Endogenous sodium potassium ATPase inhibition related biochemical cascade and the acquired immunodeficiency syndrome -Neural regulation of viral replication and immune response to the virus

Introduction

The acquired immunodeficiency syndrome is associated with an increased predilection to oncogenesis, glutamate excitotoxicity and AIDS dementia, HIV-1 related psychosis with schizophreniform or bipolar mood disorder type of presentation and autoimmune disease. A disordered isoprenoid pathway has been described in neoplasms, psychiatric disorders, immune mediated disorders and degenerative disorders.[1] The important metabolic products of the isoprenoid pathway are dolichol, ubiquinone and digoxin. Digoxin is an endogenous membrane Na+-K+ ATPase inhibitor secreted by the human hypothalamus.[2] Digoxin can regulate the transport of neutral amino acids tyrosine and tryptophan.[3] Alterations in the tryptophan metabolite - quinolinic acid has been implicated in the pathological lesions of AIDS dementia.[4] The changes in ubiquinone levels can contribute to mitochondrial dysfunction and free radical generation. Free radical mechanisms related to quinolinic acid and nitric oxide have been implicated in AIDS dementia.[4] Dolichol can modulate glycoconjugate metabolism important in viral glycoprotein processing. The study was undertaken to assess the isoprenoid pathway related biochemical cascade in the acquired immunodeficiency syndrome.

Materials and Methods

Informed consent from all the patients and normal individuals was obtained for the study. Two groups of patients were included in the study. (1) 15 cases of male acquired immunodeficiency syndrome (CD4 count below 200 cells per microliter) between the ages of 20-30 years, diagnosed by the western blot test and selected at random from the patient population attending the infectious disease department were included in the study. 15 age and sex matched healthy normal male controls between the ages of 20-30 years, selected at random from the general population of Pappanamcode ward of Trivandrum were also chosen for the study. All patients and control subjects were non-smokers (passive or active) and were not on any drugs. The blood samples were collected from the patients before starting any treatment. Activity of HMG CoA reductase of the plasma was determined using the method of Rao and Ramakrishnan by determining the ratio of HMG CoA to mevalonate.[5] For the determination of the Na+-K+ ATPase activity of the erythrocyte membrane, the procedure described by Wallach and Kamat was used.[6] Digoxin in the plasma was determined by using the procedure described by Arun et al.[7] For estimation of ubiquinone and dolichol in the plasma, the procedure described by Palmer et al was used.[8] Magnesium in the plasma was estimated by atomic absorption spectrophotometry.[9] Tryptophan, tyrosine, serotonin and catecholamines were estimated by the procedures described in Methods of Biochemical Analysis.[10] Quinolinic acid content of plasma was estimated by HPLC (C18 column micro BondapakTM 4.6 x 150 nm), solvent system 0.01 M acetate buffer (pH3.0) and methanol (6:4), flow rate 1.0 ml/minute and detection at UV 250 nm. Morphine, strychnine and nicotine were estimated by the methods described by Arun et al.[11] Details of the procedures used for the estimation of total and individual GAG, carbohydrate components of glycoproteins, activity of GAG degrading enzymes and activity of glycohydrolases are described before.[12] Serum glycolipids were estimated as described in Methods in Enzymology.[6] Cholesterol was estimated by using commercial kits supplied by Sigma Chemicals, USA. Free radical scavenging enzymes were estimated by the procedures described in Methods of Enzymatic Analysis.[13] Lipid peroxidation products and reduced glutathione were estimated by the procedures described in Methods of Biochemical Analysis.[10] Nitric oxide was estimated in the plasma by the method of Gabor and Allon.[14] Statistical analysis was done by ′ANOVA′.

Results

(1) The activity of HMG CoA reductase and the concentration of digoxin and dolichol were increased in HIV-1 infection. The concentration of serum ubiquinone, the activity of erythrocyte membrane Na+-K+ ATPase and serum magnesium were decreased. The concentration of serum tryptophan, quinolinic acid and serotonin was increased in the plasma of these patients with HIV-1 infection while that of tyrosine, dopamine and noradrenaline was decreased. Nicotine (2.23+0.19 i g/dl) and strychnine (7.44+0.43 µg/ dl) were detected in the plasma of patients with HIV-1 infection but were not detected in control serum. Morphine was not detected in the plasma of these patients but in control serum (8.52+0.51 µmg/dl) [Table - 1]. (2) The activity of free radical scavenging enzymes were decreased significantly in HIV-1 infection. The concentration of lipid peroxidation products and nitric oxide (NO) increased significantly. The concentration of reduced glutathione decreased in HIV-1 infection. [Table - 2]. (3) The concentration of total glycosaminoglycans (GAG), different glycosaminoglycan fractions, carbohydrate residues of glycoproteins, glycolipids, GAG degrading enzymes and glycohydrolases increased in the serum of patients with HIV-1 infection. The increase in the carbohydrate components-total hexose, fructose and sialic acid-in HIV-1 infection was not to the same extent suggesting qualitative change in glycoprotein structure. The concentration of total GAG and carbohydrate residues of glycoproteins in the RBC membrane decreased significantly while the membrane cholesterol: phospholipid ratio increased in HIV-1 infection [Table - 3].

Discussion

The elevated HMG CoA reductase activity correlates well with elevated digoxin synthesis and - reduced RBC membrane Na,-K+ ATPase activity.[15] The increase in endogenous digoxin, a potent inhibitor of membrane Na+-K+ ATPase, can decrease this enzyme activity. The inhibition of membrane Na+-K+ .ATPase by digoxin is known to cause an increase in intracellular calcium and a reduction in intracellular magnesium.[16] Serum magnesium was found to be reduced in AIDS. There is a increase in tryptophan and its catabolites and a reduction in tyrosine and its catabolites in the patient′s serum. This could be due to the fact that digoxin can regulate neutral aminoacid transport system, with a preferential promotion of tryptophan transport over tyrosine.[3] Endogenous nicotine and strychnine are synthesised from tryptophan and endogenous morphine from tyrosine.[11] The neurotransmitter pattern of reduced dopamine, noradrenaline and morphine and increased serotonin, strychnine and nicotine could contribute to AIDS related psychosis. Hypomagnesemia and increased levels of NMDA modulators - serotonin, quinolinic acid and strychnine can contribute to NMDA excitotoxicity, implicated in neuronal degeneration observed in AIDS dementia.[4] increased intracellular calcium can activate the T -cell calcium dependent calcineurin signal transduction pathway resulting in T cell activation and secretion of Interleukin 3, 4, 5, 6 and TNF alpha (Tumour necrosis factor alpha)[17] TNF alpha binds to its receptor TNFRI and activates the transcription factor NF-kB leading to the induction of proinflammatory genes as well as promote HIV-1 replication.[18] Polyclonal B cell activation and proliferation have been described in the early phase of HIV-1 infection and facilitates HIV-1 replication.[19] TNF alpha can also bring about apoptosis of the CD4 cell contributing its depletion as well as the neurons in AIDS dementia by inducing caspase-9, an ICE protease which also results in IL-lbeta synthesis. IL-1 beta also promotes HIV-1 protein transcription.[19] The HIV-1 virus binds only to the CD4 receptor. A high initial CD4 could predispose to HIV-1 infection and it is possible to have hyperdigoxinemia induced pre-HIV-1 infectious state.[19] Elevated neuronal levels of serotonin and quinolinic acid as well as reduced noradrenaline, morphine and dopamine can also contribute to immune activation which promotes HIV replication.[20] The retroviral sequences in the genome which are endogenous vertically transmitted proviruses are transposable and are kept silenced by DNA methylation.[21],[22] Increased secretion of hypothalamic digoxin contributes to an intracellular magnesium deficiency, which leads to a DNA methylation defect and activation/expression of HIV-1 transposons. The increased availability of dolichol for N-glycosylation of proteins and intracellular Mg++ deficiency can upregulate the synthesis of glycosaminoglycans, glycoproteins and glycolipids as well as produce qualitative change in proteoglycan and glycoprotein structure. Interaction between HS and ChS proteoglycans with neuronal glycoproteins and reduced proteolytic digestion of these complexes can lead to neuronal degeneration in AIDS dementia. The protein processing defect can result in defective glycosylation of exogenous viral glycoprotein antigens with consequent defective formation of MHC -glycoprotein antigen complex.[23] This results in defective transport of MHC class 1 - viral glycoprotein antigen complex to the antigen presenting cell surface for recognition by CD, or CD, cell producing immune evasion and HIV-1 persistence. The upregulation of isoprenoid pathway can lead to increased cholesterol synthesis and magnesium deficiency can inhibit phospholipids synthesis resulting in an increased′ membrane cholesterol: phospholipid ratio. The concentration of glycoconjugates decreased in the RBC membrane in AIDS suggesting a defective incorporation into RBC membrane, consequent to inhibition of membrane trafficking enzymes - lipid kinases and GTPases in the presence of magnesium deficiency. Altered membrane structure can contribute to (i) membrane Na+-K+ ATPase inhibition and (ii) also reduced lysosomal stability with leakage of GAG degrading enzymes and glycohydrolases in serum and altered glycoconjugate metabolism. In AIDS there is a mitochondrial dysfunction and increased generation of free radicals important in pathogenesis of HIV syndrome related oncogenesis neuronal degeneration and immune activation consequent to (i). digoxin induced decreased tyrosine availability leads to inhibition of ubiquinone synthesis (ii) increased intracellular calcium related opening up of the mitochondrial PT pore and reduced intracellular magnesium inhibiting ATP synthase.[24] (iii) increased intracellular calcium inducing NO synthase and liberating NO. There is reduced free radical scavenging owing to ubiquinone and reduced glutathione deficiency and decrease in free radical scavenging enzymes. In AIDS there is an oncogenic tendency owing to (i) increased intracellular calcium activating phospholipase C beta - protein kinase C - MAP kinase cascade. (ii) The decreased intracellular magnesium can produce dysfunction of G T Pase activity of the alpha-subunit of G protein and ras-oncogene activation. (iii) The activation of P53 is impaired owing to intracellular magnesium deficiency producing a phosphorylation defect. Le Vay has reported structural changes in hypothalamus in homosexuals predisposed to AIDS suggesting increased hypothalamic digoxin secretion.[25]