Substrate specificity of phosphorylase kinase: effects of heparin and calcium

Calcium-dependent insulin resistance in hepatocytes: mathematical model

Hepatocyte insulin resistance is one of the early factors of developing type II diabetes. If insulin resistance is treated early, type II diabetes could be prevented. In recent years, scientists have been conducting extensive research on the underlying issues on a cellular and molecular level. It was found that the modulation of IP₃-receptors, the mitochondrial ability to form the mitochondria-associated membranes (MAMs) and the endoplasmic reticulum stress during Ca²⁺ signaling play a key role in hepatocyte being able to maintain euglycemia and provide metabolic flexibility. However, researchers cannot agree on what factor is the key one in resulting in insulin resistance. In this work, we propose a mathematical model of Ca²⁺ signaling. We included in the model all the major contributors of a proper Ca²⁺ signaling during both the fasting and the postprandial state. Our modeling results are in good agreement with available experimental data. The analysis of modeling results suggests that MAMs dysfunction alone cannot result in abnormal Ca²⁺ signaling and the wrong modulation of IP₃-receptors is a more definite reason. However, both the MAMs dysfunction and the IP₃ signaling dysregulation combined can lead to a robust Ca²⁺ signal and improper glucose release. In addition, our model results suggest a strong dependence of Ca²⁺ oscillations pattern on morphological characteristics of the ER and the mitochondria.

Calcium Homeostasis and Organelle Function in the Pathogenesis of Obesity and Diabetes

A number of chronic metabolic pathologies, including obesity, diabetes, cardiovascular disease, asthma, and cancer, cluster together to present the greatest threat to human health. As research in this field has advanced, it has become clear that unresolved metabolic inflammation, organelle dysfunction, and other cellular and metabolic stresses underlie the development of these chronic metabolic diseases. However, the relationship between these systems and pathological mechanisms is poorly understood. Here we discuss the role of cellular Ca(2+) homeostasis as a critical mechanism integrating the myriad of cellular and subcellular dysfunctional networks found in metabolic tissues such as liver and adipose tissue in the context of metabolic disease, particularly in obesity and diabetes.

Identification of the glycogenic compound 5-iodotubercidin as a general protein kinase inhibitor

Addition of micromolar concentrations of the adenosine derivative 5-iodotubercidin (Itu) initiates glycogen synthesis in isolated hepatocytes by causing inactivation of phosphorylase and activation of glycogen synthase [Flückiger-Isler and Walter (1993) Biochem. J. 292, 85-91]. We report here that Itu also antagonizes the effects of saturating concentrations of glucagon and vasopressin on these enzymes. The Itu-induced activation of glycogen synthase could not be explained by the removal of phosphorylase a (a potent inhibitor of the glycogen-associated synthase phosphatase). When tested on purified enzymes, Itu did not affect the activities of the major Ser/Thr-specific protein phosphatases (PP-1, PP-2A, PP-2B and PP-2C), but it inhibited various Ser/Thr-specific protein kinases as well as the tyrosine kinase activity of the insulin receptor (IC50 between 0.4 and 28 microM at 10-15 microM ATP). Tubercidin, which did not affect glycogen synthase or phosphorylase in liver cells, was 300 times less potent as a protein kinase inhibitor. Kinetic analysis of the inhibition of casein kinase-1 and protein kinase A showed that Itu acts as a competitive inhibitor with respect to ATP, and as a mixed-type inhibitor with respect to the protein substrate. We propose that Itu inactivates phosphorylase and activates glycogen synthase by inhibiting phosphorylase kinase and various glycogen synthase kinases. Consistent with the broad specificity of Itu in vitro, this compound decreased the phosphorylation level of numerous phosphopolypeptides in intact liver cells. Our data suggest that at least some of the biological effects of Itu can be explained by an inhibition of protein kinases.

Decreased activity and impaired hormonal control of protein phosphatases in rat livers with a deficiency of phosphorylase kinase

1. Livers from gsd/gsd rats, which do not express phosphorylase kinase activity, also contain much less particulate type-1 protein phosphatases. In comparison with normal Wistar rats, the glycogen/microsomal fraction contained 75% less glycogen-synthase phosphatase and 60% less phosphorylase phosphatase activity. This was largely due to a lower amount of the type-1 catalytic subunit in the particulate fraction. In the cytosol, the synthase phosphatase activity was also 50% lower, but the phosphorylase phosphatase activity was equal. 2. Both Wistar rats and gsd/gsd rats responded to an intravenous injection of insulin plus glucose with an acute increase (by 30-40%) in the phosphorylase phosphatase activity in the liver cytosol. In contrast, administration of glucagon or vasopressin provoked a rapid fall (by about 25%) in the cytosolic phosphorylase phosphatase activity in Wistar rats, but no change occurred in gsd/gsd rats. 3. Phosphorylase kinase was partially purified from liver and subsequently activated. Addition of a physiological amount of the activated enzyme to a liver cytosol from Wistar rats decreased the V of the phosphorylase phosphatase reaction by half, whereas the non-activated kinase had no effect. The kinase preparations did not change the activity of glycogen-synthase phosphatase, which does not respond to glucagon or vasopressin. Furthermore, the phosphorylase phosphatase activity was not affected by addition of physiological concentrations of homogeneous phosphorylase kinase from skeletal muscle (activated or non-activated). 4. It appears therefore that phosphorylase kinase plays an essential role in the transduction of the effect of glucagon and vasopressin to phosphorylase phosphatase. However, this inhibitory effect either is specific for the hepatic phosphorylase kinase, or is mediated by an unidentified protein that is a specific substrate of phosphorylase kinase.