Effects of hypoxia on lipolysis in isolated rat myocardial cells

Cancer cachexia has many symptoms but only one cause: anoxia

During nearly 100 years of research on cancer cachexia (CC), science has been reciting the same mantra: it is a multifactorial syndrome. The aim of this paper is to show that the symptoms are many, but they have a single cause: anoxia. CC is a complex and devastating condition that affects a high proportion of advanced cancer patients. Unfortunately, it cannot be reversed by traditional nutritional support and it generally reduces survival time. It is characterized by significant weight loss, mainly from fat deposits and skeletal muscles. The occurrence of cachexia in cancer patients is usually a late phenomenon. The conundrum is why do similar patients with similar tumors, develop cachexia and others do not? Even if cachexia is mainly a metabolic dysfunction, there are other issues involved such as the activation of inflammatory responses and crosstalk between different cell types.  The exact mechanism leading to a wasting syndrome is not known, however there are some factors that are surely involved, such as anorexia with lower calorie intake, increased glycolytic flux, gluconeogenesis, increased lipolysis and severe tumor hypoxia. Based on this incomplete knowledge we put together a scheme explaining the molecular mechanisms behind cancer cachexia, and surprisingly, there is one cause that explains all of its characteristics: anoxia.  With this different view of CC we propose a treatment based on the physiopathology that leads from anoxia to the symptoms of CC.  The fundamentals of this hypothesis are based on the idea that CC is the result of anoxia causing intracellular lactic acidosis. This is a dangerous situation for cell survival which can be solved by activating energy consuming gluconeogenesis. The process is conducted by the hypoxia inducible factor-1α. This hypothesis was built by putting together pieces of evidence produced by authors working on related topics.

Hypoxia, Oxidative Stress and Fat

Metabolic disturbances in white adipose tissue in obese individuals contribute to the pathogenesis of insulin resistance and the development of type 2 diabetes mellitus. Impaired insulin action in adipocytes is associated with elevated lipolysis and increased free fatty acids leading to ectopic fat deposition in liver and skeletal muscle. Chronic adipose tissue hypoxia has been suggested to be part of pathomechanisms causing dysfunction of adipocytes. Hypoxia can provoke oxidative stress in human and animal adipocytes and reduce the production of beneficial adipokines, such as adiponectin. However, time-dose responses to hypoxia relativize the effects of hypoxic stress. Long-term exposure of fat cells to hypoxia can lead to the production of beneficial substances such as leptin. Knowledge of time-dose responses of hypoxia on white adipose tissue and the time course of generation of oxidative stress in adipocytes is still scarce. This paper reviews the potential links between adipose tissue hypoxia, oxidative stress, mitochondrial dysfunction, and low-grade inflammation caused by adipocyte hypertrophy, macrophage infiltration and production of inflammatory mediators.

Decreased serum glucose and glycosylated hemoglobin levels in patients with Chuvash polycythemia: a role for HIF in glucose metabolism

In Chuvash polycythemia, a homozygous 598C>T mutation in the von Hippel-Lindau gene (VHL) leads to an R200W substitution in VHL protein, impaired degradation of α-subunits of hypoxia-inducible factor (HIF)-1 and HIF-2, and augmented hypoxic responses during normoxia. Chronic hypoxia of high altitude is associated with decreased serum glucose and insulin concentrations. Other investigators reported that HIF-1 promotes cellular glucose uptake by increased expression of GLUT1 and increased glycolysis by increased expression of enzymes such as PDK. On the other hand, inactivation of Vhl in murine liver leads to hypoglycemia associated with a HIF-2-related decrease in the expression of the gluconeogenic enzyme genes Pepck, G6pc, and Glut2. We therefore hypothesized that glucose concentrations are decreased in individuals with Chuvash polycythemia. We found that 88 Chuvash VHL ( R200W ) homozygotes had lower random glucose and glycosylated hemoglobin A1c levels than 52 Chuvash subjects with wild-type VHL alleles. Serum metabolomics revealed higher glycerol and citrate levels in the VHL ( R200W ) homozygotes. We expanded these observations in VHL ( R200W ) homozygote mice and found that they had lower fasting glucose values and lower glucose excursions than wild-type control mice but no change in fasting insulin concentrations. Hepatic expression of Glut2 and G6pc, but not Pdk2, was decreased, and skeletal muscle expression of Glut1, Pdk1, and Pdk4 was increased. These results suggest that both decreased hepatic gluconeogenesis and increased skeletal uptake and glycolysis contribute to the decreased glucose concentrations. Further study is needed to determine whether pharmacologically manipulating HIF expression might be beneficial for treatment of diabetic patients.

Inhibition by 5-N-(4-chlorobenzyl)-2',4'-dimethylbenzamil of Na+/Ca2+ exchange and L-type Ca2+ channels in isolated cardiomyocytes

The inhibitory effect of the amiloride derivative 5-N-(4-chlorobenzyl)-2',4'-dimethylbenzamil (CBDMB) on calcium (Ca2+) uptake via sarcolemmal sodium-calcium (Na+/Ca2+) exchange and L-type Ca2+ channels was investigated in isolated adult rat ventricular cardiomyocytes under depolarizing conditions in cells preincubated with 1 mM ouabain or 137 mM lithium (Li+), respectively. Fifteen or 120 min. preincubation with CBDMB inhibited Ca2+ uptake via Na+/ Ca2+ exchange in Na(+)-loaded depolarized cells completely at 100 microM with an IC50 of 21 microM. After 120 min. preincubation, CBDMB inhibited Ca2+ uptake via L-type Ca2+ channels by 75.1 +/- 8.1% (mean and S.E.M.) and IC50 of 4 microM, whereas no significant inhibition was observed after 15 min. preincubation. (+)-Isradipine (10 microM) inhibited high potassium (K+) induced Ca2+ uptake via L-type Ca2+ channels by 35% after 15 min. and by 70% after 120 min. preincubation. Inhibition by CBDMB of specific (+)-[3H]isradipine binding to L-type Ca2+ channels showed similar concentration dependency as inhibition of Ca2+ uptake via L-type Ca2+ channels. In conclusion, CBDMB inhibits sarcolemmal Na+/Ca2+ exchange in rat ventricular cardiomyocytes rapidly. However, after longer preincubation periods, L-type Ca2+ channels are inhibited as well and with higher potency than Na+/Ca2+ exchange.

Stable guanosine 5'-triphosphate-analogues inhibit specific (+)-[3H]isradipine binding in rat hearts by a Ca(2+)-lowering, G protein-independent mechanism

We investigated if and how stable guanosine 5'-triphosphate-analogues affect (+)-[3H]isradipine binding in rat hearts. Gpp(NH)p and GTP-gamma-S inhibit specific (+)-[3H]isradipine binding in membranes and cell-homogenates by reducing the binding density without changing the Kd of the k-1. Inhibition by Gpp(NH)p was less in crude tissue homogenates than in membranes apparently due to a soluble factor. Pretreatment of cardiomyocytes with cholera toxin or the presence of the protein kinase A inhibitor, PKI6-22, did not influence the effect of 10(-3) M Gpp(NH)p on binding. The inhibitory effect of 10(-3) M Gpp(NH)p was not significantly altered in membranes from in vivo pertussis toxin treated rats. The addition of 10(-3) M Ca2+ or Mg2+ abolished the inhibitions. Gpp(NH)p in the concentration that inhibits binding, reduced the free concentration of Ca2+. The Ca(2+)-lowering effect of 10(-3) M Gpp(NH)p produced 70%, 60% and 100% of the inhibition in membranes, sonicated and unsonicated cell homogenates. Thus, Gpp(NH)p inhibited specific (+)-[3H] isradipine binding mainly by lowering the free concentration of Ca2+ by chelation and not by activation of cholera toxin or pertussis toxin-sensitive G proteins or protein kinase A.

Formation of pivaloylcarnitine in isolated rat heart cells

Pivaloyl-containing antibiotics and pivalic acid in man or rat have been reported to cause increased urinary carnitine loss secondary to pivaloylcarnitine generation. Pivaloylcarnitine concentration was especially high in heart after administration of pivalic acid or pivampicillin in vivo. Formation of pivaloylcarnitine was therefore studied in isolated rat heart cells in the presence of sodium pivalate. Formation of pivaloylcarnitine in rat heart cells increased with incubation time, after a lag time from 0 to 2 h and linearly up to 6 h. The formation increased with increasing concentration of sodium pivalate, followed Michaelis-Menten kinetics with an apparent Km = 348 +/- 10 microM and Vmax = 116 +/- 20 pmol.mg protein-1.h-1. Bromoacetylcarnitine inhibited the pivaloylcarnitine formation to Ki = 116 +/- 43 microM and Vmax = 107 +/- 14 pmol.mg protein-1.h-1. The uptake of carnitine in heart cells was suppressed 62-74% by deoxycarnitine (40 microM) and D-carnitine (200 microM), and 95% by NaF (5 mM), NaN3 (500 microM) or at temperature 4 degrees C. Pivaloylcarnitine inhibited carnitine uptake to 33-35% of the controls, while sodium pivalate did not. More than 90% of intracellular pivaloylcarnitine was released from the heart cells after 18 h of incubation in the absence of sodium pivalate and L-carnitine. These data show that pivalate is readily converted to pivaloylcarnitine in heart cells, in contrast to the limited conversion in hepatocytes.

Hypoxia-stimulated glycerol production from the isolated, perfused rat heart is mediated by non-adrenergic mechanisms

Factors controlling hypoxia-induced myocardial glycerol release were studied in isolated, perfused rat hearts. A constant coronary flow rate 10 ml g-1 min-1 was maintained. The perfusion buffer was gassed with O2-N2 mixtures containing 5% CO2. The O2:N2 ratios were normoxia 95:0, hypoxia 30:65, and severe hypoxia 10:85 (v/v). Glycerol and lactate release were stimulated during a 30-min period of either hypoxia or severe hypoxia but remained constant during normoxia. Tissue glycerol-3-phosphate levels were increased after 30 min hypoxia compared with after a similar period of normoxic perfusion (p < 0.01) and further increased after severe hypoxia (p < 0.01 vs hypoxia). beta-Adrenoceptors remained sensitive to isoprenaline during hypoxia, demonstrated by an increase in glycerol release over a 30-min period of isoprenaline infusion from 897 +/- 317 to 1771 +/- 307 nmol g-1 wet weight (p < 0.05). The isoprenaline-induced increase in glycerol release during hypoxia was inhibited by both atenolol and timolol (1 x 10(-5) M). In contrast, beta-adrenoceptor blockade using these drugs failed to reduce glycerol release induced by either hypoxia or severe hypoxia. Both drugs attenuated the rise in glycerol-3-phosphate during hypoxia. Chronic denervation by pretreatment with 6-hydroxydopamine reduced hypoxia-stimulated glycerol release by only 30%. Thus, a major part of hypoxia-induced glycerol release is mediated by non-adrenergic mechanisms. The results of this study bring into question the validity of the use of glycerol production during hypoxia as a reliable measure of myocardial lipolysis.

Effect of stimulation and veratrine on total cellular calcium in rat and guinea-pig ventricular myocytes

In rat cardiac myocytes, calcium efflux by Na+/Ca(2+)-exchange is expected only during ventricular systole following initial action potential repolarization. In contrast, in guinea-pigs, calcium influx via Na+/Ca(2+)-exchange is expected only during the initial portion of the action potential. Thus electrical stimulation is expected to result in reduced intracellular calcium ([Ca2+]i) in rat and an increase in guinea pig. We tested this hypothesis by measuring total cellular calcium ([Ca]tot) using 45Ca following stimulation of isolated rat and guinea-pig ventricular myocytes. Many studies have also emphasized that the rate and the direction of Na+/Ca(2+)-exchange across the sarcolemma are in part dependent on the magnitude of the transsarcolemmal sodium gradient. Thus, increasing intracellular sodium ([Na+]i) is expected to result in an increased [Ca2+]i. This hypothesis was also tested by measuring [Ca]tot following veratrine administration. Enzymatically isolated rat and guinea-pig ventricular myocytes were divided into two groups; non-stimulated and stimulated (1 Hz). The concentration-dependent effects of veratrine (1,10,100 micrograms/ml) on [Ca]tot were determined in both these groups. In the absence of veratrine, non-stimulated rat myocytes had a significantly higher [Ca]tot than did stimulated ones. Non-stimulated guinea-pig myocytes had a significantly lower [Ca]tot when compared with stimulated ones. Veratrine increased [Ca]tot in both species in a concentration-dependent fashion. In addition, following veratrine the difference between [Ca]tot in non-stimulated and stimulated rat myocytes was no longer significant. These results support those of others who have demonstrated that stimulation is associated with a gain of cellular calcium in both rabbit and guinea-pig ventricle and a calcium loss in rat ventricle.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial cell vulnerability to exogenous phospholipase attack

Myocardial cell vulnerability to phospholipase C (PC-PLC) attack was investigated in three different preparations of rat myocardial cells: triacylglycerol (TG)-loaded, hypothermic/rewarmed and energy depleted myocytes. The attack by PC-PLC was evaluated as PC-PLC induced glycerol output due to the combined action of phospholipase C and intracellular lipases. PC-PLC induced glycerol output was significantly higher (p < 0.05) in all three myocyte preparations, compared to their respective controls. Cell morphology (% rod shaped myocytes) of TG-loaded or hypothermic/rewarmed myocytes was not different from their controls, whereas energy depleted myocytes almost exclusively were rounded up, due to hypercontraction of the myofilaments. Hypothermic/rewarmed and energy depleted myocytes showed a significantly higher release of lactate dehydrogenase (LDH), compared to their controls although the difference was much more pronounced in the latter. Finally, the cellular contents of ATP were maintained both in TG-loaded and hypothermic rewarmed myocytes, while energy depleted myocytes contained only about 25% of the normal ATP level. These results demonstrate that attack from exogenously added phospholipases can occur, not only in seriously damaged cardiac myocytes, but in myocytes with a more subtle damage as well.