Effects of alloxan diabetes on the turnover of rat liver glycogen synthase. Comparison with liver phosphorylase

Dynamic Metabolic Zonation of the Hepatic Glucose Metabolism Is Accomplished by Sinusoidal Plasma Gradients of Nutrients and Hormones

Being the central metabolic organ of vertebrates, the liver possesses the largest repertoire of metabolic enzymes among all tissues and organs. Almost all metabolic pathways are resident in the parenchymal cell, hepatocyte, but the pathway capacities may largely differ depending on the localization of hepatocytes within the liver acinus-a phenomenon that is commonly referred to as metabolic zonation. Metabolic zonation is rather dynamic since gene expression patterns of metabolic enzymes may change in response to nutrition, drugs, hormones and pathological states of the liver (e.g., fibrosis and inflammation). This fact has to be ultimately taken into account in mathematical models aiming at the prediction of metabolic liver functions in different physiological and pathological settings. Here we present a spatially resolved kinetic tissue model of hepatic glucose metabolism which includes zone-specific temporal changes of enzyme abundances which are driven by concentration gradients of nutrients, hormones and oxygen along the hepatic sinusoids. As key modulators of enzyme expression we included oxygen, glucose and the hormones insulin and glucagon which also control enzyme activities by cAMP-dependent reversible phosphorylation. Starting with an initially non-zonated model using plasma profiles under fed, fasted and diabetic conditions, zonal patterns of glycolytic and gluconeogenetic enzymes as well as glucose uptake and release rates are created as an emergent property. We show that mechanisms controlling the adaptation of enzyme abundances to varying external conditions necessarily lead to the zonation of hepatic carbohydrate metabolism. To the best of our knowledge, this is the first kinetic tissue model which takes into account in a semi-mechanistic way all relevant levels of enzyme regulation.

The relative importance of kinetic mechanisms and variable enzyme abundances for the regulation of hepatic glucose metabolism--insights from mathematical modeling

Background

Adaptation of the cellular metabolism to varying external conditions is brought about by regulated changes in the activity of enzymes and transporters. Hormone-dependent reversible enzyme phosphorylation and concentration changes of reactants and allosteric effectors are the major types of rapid kinetic enzyme regulation, whereas on longer time scales changes in protein abundance may also become operative. Here, we used a comprehensive mathematical model of the hepatic glucose metabolism of rat hepatocytes to decipher the relative importance of different regulatory modes and their mutual interdependencies in the hepatic control of plasma glucose homeostasis.

Results

Model simulations reveal significant differences in the capability of liver metabolism to counteract variations of plasma glucose in different physiological settings (starvation, ad libitum nutrient supply, diabetes). Changes in enzyme abundances adjust the metabolic output to the anticipated physiological demand but may turn into a regulatory disadvantage if sudden unexpected changes of the external conditions occur. Allosteric and hormonal control of enzyme activities allow the liver to assume a broad range of metabolic states and may even fully reverse flux changes resulting from changes of enzyme abundances alone. Metabolic control analysis reveals that control of the hepatic glucose metabolism is mainly exerted by enzymes alone, which are differently controlled by alterations in enzyme abundance, reversible phosphorylation, and allosteric effects.

Conclusion

In hepatic glucose metabolism, regulation of enzyme activities by changes of reactants, allosteric effects, and reversible phosphorylation is equally important as changes in protein abundance of key regulatory enzymes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0237-6) contains supplementary material, which is available to authorized users.

The action of aminoguanidine on the liver of trained diabetic rats

Background

This study evaluated the effect of aminoguanidine on liver of diabetic rats subject to physical exercises using histological and histochemical techniques.

Methods

The rats used in this study were divided into five groups: sedentary control, sedentary diabetic, trained diabetic, sedentary diabetic and treated with aminoguanidine, trained diabetic and treated with aminoguanidine.

Results

The results showed no effect of aminoguanidine on the liver tissue, although there was improvement with exercise training showing cytological, morpho-histological and histochemical alterations in liver cells of animals from groups trained diabetic and/or treated diabetic compared to those individuals in the sedentary control and sedentary diabetic. These changes included: hepatocytes hypertrophy, presence and distribution of polysaccharides in the hepatocytes cytoplasm and, especially, congestion of the liver blood vessels.

Conclusion

Our results suggest that aminoguanidine is not hepatotoxic, when used at dosage of 1 g/L for the treatment of diabetes complications, and confirmed that the practice of moderate physical exercise assuaged the damage caused by diabetes without the use of insulin.

Electronic supplementary material

The online version of this article (doi:10.1186/2251-6581-12-40) contains supplementary material, which is available to authorized users.

Morphology and protein content of hepatocytes in type I diabetic rats submitted to physical exercises

The importance of physical exercise practice in the treatment of diabetes has been reported in many studies recently, but only limited data can be found regarding its benefits on liver morphology and protein content of hepatocytes. In order to assess the changes arising from the development of type I diabetes and the benefits of a training protocol, Wistar rats were divided into four groups: sedentary control (SC), trained control (TC), sedentary diabetic (SD) and trained diabetic (TD). The training protocol consisted of swimming for 60 min a day, 5 days/week, during 8 weeks. Liver samples were collected, processed and analyzed by histochemical and ultrastructural techniques. Biochemical tests were also conducted to examine the protein content and quantity of DNA in the liver. In morphological assessment, the presence of areas of cytoplasmic basophilia observed in control subjects was not visualized in sedentary diabetics. It was related to differences in the amount of mitochondria in the cytosol. The mitochondrial structure has not undergone relevant changes, and the number of rough endoplasmic reticulum cisterns was clearly inferior in sedentary diabetics, suggesting lower protein production. However, the biochemical analysis of protein content indicated no statistical differences between groups. The exercise, in turn, was not responsible for major changes in these characteristics. On the whole, the morphological damages arising from type I diabetes were noteworthy. Nevertheless, regular physical training was not responsible for significant improvements in some respects, making evident the need for combined application of a distinct form of treatment.

Expression of motilin in the hypothalamus and the effect of central erythromycin on gastric motility in diabetic rats

OBJECTIVE

To investigate the expression of motilin-immunoreactive neurons in the hypothalamus and the effect of central administration of erythromycin (EM) on the regulation of gastric motility in diabetic rats.

METHODS

The motilin immunoreactive neurons in the hypothalamus and the hippocampus were detected by immunohistochemistry with rabbit anti-motilin polyclonal antibody. To measure the gastric motility, force transducers were surgically affixed to the gastric serosa. A microinjection syringe was connected via a plastic tube to an injection cannula, which was connected with a stainless steel guide cannula. The syringe was inserted into the right lateral cerebral ventricle for microinjecting the chemicals.

RESULTS

Diabetic mellitus was successfully induced in cohorts of rats. Motilin-immunoreactive neurons significantly increased in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus in the diabetic rats. Intracerebroventricular (i.c.v.) administration of EM, a motilin receptor agonist, stimulated the gastric motility of diabetic rats. EM (91.56 nmol, i.c.v.) dose-dependently increased the amplitude by (174.82 +/- 48.62)% (P<0.05), and increased the frequency by (70.43 +/- 27.11)% (P < 0.05) in 5 min. The stimulatory effect lasted more than 15 min to the end of the measurement, and can be blocked partially by the prior treatment of motilin receptor antagonist GM-109.

CONCLUSION

Motilin-immunoreactive neurons are increased in the PVN and SON of the hypothalamus in diabetic rats. Centrally administered EM may regulate gastric motility by binding to the central motilin receptors, and central motilin might be involved in regulation of gastric motility in diabetic rats.

POSSIBLE ANTIDIABETIC AND ANTIHYPERLIPIDAEMIC EFFECT OF FERMENTED PARKIA BIGLOBOSA (JACQ) EXTRACT IN ALLOXAN-INDUCED DIABETIC RATSs

1. The hypoglycaemic effect of fermented seeds of Parkia biglobosa (PB; African locust bean), a natural nutritional condiment that features frequently in some African diets as a spice, was investigated in the present study in alloxan-induced diabetic rats. Its effect was compared with that of glibenclamide (Daonil; Sanofi-Aventis, Paris, France), a reference antidiabetic drug. The effects of PB on lipid profiles were also examined. 2. In order to assess the hypoglycaemic and hypolipidaemic effects of aqueous and methanolic extracts of PB on experimental animals, fasting plasma glucose (FPG), total cholesterol, triglyceride, high-density lipoprotein (HDL) and low-density lipoprotein (LDL) were determined. In addition, the weight of each animal was determined to assess any possible weight gain or loss in the experimental animals (diabetic rats treated with Daonil (group C), the aqueous extract of PB (group D) or the methanolic extract of PB (group E)) compared with control groups (non-diabetic (group A) and non-treated diabetic (group B)). 3. A single dose of 120 mg/kg, i.v., alloxan administered to rats resulted in significant increases in the FPG (P < 0.001) of test animals compared with controls. However, dietary supplementation with PB (6 g/kg extract for 4 weeks administered orally using an intragastric tube) ameliorated the alloxan-induced diabetes in a manner comparable with that of the reference antidiabetic drug glibenclamide. Aqueous and methanolic extracts of PB (6% w/w) elicited 69.2% and 64.4% reductions, respectively, in FPG compared with 70.4% in 0.01 mg/150 g glibenclamide-treated rats. 4. Although animals treated with the aqueous extract of PB gained weight in manner similar to normal controls, animals given the methanolic extract and glibenclamide lost weight in manner similar to non-treated diabetic rats. In addition, high levels of HDL and low LDL were observed in animals treated with the aqueous extract of PB, a pattern similar to that seen in normal controls. Low levels of HDL and high levels of LDL were observed in animals treated with the methanolic extract of PB, a pattern similar to that seen in non-treated diabetic controls. 5. The results of the present study demonstrate that both aqueous and methanolic extracts of fermented seeds of PB exert a hypoglycaemic effect; hence, PB has an antidiabetic property. However, only the aqueous extract of PB ameliorated the loss of bodyweight usually associated with diabetes. Although the aqueous extract has a favourable lipid profile, which is probably an indication of its possible anti-arteriogenic property (hypertension and ischaemic heart diseases being common complications in diabetes mellitus), the methanolic extract shows possible contraindication to ischaemic heart diseases.

Anti-diabetic activity of the semi-purified fractions of Averrhoa bilimbi in high fat diet fed-streptozotocin-induced diabetic rats

The present study was designed to investigate the hypoglycemic and hypolipidemic activities of the semi-purified fractions of an ethanolic leaf extract of Averrhoa bilimbi (ABe) in high fat diet (HFD)-streptozotocin (STZ)-induced diabetic rats. Male Sprague-Dawley rats aged 10 weeks (200-250 g) were fed with a high fat diet obtained from Glen Forrest stock feeders (Western Australia) for 2 weeks prior to intraperitoneal injection with streptozotocin (STZ, 50 mg/kg). The leaves of A.bilimbi were exhaustively extracted with 80% ethanol, concentrated at 40 degrees C using a rotavapor and partitioned successively with butanol, ethylacetate and hexane to get aqueous (AF), butanol (BuF), ethylacetate (EF), and hexane fractions (HF). The fractions were freeze-dried to obtain powders of each. To investigate the effect of long term administration of the hypoglycemic fractions, diabetic animals were treated with vehicle (distilled water), AF (125 mg/kg), or BuF (125 mg/kg), twice a day for 14 days. The long term administration of AF and BuF at a dose of 125 mg/kg significantly (P < 0.05) lowered blood glucose and triglyceride concentrations when compared to the vehicle. The hepatic glycogen content was significantly higher (P < 0.05) in AF-treated rats when compared to diabetic control, however no change was found in the BuF-treated rats. Moreover, AF as well as BuF did not cause any significant change in the total cholesterol and HDL-cholesterol. There was also no difference in liver thiobarbituric acid reactive substances (TBARS) and cytochrome P450 values between AF, BuF and vehicle-treated control rats. In conclusion, the results indicate that AF is more potent than BuF in the amelioration of hyperglycemia and hyperlipidemia in HFD fed-STZ diabetic rats. Hence, AF is a potential source for the isolation of active principle(s) for oral anti-diabetic therapy.

Diet and diabetic state modify glycogen synthase activity and expression in rat hepatocytes

Glycogen synthase (GS), a key regulatory enzyme in glycogen synthesis, is controlled by multisite phosphorylation and allosteric regulation and is activated by insulin. This study investigated changes in GS activity and expression in hepatocytes isolated from rats under altered nutritional and diabetic conditions. Experiments were carried out in healthy rats fed a chow diet, rats on high simple sugar (60% of energy from fructose and sucrose) or high fat (46% of energy from fat) diet, and in rats with streptozotocin induced diabetes. In the presence of insulin, activated GS activity (GS(I) form) was increased by 89% in hepatocytes isolated from healthy rats. The stimulatory effect of insulin on GS activity and expression was blunted by cycloheximide and actinomycin treatment. In rats fed a high simple sugar or high fat diet, insulin stimulation of GS(I) in isolated hepatocytes was impaired and GS expression was significantly lower in rats fed the high fat diet in comparison to controls. GLUT-2 protein expression was significantly lowered by both the high fat and high simple sugar diets. In hepatocytes isolated from diabetic rats, total GS activity (GS(T)) was lower than in hepatocytes from healthy animals. Insulin added to the incubation medium did not stimulate GS activity, demonstrating impaired sensitivity to insulin in diabetic rats. However, insulin administration significantly increased GS expression indicating that a defect in synthase phosphorylation may be responsible for impaired GS activity in the diabetic state. The results presented in this study further confirm that GS activity is affected by both dietary and hormonal factors which can be measured in a rat hepatocyte model.

Insulin replacement therapy in diabetic rats using an osmotic pump normalizes expression of enzymes key to hepatic carbohydrate metabolism

Intensively treating type I diabetics with continuous subcutaneous insulin infusions or multiple daily insulin injections to normalize mean blood glucose concentrations significantly reduces the onset of secondary diabetic complications when compared to conventionally treated diabetics. Our studies focused on characterizing hepatic enzyme expression in intensively and conventionally treated diabetic rats. Alloxan-induced diabetic rats were conventionally treated with insulin injected twice daily or intensively treated with similar daily dosages of insulin administered via a surgically implanted osmotic pump. Our results demonstrate a significant difference in hepatic enzyme expression when these treatment regimes are compared. In conventionally treated diabetic rats, phosphoenolpyruvate carboxykinase (PEPCK) protein and mRNA levels remained slightly elevated when compared to normal animals, glycogen phosphorylase (GP) protein levels were still slightly decreased, and glycogen synthase (GS) protein and mRNA levels remained at the elevated levels observed in untreated diabetics. In contrast, the protein and mRNA levels of all three enzymes were normalized in the insulin pump-treated animals. These results suggest that intensive insulin therapy improves glycemia directly by normalizing hepatic gene expression while conventional insulin therapy normalizes plasma glucose concentrations indirectly.

Improved hepatocyte culture system for studying the regulation of glycogen synthase and the effects of diabetes

When 3-4-week-old rats (young rats) are used as a source of hepatocytes, primary culture cells express the adult, differentiated, liver-specific isoform of glycogen synthase. Synthase enzyme protein levels are relatively stable over a 3 day culture period in young but not in adult (> 150 g rat) hepatocyte cultures. Corresponding synthase enzyme activity and mRNA levels decrease over time in culture in adult but not in young hepatocyte cultures. Young rat hepatocytes also have the ability to proliferate in chemically defined medium in the absence of added mitogens. A diabetes-induced increase in total synthase activity has been demonstrated by our lab and others, using cultured hepatocytes, liver homogenates, and perfused livers. In the present study, utilizing synthase-specific antibody and primary cultures of cells from young normal and alloxan diabetic rats, we found that greater total synthase activity in the diabetic cells was associated with higher levels of enzyme protein. Immuneprecipitation of 35S methionine-labeled freshly plated cells demonstrates an increase in the rate of protein synthesis in diabetic as compared with normal cells. Synthase mRNA levels are correspondingly increased in the diabetic relative to normal cells. Chronic exposure of young, normal hepatocytes to increasing levels of glucose induces a dose-dependent increase in total synthase activity, total synthase protein, and synthase message levels. By comparison, cells from diabetic animals do not respond by any of these measures to increased glucose concentrations. We conclude that this defined primary culture system represents a useful model for investigating the regulation of hepatic glycogen synthase and the defects which occur as a result of diabetes.

The effects of streptozotocin-induced diabetes and insulin supplementation on expression of the glycogen phosphorylase gene in rat liver

We have previously observed that the chronic effects of streptozotocin-induced diabetes cause a decrease in the total hepatic glycogen phosphorylase activity with a corresponding reduction in the phosphorylase protein levels. These effects were normalized by insulin administration to diabetic rats. There was no change in the total glycogen synthase activity as a result of diabetes or insulin supplementation. These results are extended to examine the effects of diabetes and insulin administration to diabetic animals on the expression of phosphorylase and glycogen synthase enzymes. The expression (i.e. mRNA levels) of phosphorylase was down-regulated (45% of normal levels) in diabetic livers, and this was normalized by insulin supplementation to diabetic animals. Diabetes or insulin supplementation to diabetic rats showed no effect on the transcription rate of phosphorylase. As expected, diabetes (or insulin administration to diabetic animals) did not cause any alteration in the mRNA levels or in the transcription rate of hepatic glycogen synthase. The stability of phosphorylase mRNA was then examined using hepatocytes prepared from normal and diabetic rats. Diabetes caused a decrease in the half-life of phosphorylase mRNA from 14 h in normal hepatocytes to 6.5 h in diabetic hepatocytes. Insulin supplementation to the medium of diabetic hepatocytes increased the half-life of phosphorylase mRNA to a level comparable with normal values. This study indicates that the chronic effect of insulin on the activation of the total hepatic phosphorylase activity (and protein) is mediated through the stabilization of its mRNA levels.

Effects of glucose on the activation and translocation of glycogen synthase in diabetic rat hepatocytes

Incubation of hepatocytes from diabetic rats with glucose results in the translocation of glycogen synthase from soluble fractions to fractions which sediment at 9200 g. The extent of the translocation correlates positively with the intracellular concentration of glucose 6-phosphate. No difference was found between healthy and diabetic rats in the capacity of glycogen synthase to translocate to pellets in response to an increase in glucose 6-phosphate. In diabetic hepatocytes, glycogen synthase in the supernatant fractions was not activated upon incubation of the cells with glucose, whereas this sugar was able to activate the enzyme found in the fractions that could be pelleted. In the 9200-g pellets, the glycogen synthase activity ratio (-glucose 6-phosphate/+glucose 6-phosphate) from both healthy and diabetic animals correlated with the intracellular glucose 6-phosphate levels. In the supernatants, the glycogen synthase activity ratio from healthy cells also correlated with glucose 6-phosphate levels. In contrast, in diabetic cells the activation state of the soluble enzyme remained essentially unchanged despite the accumulation of glucose 6-phosphate.

Peptide sequences that target cytosolic proteins for lysosomal proteolysis

Lysosomes take up and degrade intracellular proteins in cultured cells in response to serum deprivation, and in tissues of organisms in response to starvation. One mechanism by which proteins enter lysosomes for subsequent degradation requires that substrate proteins contain peptide sequences biochemically related to Lys-Phe-Glu-Arg-Gln (KFERQ).

Glycogen synthase turnover and phosphorylation in rat H4IIE hepatoma cells

Glycogen synthase was isolated from rat H4IIE hepatoma cells by the use of specific antibodies. Immunoprecipitates from cells grown in the presence of [35S]methionine contained two 35S-labeled polypeptides, designated GS1 and GS2, separable by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Labeling of both species was half-maximal after 3 h and remained constant up to 48 h. When cells were incubated with [32P]-phosphate, 32P was incorporated into both species with similar kinetics, half-maximal labeling occurring after 2-3 h. The steady-state ratio 32P/35S was significantly higher for the lower mobility GS2 polypeptide. Pulse-chase experiments showed that the two subunits followed similar kinetics with respect to 35S-labeling. However, the turnover of 32P on the GS2 subunit was significantly faster (t1/2 approximately 30 min) than that on the GS1 subunit (t1/2 approximately 2 h). We suggest that the two polypeptides represent different phosphorylation states of the glycogen synthase subunit and are rapidly interconverted.

Phosphorylase synthesis in diabetic hepatocytes and cardiomyocytes

Whereas total cardiac glycogen phosphorylase activity appears to be unaffected by severe insulin deficiency, a diabetes-induced decreased in hepatic glycogen phosphorylase activity has been demonstrated by our laboratory and others using liver extracts, isolated perfused liver, and cultured hepatocytes. The loss of activity in diabetic liver can be correlated with a drop in protein levels. Using primary cultures of cells from normal and diabetic rats and phosphorylase specific antibodies, we found a corresponding decrease in phosphorylase synthesis in diabetic hepatocytes cultured for 2 days in a serum-free, chemically defined medium. When hepatocytes are cultured in the presence of insulin, triiodothyronine, and cortisol, there is a significant recovery in the rate of phosphorylase synthesis after 3 days. Over the 3-day time period, there is no significant difference in the rate of phosphorylase degradation in normal compared with diabetic hepatocytes. Total protein synthesis in both hepatocytes and cardiomyocytes is unaffected by diabetes, as is phosphorylase synthesis in cultured cardiomyocytes.

The family of glycogen phosphorylases: structure and function

Glycogen phosphorylase plays a central role in the mobilization of carbohydrate reserves in a wide variety of organisms and tissues. While rabbit muscle phosphorylase remains the most studied and best characterized of phosphorylases, recombinant DNA techniques have led to the recent appearance of primary sequence data for a wide variety of phosphorylase enzymes. The functional properties of rabbit muscle phosphorylases are reviewed and then compared to properties of phosphorylases from other tissues and organisms. Tissue expression patterns and the chromosomal localization of mammalian phosphorylases are described. Differences in functional properties among phosphorylases are related to new structural information. Evolutionary relationships among phosphorylases as afforded by comparative analysis of proteins and gene sequences are discussed.

Insulin-dependent changes in lysosomal cathepsin D activity in rat liver, kidney, brain and heart

Streptozotocin-induced diabetes resulted in a decrease in the cathepsin D activity (free and total) in rat liver, kidney, brain and heart with a concomitant increase in tissue protein content and amino acids pool size. Treatment with insulin brought about the restoration of the cathepsin D activity to normal or greater than normal levels; tissue protein content and amino acids pool size also returned to normal values.

Isolation of partial cDNAs for rat liver and muscle glycogen phosphorylase isozymes

cDNA clones for the rat liver and muscle glycogen phosphorylase isozymes have been isolated using isozyme-selective antibodies and libraries prepared in the expression vector, lambda gt11. A 1.2 kb cDNA coding for the carboxy-terminal domain of rat liver phosphorylase was found to have 82% homology with the amino acid sequence of rabbit muscle phosphorylase. Limited sequencing of rat muscle phosphorylase cDNA indicated a 95% homology with the rabbit muscle enzyme. The rat liver clone has eight additional amino acid residues at the COOH-terminus compared to the rat muscle clone. Furthermore, 17 of 26 (65%) residues between amino acids 815-840 differ between liver and muscle isozymes. The similarity in enzymatic properties and conservation of structure except at the COOH-terminus suggest that the liver and muscle phosphorylase isozymes do not exist in order to have significant differences in the regulation of glycogen breakdown in the two tissues. Rather, the phosphorylase isozymes probably evolved for tissue-specific transcriptional regulation of the genes in liver and muscle.

Phosphorylation of rat liver glycogen synthase bound to the glycogen particle

Rat liver glycogen synthase bound to the glycogen particle was partially purified by repeated high-speed centrifugation. This synthase preparation was labeled with 32P by incubations with cAMP-dependent protein kinase and cAMP-independent synthase (casein) kinase-1 in the presence of [gamma-32P]ATP. The phosphorylated synthase was separated from other proteins in the glycogen pellet by immunoprecipitation with rabbit anti-rat liver glycogen synthase serum. Analysis of the immunoprecipitates by sodium dodecyl sulfate-gel electrophoresis showed that synthase subunits of Mr 85,000 and 80,000 were present in varying proportions. The 32P-labeled synthase in the immunoprecipitate was digested with trypsin, and the resulting peptides were analyzed by isoelectric focusing. Synthase bound to the glycogen particle was phosphorylated by cAMP-dependent protein kinase at more sites and by cAMP-independent synthase (casein) kinase-1 at less sites than when the homogeneous synthase was incubated with these kinases. Phosphorylation of synthase in the glycogen pellet by either cAMP-dependent protein kinase or cAMP-independent synthase (casein) kinase-1 did not cause a significant inactivation as has been observed when the synthase was incubated with these kinases. Inactivation of synthase in the glycogen pellet, however, can be achieved by the combination of both kinases. This inactivation appears to result from the phosphorylation of a new site by cAMP-independent synthase (casein) kinase-1 neighboring a site previously phosphorylated by cAMP-dependent protein kinase.