is identified as RSK3. In addition, RSK3 can be activated by JNK in vitro. Based upon these data a signal linkage map for activation of glycogen synthase in vivo in skeletal muscle can be constructed in which JNK mediates activation of glycogen synthase via RSK3.

Insulin action impaired by deficiency of the G-protein subunit G ialpha2.

Integration of information between tyrosine kinase and G-protein-mediated pathways is necessary, but remains poorly understood. Here we use cells from transgenic mice harbouring inducible expression of RNA antisense to the gene encoding G ialpha2 to show that G ialpha2 is critical for insulin action. G ialpha2 deficiency in adipose tissue and liver produces hyperinsulinaemia, impaired glucose tolerance and resistance to insulin in vivo. Insulin resistance affects glucose-transporter activity and recruitment, counterregulation of lipolysis, and activation of glycogen synthase, all of which are cardinal responses to insulin. G ialpha2 deficiency increases protein-tyrosine phosphatase activity and attenuates insulin-stimulated tyrosine phosphorylation of IRS (insulin-receptor substrate 1) in vivo. G ialpha2 deficiency creates a model for insulin resistance characteristic of noninsulin-dependent diabetes mellitus (NIDDM), implicating G ialpha2 as a positive regulator of insulin action.

Suppression of Gi alpha 2 enhances phospholipase C signalling.

G-proteins mediate transmembrane signalling from a populous group of cell-surface receptors to a smaller group of effectors that includes adenylate cyclase, various ion channels and phospholipase C. Stem cells (F9 teratocarcinoma) or rat osteosarcoma 17/2.8 cells in which Gi alpha 2 expression is abolished by antisense RNA display markedly elevated basal inositol 1,4,5-trisphosphate accumulation and a potentiated phospholipase C response to stimulatory hormones. Expression of the Q205L mutant of Gi alpha 2, which is constitutively active, was found to block persistently hormonally stimulated phospholipase C activity, implicating Gi alpha 2 as an inhibitory regulator of phospholipase C signalling. Analysis using Gi alpha 2-deficient adipocytes of transgenic mice provided further evidence for a role for Gi alpha 2 in phospholipase C regulation, demonstrating in vivo that loss of Gi alpha 2 elevates basal, and markedly potentiates hormonally stimulated, phospholipase C activity. This report demonstrates for the first time that a single G-protein, G12, can regulate two distinct signalling pathways, i.e. adenylate cyclase and phospholipase C.

Induction of G alpha i2-specific antisense RNA in vivo inhibits neonatal growth.

Guanosine triphosphate-binding regulatory proteins (G proteins) are key elements in transmembrane signaling and have been implicated as regulators of more complex biological processes such as differentiation and development. The G protein G alpha i2 is capable of mediating the inhibitory control of adenylylcyclase and regulates stem cell differentiation to primitive endoderm. Here an antisense RNA to G alpha i2 was expressed in a hybrid RNA construct whose expression was both tissue-specific and induced at birth. Transgenic mice in which the antisense construct was expressed displayed a lack of normal development in targeted organs that correlated with the absence of G alpha i2. The loss of G alpha i2 expression in adipose tissue of the transgenic mice was correlated with a rise in basal levels of adenosine 3',5'-monophosphate (cAMP) and the loss of receptor-mediated inhibition of adenylylcyclase. These data expand our understanding of G protein function in vivo and demonstrate the necessity for G alpha i2 in the development of liver and fat.

Gi alpha 2 mediates the inhibitory regulation of adenylylcyclase in vivo: analysis in transgenic mice with Gi alpha 2 suppressed by inducible antisense RNA.

The role of the GTP-binding regulatory protein (G-protein) Gi alpha 2 in vivo was explored using transgenic mice in which the alpha-subunit of Gi alpha 2 was suppressed by antisense RNA. Rat hepatoma FTO-2B cells provide an ideal test system for constructs employing the expression vector pPCK-AS, designed to express antisense RNA at birth under the control of the phosphoenolpyruvate carboxykinase (PEPCK) promoter. Cells transfected with the expression vector containing a sequence antisense to Gi alpha 2 (pPCK-ASGi alpha 2) displayed expression of RNA antisense to Gi alpha 2 that, like transcription of the PEPCK gene, was inducible by cyclic AMP. Expression of RNA antisense to Gi alpha 2 and suppression of the expression of Gi alpha 2, but not Gsa and Gi alpha 3, was observed in the transfected FTO-2B cells. BDF1 mice carrying the transgene displayed suppression of Gi alpha 2 in liver and fat, two targets for tissue-specific expression of the PEPCK gene. The loss of Gi alpha 2 in white adipocytes of transgenic mice resulted in 3.1-fold elevation of basal cyclic AMP accumulation. Cyclic AMP accumulation in response to stimulation by epinephrine (10 microM) was normal in adipocytes of transgenic mice, demonstrating no alteration in the stimulatory adenylylcyclase capacity in the Gi alpha 2-deficient cells. The inhibitory adenylylcyclase pathway, in sharp contrast, was severely blunted in response to challenge by the inhibitory A1-purinergic agonist, (-)R-N6-phenylisopropyladenosine. These studies illuminate a critical role of Gi alpha 2 in the inhibitory adenylylcyclase signaling pathway in vivo.