Multi-site phosphorylation of the inhibitory guanine nucleotide regulatory protein Gi-2 occurs in intact rat hepatocytes

Gα12/13 signaling in metabolic diseases

As the key governors of diverse physiological processes, G protein-coupled receptors (GPCRs) have drawn attention as primary targets for several diseases, including diabetes and cardiovascular disease. Heterotrimeric G proteins converge signals from ~800 members of the GPCR family. Among the members of the G protein α family, the Gα₁₂ family members comprising Gα₁₂ and Gα₁₃ have been referred to as gep oncogenes. Gα_12/13 levels are altered in metabolic organs, including the liver and muscles, in metabolic diseases. The roles of Gα_12/13 in metabolic diseases have been investigated. In this review, we highlight findings demonstrating Gα_12/13 amplifying or dampening regulators of phenotype changes. We discuss the molecular basis of G protein biology in the context of posttranslational modifications to heterotrimeric G proteins and the cell signaling axis. We also highlight findings providing insights into the organ-specific, metabolic and pathological roles of G proteins in changes associated with specific cells, energy homeostasis, glucose metabolism, liver fibrosis and the immune and cardiovascular systems. This review summarizes the currently available knowledge on the importance of Gα_12/13 in the physiology and pathogenesis of metabolic diseases, which is presented according to the basic understanding of their metabolic actions and underlying cellular and molecular bases.

Understanding the activities of two members of a vital category of proteins called G proteins, which initiate metabolic changes when signaling molecules bind to cells, could lead to new therapies for many diseases. Researchers in South Korea and Japan, led by Sang Geon Kim at Seoul National University, review the significance of the Gα12 and Gα13 proteins in diseases characterised by significant changes in metabolism, including liver conditions and disorders of the cardiovascular and immune systems. Specific roles for the proteins have been identified by a variety of methods, including studying the effect of disabling the genes that code for them in mice. Recent insights suggest that drugs interfering with the activity of these Gα proteins might help treat many conditions in which the molecular signalling networks involving the proteins are disrupted.

G protein subunit phosphorylation as a regulatory mechanism in heterotrimeric G protein signaling in mammals, yeast, and plants

Heterotrimeric G proteins composed of Gα, Gβ, and Gγ subunits are vital eukaryotic signaling elements that convey information from ligand-regulated G protein-coupled receptors (GPCRs) to cellular effectors. Heterotrimeric G protein-based signaling pathways are fundamental to human health [Biochimica et Biophysica Acta (2007) 1768, 994-1005] and are the target of >30% of pharmaceuticals in clinical use [Biotechnology Advances (2013) 31, 1676-1694; Nature Reviews Drug Discovery (2017) 16, 829-842]. This review focuses on phosphorylation of G protein subunits as a regulatory mechanism in mammals, budding yeast, and plants. This is a re-emerging field, as evidence for phosphoregulation of mammalian G protein subunits from biochemical studies in the early 1990s can now be complemented with contemporary phosphoproteomics and genetic approaches applied to a diversity of model systems. In addition, new evidence implicates a family of plant kinases, the receptor-like kinases, which are monophyletic with the interleukin-1 receptor-associated kinase/Pelle kinases of metazoans, as possible GPCRs that signal via subunit phosphorylation. We describe early and modern observations on G protein subunit phosphorylation and its functional consequences in these three classes of organisms, and suggest future research directions.

Mitotic activation of the DISC1-inducible cyclic AMP phosphodiesterase-4D9 (PDE4D9), through multi-site phosphorylation, influences cell cycle progression

In Rat-1 cells, the dramatic decrease in the levels of both intracellular cyclic 3'5' adenosine monophosphate (cyclic AMP; cAMP) and in the activity of cAMP-activated protein kinase A (PKA) observed in mitosis was paralleled by a profound increase in cAMP hydrolyzing phosphodiesterase-4 (PDE4) activity. The decrease in PKA activity, which occurs during mitosis, was attributable to PDE4 activation as the PDE4 selective inhibitor, rolipram, but not the phosphodiesterase-3 (PDE3) inhibitor, cilostamide, specifically ablated this cell cycle-dependent effect. PDE4 inhibition caused Rat-1 cells to move from S phase into G2/M more rapidly, to transit through G2/M more quickly and to remain in G1 for a longer period. Inhibition of PDE3 elicited no observable effects on cell cycle dynamics. Selective immunopurification of each of the four PDE4 sub-families identified PDE4D as being selectively activated in mitosis. Subsequent analysis uncovered PDE4D9, an isoform whose expression can be regulated by Disrupted-In-Schizophrenia 1 (DISC1)/activating transcription factor 4 (ATF4) complex, as the sole PDE4 species activated during mitosis in Rat-1 cells. PDE4D9 becomes activated in mitosis through dual phosphorylation at Ser585 and Ser245, involving the combined action of ERK and an unidentified 'switch' kinase that has previously been shown to be activated by H2O2. Additionally, in mitosis, PDE4D9 also becomes phosphorylated at Ser67 and Ser81, through the action of MK2 (MAPKAPK2) and AMP kinase (AMPK), respectively. The multisite phosphorylation of PDE4D9 by all four of these protein kinases leads to decreased mobility (band-shift) of PDE4D9 on SDS-PAGE. PDE4D9 is predominantly concentrated in the perinuclear region of Rat-1 cells but with a fraction distributed asymmetrically at the cell margins. Our investigations demonstrate that the diminished levels of cAMP and PKA activity that characterise mitosis are due to enhanced cAMP degradation by PDE4D9. PDE4D9, was found to locate primarily not only in the perinuclear region of Rat-1 cells but also at the cell margins. We propose that the sequestration of PDE4D9 in a specific complex together with AMPK, ERK, MK2 and the H2O2-activatable 'switch' kinase allows for its selective multi-site phosphorylation, activation and regulation in mitosis.

On studying protein phosphorylation patterns using bottom-up LC–MS/MS: the case of human α-casein

Most proteomics studies involving mapping post-translational modifications, such as the phosphorylation of serine and threonine, are performed today using the 'bottom-up' approach. This approach involves enzymatic cleavage of proteins, most often by trypsin, with subsequent nano-LC-MS/MS. The occupancy rates of phosphosites in proteins may differ by orders of magnitude, and thus the occupancy rate must be reported for each occupied phosphosite. To highlight potential pitfalls in quantifying the occupancy rates, alpha(s1)-casein from human milk was selected as a model molecule representing moderately phosphorylated proteins. For this purpose, human milk from one Caucasian woman in the eighth month of lactation was used. The phosphorylation level of caseins is believed to have major implications for the formation of micelles that are involved in delivering valuable calcium phosphate and other minerals to the new-born. Human alpha(s1)-casein has been reported to be much less phosphorylated than ruminant caseins, which may indicate a different function of caseins in humans. Revealing the phosphorylation pattern in human casein can thus shed light on its function. The current study found that the sequence region between the residues Ser70 and Ser76 in human alpha(s1)-casein is in fact phosphorylated, contrary to previous knowledge. The site of the most abundant phosphorylation is Ser75, in agreement with the known action of the mammary gland casein kinase. There is evidence for the second phosphorylation in that region, possibly at Ser73. Earlier reported positions of phosphorylations at Ser18 and Ser26 are also confirmed, but not the dominance of Ser18 phosphorylation. The occupancy rates at Ser18, Ser26 and Ser75 are estimated to be (7 +/- 2), (20 +/- 6) and (27 +/- 9)%, respectively. Owing to differences in the ionization efficiency between phosphorylated and unphosphorylated peptides a 30% error margin is added to the occupancy rates. The highlighted pitfalls of the bottom-up strategy include the sensitivity of enzymes to proximal acidic and phosphorylated residues and the presence of multiple isoforms, including unexpected ones, of the tryptic peptides. The utility of the earlier introduced PhosTS_hunter and ModifiComb approaches for evading the latter pitfall is demonstrated.

Regulation of F-actin and endoplasmic reticulum organization by the trimeric G-protein Gi2 in rat hepatocytes. Implication for the activation of store-operated Ca2+ inflow

The roles of the heterotrimeric G-protein, G(i2), in regulating the actin cytoskeleton and the activation of store-operated Ca(2+) channels in rat hepatocytes were investigated. Galpha(i2) was principally associated with the plasma membrane and microsomes. Both F-actin and Galpha(i2) were detected by Western blot analysis in a purified plasma membrane preparation, the supernatant and pellet obtained by treating the plasma membrane with Triton X-100, and after depolymerization and repolymerization of F-actin in the Triton X-100-insoluble pellet. Actin in the Triton X-100-soluble supernatant co-precipitated with Galpha(i2) using either anti-Galpha(i2) or anti-actin antibodies. The principally cortical location of F-actin in hepatocytes cultured for 0.5 h changed to a pericanalicular distribution over a further 3.5 h. Some Galpha(i2) co-localized with F-actin at the plasma membrane. Pretreatment with pertussis toxin ADP-ribosylated 70-80% of Galpha(i2) in the plasma membrane and microsomes, prevented the redistribution of F-actin, caused redistribution and fragmentation of the endoplasmic reticulum, and inhibited vasopressin-stimulated Ca(2+) inflow. It is concluded that (i) a significant portion of hepatocyte Galpha(i2) associates with, and regulates the arrangement of, cortical F-actin and the endoplasmic reticulum and (ii) either or both of these regulatory roles are likely to be required for normal vasopressin activation of Ca(2+) inflow.

Effects of pertussis toxin on extracellular signal-regulated kinase activation in hepatocytes by hormones and receptor-independent agents: evidence suggesting a stimulatory role of G(i) proteins at a level distal to receptor coupling

It was previously found that pertussis toxin (PTX) pretreatment inhibits the activation of extracellular signal-regulated kinases ERK1 (p44(mapk)) and ERK2 (p42(mapk)) in hepatocytes in response to either agonists that bind to heptahelical receptors or epidermal growth factor (EGF), suggesting a role of G(i) proteins in stimulatory mechanisms for ERK1/2. The present work shows that ERK1/2 is activated in a PTX-sensitive way not only by vasopressin, angiotensin II, prostaglandin (PG) F(2alpha), alpha(1)-adrenergic stimulation, and EGF but also by agents whose actions bypass receptors and stimulate protein kinase C (PKC) and/or elevate intracellular Ca(2+), such as 12-O-tetradecanoyl phorbol-13-acetate (TPA), exogenous phosphatidylcholine-specific phospholipase C (PC-PLC, from Bacillus cereus), thapsigargin, and the Ca(2+) ionophore A23187. Under the same conditions, PTX did not affect agonist stimulation of phosphoinositide-specific phospholipase C (PI-PLC) (IP(3) generation), and did not reduce the activation by these agents of phospholipase D (PLD). The results suggest that in hepatocytes a PTX-sensitive mechanism, presumably involving G(i) proteins, exerts a stimulatory effect on ERK at a level distal to receptor coupling, acting either as an integral part of the signaling pathway(s) or by a permissive, synergistic regulation.

Purification of A1 adenosine receptor-G-protein complexes: effects of receptor down-regulation and phosphorylation on coupling

We examined the effects of exposing A1 adenosine receptors (A1ARs) to an agonist on the stability and phosphorylation state of receptor-guanine nucleotide-binding regulatory protein (R-G-protein) complexes. Non-denatured recombinant human A1ARs extended on the N-terminus with hexahistidine (His6) and the FLAG (Asp-Tyr-Lys-Asp-Asp-Asp-Asp-Lys) epitope (H/F) were purified to near homogeneity from stably transfected Chinese-hamster ovary (CHO)-K1 cells. Purified receptors have pharmacological properties similar to receptors in membranes. G-proteins were co-purified with 15+/-2% of H/F-A1AR unless receptor-G-protein (R-G) complexes were uncoupled by pre-treating cell membranes with GTP. By silver staining, purified A1AR-G-protein complexes contain receptors, G-protein alpha and beta subunits and an unidentified 97 kDa protein. Pretreating intact cells with N6-cyclopentyladenosine (CPA) for 24 h decreased both the total number of receptors measured in membranes and the number of purified A1ARs by about 50%. In contrast, pretreating cells with CPA decreased the number of R-G complexes measured in membranes (54+/-6%) significantly less than it decreased the number of purified R-G complexes (78+/-3%) as detected by 125I-N6-(4-aminobenzyl)adenosine binding or by Western blotting Gialpha2. The effect of CPA to decrease the fraction of receptors purified as R-G complexes was not associated with any change in low-level A1AR phosphorylation (found on serine), or low-level phosphorylation of G-protein alpha or beta subunits or the 97 kDa protein. These experiments reveal a novel aspect of agonist-induced down-regulation, namely a diminished stability of receptor-G-protein complexes that is manifested as uncoupling during receptor purification.

N-Methyl-D-aspartate attenuates opioid receptor-mediated G protein activation and this process involves protein kinase C

The effects of N-methyl-D-aspartate (NMDA) on opioid receptor-mediated G protein activation were explored in neuroblastoma X glioma hybrid (NG108-15) cells. Treatment of the cells with NMDA resulted in a remarkable attenuation of [35S]guanosine-5'-O-(3-thio)triphosphate binding stimulated by [D-Pen2,D-Pen5]-enkephalin (DPDPE), a delta-opioid receptor agonist. The effects of NMDA were dose and time dependent with an IC50 value of 5 nM and could be blocked by NMDA receptor antagonists. After NMDA treatment, the DPDPE dose-response curve shifted to the right (EC50 value increased approximately 7-fold, from 6 to 40 nM), and the maximal response induced by DPDPE was reduced by approximately 60%. The effects of NMDA were reversible, and the DPDPE response could recover within 60 min. The functional responses of delta-, mu-, and kappa-opioid receptors in primarily cultured neurons also were attenuated significantly by NMDA treatment. The inhibitory effects of NMDA on opioid receptor-mediated G protein activation could be blocked by coadministration of the protein kinase C (PKC) inhibitors or by elimination of the extracellular Ca2+. Correspondingly, NMDA treatment of NG108 cells significantly elevated cellular PKC activity and stimulated Gialpha2 phosphorylation. Transient transfection into NG108-15 cells of the wild-type Gialpha2 and a mutated Gialpha2 (Ser144Ala) resulted in a 2-fold increase in DPDPE-stimulated G protein activation. The DPDPE responses were greatly inhibited by NMDA treatment in the wild-type Gialpha2-transfected cells but much less affected in the mutant Gialpha2-transfected cells. In summary, NMDA attenuates opioid receptor/G protein coupling, and this process requires activation of PKC.

Potentiation of receptor-mediated cAMP production: role in the cross-talk between vasopressin V1a and V2 receptor transduction pathways

Cross-talk between the phospholipase C and adenylyl cyclase signalling pathways was investigated in Chinese hamster ovary (CHO) cells transfected with the V1a and V2 vasopressin receptors. Cell lines expressing V1a, V2, or both V1a and V2 receptors, were established and characterized. Stimulation of V2 receptors by vasopressin induced a dose-dependent increase in cAMP accumulation, whereas stimulation of V1a receptor resulted in an increase in intracellular calcium without any change in basal cAMP. The simultaneous stimulation of V2 and V1a receptors by vasopressin elicited an intracellular cAMP accumulation which was twice that induced by stimulation of V2 receptor alone with deamino-[d-Arg8]vasopressin. This potentiation between V1a and V2 receptors was mimicked by activation of protein kinase C (PKC) with PMA, and was suppressed when PKC activity was inhibited by bisindolylmaleimide. The potentiation was observed in the presence or absence of 1 mM 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, implying that an alteration in cAMP hydrolysis was not involved. Vasopressin, as well as PMA, had no effect on the forskolin-induced cAMP accumulation, suggesting that PKC did not directly stimulate the cyclase activity. On the other hand, vasopressin, like PMA, potentiated the cAMP accumulation induced by cholera toxin, an activator of Galphas protein. These results suggest that, in CHO cells, vasopressin V1a receptor potentiates the cAMP accumulation induced by the V2 receptor through a PKC-dependent increase in the coupling between Gs protein and adenylyl cyclase.

1 alpha,25-(OH)2-vitamin D3 stimulates the adenylyl cyclase pathway in muscle cells by a GTP-dependent mechanism which presumably involves phosphorylation of G alpha i

To further understand the mechanism underlying 1,25(OH)2D3 activation of the cAMP pathway, the effect of the hormone on adenylyl cyclase (AC), GTPase and protein kinase A (PKA) activities as well as on the phosphorylation of G alpha i was studied in membranes from chick skeletal muscle cells. The sterol stimulated AC activity in a dose (0.1-10 nM) and time (1-5 min.) dependent fashion, provided GTP (10 microM) was present in the assay. High affinity GTPase activity was unaffected by the hormone. In the absence of GTP or in the presence of Mn2+ (20 mM), 1,25(OH)2D3 effects on AC were abolished. PKA activity was increased (+120%) in cells pretreated (1 nM, 5 min.) with the sterol. Moreover, immunoprecipitation of G alpha i from [32P]-labeled myoblast membranes showed that 5 min. exposure to 1 nM 1,25(OH)2D3 increased (1.5-2 fold) the phosphorylation of its alpha subunit. The present data suggest that in muscle cells, 1,25(OH)2D3 activates AC by a non direct, GTP-dependent action which could imply amelioration of Gi function by sterol-induced alpha i phosphorylation.

Corticosterone alters G protein alpha-subunit levels in the rat hippocampus

The hypothalamic-pituitary-adrenal axis regulates the synthesis and secretion of corticosteroid hormones. The hippocampus, a component of the limbic system, contains the highest concentration of corticosteroid receptors in the brain and may play an important role in regulating hypothalamic-pituitary-adrenal axis activity and mediating physiological responses to stress. The corticosteroid hormone corticosterone alters the response elicited by activation of several different G protein-linked neurotransmitter receptors in the hippocampus. In the present study we used Western blot and immunohistochemical techniques to determine the effects of chronic adrenalectomy (ADX), low basal (CT) and high (HCT) corticosterone treatments on Gs, Gi1 and 2 and Go alpha-subunit levels and intracellular location in the rat hippocampus. CT treatment increased Gs alpha-subunit levels and HCT treatment increased the levels of Gs, Gi1 and 2 and Go alpha-subunits when compared to sham as detected on Western blots. No change in the intracellular location of the G protein alpha-subunits was detected using immunohistochemistry. Based on our results, we conclude that corticosterone alters G protein alpha-subunit levels in the rat hippocampus without altering their intracellular location. These results provide an important piece of information towards understanding how corticosteroids alter G protein-linked neurotransmitter receptor-mediated responses.

Streptozotocin-induced diabetes elicits the phosphorylation of hepatocyte Gi2 alpha at the protein kinase C site but not at the protein kinase A-controlled site

Streptozotocin-induced diabetes caused a profound increase in the steady-state level of phosphorylation of the alpha-subunit of the adenylate cyclase inhibitory protein Gi2 in hepatocytes. Unlike hepatocytes from control animals, those from streptozotocin-diabetic animals showed no increase in the phosphorylation of Gi2 alpha in response to a challenge with the protein kinase C activator phorbol myristate acetate. However, a stimulatory effect of 8-bromo-cAMP on Gi2 alpha phosphorylation was evident in hepatocytes from diabetic animals but this was severely reduced compared with that observed in hepatocytes from normal animals. Two-dimensional tryptic phosphopeptide mapping showed that Gi2 alpha in resting hepatocytes from diabetic animals was phosphorylated exclusively at the protein kinase C site (C-site) but no labelling was evident at the protein kinase A-regulated site (AN-site). Treatment of hepatocytes from diabetic animals with phorbol myristate acetate did not change this pattern of labelling. In contrast, challenge of hepatocytes from diabetic animals with 8-bromo-cAMP led to the appearance of a new labelled phosphopeptide that was consistent with labelling at the AN-site. Analysis of the C-site and AN-site phosphopeptides from hepatocytes of diabetic animals treated with 8-bromo-cAMP showed that the increase in labelling of Gi2 alpha caused by this ligand could be attributed almost entirely to labelling at the AN-site. Thus streptozotocin diabetes appears to cause enhanced labelling of hepatocyte Gi2 alpha by exclusively increasing phosphorylation at the C-site. It is suggested that the increased labelling at the C-site reflects an augmentation of the protein kinase C signalling system in hepatocytes from streptozotocin-induced diabetic animals. This may have wide-spread functional consequences for these cells and may result either from an increased protein kinase C activity and/or a reduction in protein phosphatase 1 and/or 2A activity.

Insulin and vasopressin elicit inhibition of cholera-toxin-stimulated adenylate cyclase activity in both hepatocytes and the P9 immortalized hepatocyte cell line through an action involving protein kinase C

Incubation of hepatocytes or the SV40-DNA-immortalized hepatocyte P9 cell line with cholera toxin led to a time-dependent activation of adenylate cyclase activity, which occurred after a defined lag period. When added together with cholera toxin, each of the hormones insulin and vasopressin was capable of attenuating the maximum stimulatory effect achieved by cholera toxin over a period of 60 min through a process which could be blocked by the compounds staurosporine and chelerythrine. Attenuating effects on cholera-toxin-stimulated adenylate cyclase activity could also be elicited by using either the protein kinase C (PKC)-stimulating phorbol ester PMA (phorbol 12-myristate 13-acetate) or the protein phosphatase inhibitor okadaic acid. Alkaline phosphatase treatment of membranes reversed the inhibitory effect of PMA. Cholera toxin also stimulated the adenylate cyclase activity of intact CHO (Chinese-hamster ovary) and NIH-3T3 cells, but this activity was insensitive to the addition of PMA. Overexpression of various PKC isoforms in CHO cell lines did not confer sensitivity to inhibition by PMA upon cholera-toxin-stimulated adenylate cyclase activity. Rather, overexpression of the gamma isoform of PKC allowed PMA to stimulate adenylate cyclase activity in CHO cells. It is suggested that the PKC-mediated phosphorylation of a membrane protein attenuates cholera-toxin-stimulated adenylate cyclase activity in hepatocytes and P9 cells. The cellular selectivity of such an action may be due to the target for this inhibitory action of PKC being a particular isoform of adenylate cyclase which provides the major activity in hepatocytes and P9 cells, but not in either CHO or NIH-3T3 cells.

Induction of Ca2+/calmodulin-stimulated cyclic AMP phosphodiesterase (PDE1) activity in Chinese hamster ovary cells (CHO) by phorbol 12-myristate 13-acetate and by the selective overexpression of protein kinase C isoforms

The cAMP phosphodiesterase (PDE) activity of CHO cells was unaffected by the addition of Ca2+ +calmodulin (CaM), indicating the absence of any PDE1 (Ca2+/CaM-stimulated PDE) activity. Treatment with the tumour promoting phorbol ester phorbol 12-myristate 13-acetate (PMA) led to the rapid transient induction of PDE1 activity which attained a maximum value after about 13 h before slowly decreasing. Such induction was attenuated by actinomycin D. PCR primers were designed to hybridize with two regions identified as being characteristic of PDE1 forms found in various species and predicted to amplify a 601 bp fragment. RT-PCR using degenerate primers allowed an approx. 600 bp fragment to be amplified from RNA preparations of rat brain but not from CHO cells unless they had been treated with PMA. CHO cells transfected to overexpress protein kinase C (PKC)-alpha and PKC-epsilon, but not those transfected to overexpress PKC-beta I or PKC-gamma, exhibited a twofold higher PDE activity. They also expressed a PDE1 activity, with Ca2+/CaM effecting a 1.8-2.8-fold increase in total PDE activity. RT-PCR, with PDE1-specific primers, identified an approx. 600 bp product in CHO cells transfected to overexpress PKC-alpha and PKC-epsilon, but not in those overexpressing PKC-beta I or PKC-gamma. Treatment of PKC-alpha transfected cells with PMA caused a rapid, albeit transient, increase in PDE1 activity, which reached a maximum some 1 h after PMA challenge, before returning to resting levels some 2 h later. The residual isobutylmethylxanthine (IBMX)-insensitive PDE activity was dramatically reduced (approx. 4-fold) in the PKC-gamma transfectants, suggesting that the activity of the cyclic AMP-specific IBMX-insensitive PDE7 activity was selectively reduced by overexpression of this particular PKC isoform. These data identify a novel point of 'cross-talk' between the lipid and cyclic AMP signalling systems where the action of specific PKC isoforms is shown to cause the induction of Ca2+/CaM-stimulated PDE (PDE1) activity. It is suggested that this protein kinase C-mediated process might involve regulation of PDE1 gene expression by the AP-1 (fos/jun) system.

Insulin inhibits the phosphorylation of alpha-Gi-2 in intact hepatocytes

Challenge of intact hepatocytes with insulin reduced the level of phosphorylated alpha-Gi-2 found under basal (resting) conditions. At maximally effective concentrations of insulin the steady-state labelling of alpha-Gi-2 was reduced by approximately 21%. Insulin achieved this in a time- and dose-dependent fashion, exhibiting an IC50 value of 109 +/- 22 pM. The increased labelling of alpha-Gi-2 seen after challenge of cells with phorbol 12-myristate 13-acetate was also attenuated by insulin. Treatment of hepatocytes with the protein phosphatase inhibitor okadaic acid increased the labelling of alpha-Gi-2 in a fashion which was insensitive to the action of insulin. It is suggested that insulin may reduce the level of phosphorylation of alpha-Gi-2 by stimulating intracellular protein phosphatase activity and that this action may offer a molecular explanation for the ability of insulin to inhibit adenylate cyclase activity in hepatocytes by increasing the level of non-phosphorylated alpha-Gi-2.

A role for protein kinase C-mediated phosphorylation in eliciting glucagon desensitization in rat hepatocytes

An immobilized hepatocyte preparation was used to show that both vasopressin and glucagon could desensitize the ability of glucagon to increase intracellular cyclic AMP concentrations. This process was not dependent on any influx of extracellular Ca2+ and was not mediated by any rise in the intracellular level of Ca2+. The protein kinase C-selective inhibitors chelerythrine, staurosporine and calphostin C acted as potent inhibitors of the desensitization process but with various degrees of selectivity regarding their ability to inhibit the desensitizing actions of glucagon and vasopressin. The protein phosphatase inhibitor okadaic acid was just as potent as vasopressin and glucagon in causing desensitization. Treatment of hepatocyte membranes with alkaline phosphatase restored to near control levels the ability of glucagon to stimulate adenylate cyclase activity in membranes from both glucagon- and vasopressin-treated (desensitized) hepatocytes. It is suggested that the desensitization of glucagon-stimulated adenylate cyclase activity involves a reversible phosphorylation reaction with the likely target being the glucagon receptor itself.