Restoration of glucagon responsiveness in spontaneously transformed rat hepatocytes (RL-PR-C) by fusion with normal progenitor cells and rat liver plasma membranes

Spontaneously transformed RL-PR-C rat hepatocytes, unlike their normal differentiated progenitor cells, are insensitive to glucagon, although seemingly possessing large numbers of glucagon receptors and although retaining guanyl nucleotide regulatory protein-adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] system that responds to catecholamines, cholera toxin, and fluoride ions. Biochemical fusions between normal RL-PR-C hepatocytes or purified rat liver plasma membranes (whose adenylate cyclases were previously irreversibly inactivated with N-ethylmaleimide) with spontaneously transformed hepatocytes produced hybrids whose basal and fluoride-stimulated adenylate cyclase activities reflected those of the parental transformed cells but that now responded to glucagon. Using cholera toxin-catalyzed ADP-riboxylation of transformed hepatocytes to mark their guanyl nucleotide regulatory protein, fusiong such cells with N-ethylmaleimide-treated normal hepatocytes, and examining glucagon stimulation of adenylate cyclase activity in fusion hybrids produced results suggesting that the regulatory protein of the transformed cells is functionally normal. In fusion experiments between N-ethylmaleimide-treated hepatocytes and igeon erythrocytes, we found that normal, but not transformed, hepatocytes were effective in conferring glucagon sensitivity upon erythrocytes. Glucagon binding data revealed that, whereas normal RL-PR-C hepatocytes have two independent classes of binding sites, one of higher and the other of lower affinity, transformed cells possess only the low-affinity receptors. From these and previous observations, it is possible to conclude that the insensitivity of spontaneously transformed RL-PR-C hepatocytes to glucagon is due to the loss, during the transformation process, of the high-affinity glucagon receptor.

Uncoupling of the glucagon receptor-adenylate cyclase system by glucagon in cloned differentiated rat hepatocytes

The ability of glucagon to induce a state of desensitization to glucagon responsiveness has been examined in a cloned line of normal, differentiated, diploid rat hepatocytes (RL-PR-C). These cells, which respond to glucagon with increased production of cyclic AMP, become refractory to further stimulation of cyclic AMP synthesis following a 4 hour exposure period of the cells to the hormone. Refractoriness to glucagon was demonstrated over a wide range of hormone concentrations (10(-12) to 10(-6) M). In desensitized cells that were subsequently washed free of the hormone, recovery from refractoriness was complete by 20 hours. The mechanism underlying this desensitization does not appear to involve decreased receptor numbers, increased efflux of cyclic AMP from the cells, increased degradation of cyclic AMP by phosphodiesterase, or an alteration in the catalytic unit of the adenylate cellular cyclic AMP responsiveness to glucagon in normal RL-PR-C hepatocytes may involve glucagon a reversible uncoupling of glucagon receptors from adenylate cyclase. In addition, late passage, spontaneously transformed RL-PR-C hepatocytes were found to exist in a state in which glucagon receptors are permanently uncoupled from adenylate cyclase.

Cachexia of malignancy: potential role of insulin in nutritional management

Patients manifesting the syndrome of cachexia of malignancy exhibit an abnormal diabetic glucose tolerance. In our patients this has been correlated with a marked resistance to administered insulin, while insulin receptors on monocytes are normal. Lipolysis remains responsive to the effects of insulin. The oxidation of FFA, as a substrate for metabolism, has been reported to be increased, and the utilization of glucose as a metabolic fuel is reduced. Increased Cori cycle activity has been demonstrated, which produces an enhanced gluconeogenesis from lactate and amino acids; there is an expenditure of 6 ATP for the synthesis of each mole of glucose. An attempt to interrupt the Cori cycle in man, using hydrazine sulfate to inhibit the enzyme phosphoenolpyruvate carboxykinase, has not resulted in reproducible clinical benefit. However, successful treatment of the underlying tumor may produce a total reversal of the cachexia syndrome, suggesting that neoplasms have the potential to elaborate an, as yet, unidentified metabolic toxin. The use of insulin to counteract the reported abnormalities should be examined as a possible supportive measure in the total nutritional management of the cancer patient.

Cell-surface receptors in health and disease

Assessment of interaction of hormones and neurotransmitters with their cell-surface receptors can lead to an understanding of the molecular mechanisms that underlie diseases in man involving resistance and supersensitivity to these humoral agents and to pharmacological agents. They can also lead to the development of simple diagnostic procedures for such disorders, as well as the development of radioreceptor assays for humoral agents and drugs in blood and tissues, assays which combine sensitivity comparable to those involving radiommunoassay with a greater biological accuracy.

Cell-surface receptors in health and disease

Assessment of interaction of hormones and neurotransmitters with their cell-surface receptors can lead to an understanding of the molecular mechanisms that underlie diseases in man involving resistance and supersensitivity to these humoral agents and to pharmacological agents. They can also lead to the development of simple diagnostic procedures for such disorders, as well as the development of radioreceptor assays for humoral agents and drugs in blood and tissues, assays which combine sensitivity comparable to those involving radiommunoassay with a greater biological accuracy.

Hormone receptors. 7. Characteristics of insulin receptors in a new line of cloned neonatal rat hepatocytes

1. A new line of cloned, differentiated rat hepatocytes (RL-PR-C) was evaluated for its usefulness as an in vitro system for studying the regulation of the insulin receptor. 2. Insulin rapidly reversibly and specifically bound to RL-PR-C hepatocytes. Binding of tracer 125I-labeled insulin, which was competitively inhibited by native insulin as well as by proinsulin and analogs of insulin and proinsulin in proportion to their biological activity, was not influenced by glucagon, corticotropin, or human growth hormone. Anti-insulin receptor serum from a patient with Acanthosis Nigricans Type B competed with 125I-labeled insulin for binding to cell surface sites. 3. Trypsinization destroyed insulin binding sites, but these were restored by incubation under growth conditions; a 75% restoration of binding sites was achieved by one cell population doubling. 4. RL-PR-C hepatocytes responded to insulin binding by an increase in glycogen synthesis from glucose. The insulin effect was maximal at 85 nM, but was detectable at lower, more physiological, concentrations. 5. Chronic exposure (for at least 3h) of hepatocytes to insulin (10(-10)--(10(-8) M) reduced by up to 60% the number of binding sites for insulin (down-regulation). Down-regulation was prevented by cycloheximide at concentration (10 micron) sufficient to inhibit markedly protein synthesis from tracer isoleucine. Recovery from down-regulation induced by native insulin at 10(-7 M or lower concentrations was complete by 18 h under growth conditions. 6. Although RL-PR-C hepatocytes spontaneously transform after about 90 population doublings, no significant differences between normal and transformed cells were observed in insulin binding characteristics and in interaction of cells with anti-insulin receptor serum. However, transformed cells exhibited a substantially reduced (maximum of 20%) down-regulation response to insulin. 7. RL-PR-C rat hepatocytes appear, for these reasons, to be a useful model system for studying the regulation of the insulin receptor.

Hormone receptors: VI. On the nature of the binding of glucagon and insulin to human circulating mononuclear leukocytes

Several characteristics of the binding of insulin and glucagon to human circulating mononuclear leukocytes have been studied. Functional analysis (latex bead ingestion) revealed that cell mixtures, as prepared according to Boyum and used generally in studies of insulin resistance in humans, consist of 20-29% phagocytic monocytes, with the remainder being lymphocytes. Partial separation of monocytes from lymphocytes on columns of Sephadex G-10, followed by correlation of insulin binding with cell type, confirms that the monocyte is the binding species. Insulin influenced neither glucose uptake nor the further conversion of glucose to lipids and CO2 by the leukocytes. The transport of alpha-aminoisobutyrate, a nonmetabolizable amino acid, into these cells was also unaffected by insulin. Monocyte/lymphocyte mixtures specifically bound glucagon and prostaglandin E1. At physiological concentrations of these hormones, steady states were reached in 15 min and 45 min, respectively. In contrast to the 8-10-fold increases in cellular cyclic AMP produced by prostaglandins, the effect of glucagon was very small but apparently real. Under appropriate preincubation conditions, sodium azide and iodoacetamide inhibited phagocytosis and insulin binding in parallel. The binding of glucagon was unaffected by these agents. Although both antimycin A and actinomycin D inhibited phagocytosis of the monocytes, only the former inhibited insulin binding; there was only a slight effect on glucagon binding. We would conclude that the binding of insulin to human circulating monocytes, although reflective of insulin resistance in diabetes mellitus and obesity, may not be to traditional receptors. In contrast, the binding of glucagon to lymphocyte/monocyte mixtures may be to function-linked receptors.

Hormone receptors, 5. Binding of glucagon and insulin to human circulating mononuclear cells in diabetes mellitus

Preparations of human peripheral mononuclear leukocytes, containing over 85% lymphocytes, specifically bound glucagon and insulin. Binding of physiological concentrations of both hormones was seen to be substantially diminished when cells were derived from either adult-type diabetics or individuals with a unilateral genetic history of diabetes.

Synthesis of fatty acyl CoA and other thiol esters using N-hydroxysuccinimide esters of fatty acids

N-Hydroxysuccinimide esters of long-chain fatty acids have been used to synthesize the CoA and thioglycolic acid thiol esters of palmitic and 3-ketopalmitic acids in high yield and with a minimum of untoward side reactions.

Synthesis and characterization of 3-ketohexadecanoic acid-1-14-C, DL-3-hydroxyhexadecanoic acid-1-14-C, and trans-2-hexadecenoic acid-1-14-C

The chemical synthesis and characterization of three intermediates in the Beta oxidation of palmitic acid-1-(14)C by rat liver mitochondria, namely, 3-ketohexadecanoic acid-1-(14)C, DL-3-hydroxyhexadecanoic acid-1-(14)C, and trans-2-hexadecenoic acid-1-(14)C, are described.