Lecture 76: Pharmacology - Metabolic Energetics & Drug Metabolism in Kidneys PDF

Lecture 76: Pharmacology - Metabolic Energetics & Drug Metabolism in Kidneys Substrate supplies & respiratory rates in different kidney regions: Why kidney energetics are important: kidneys have high O2 consumption (2nd to heart), high blood flow, are particularly susceptible to hypoxic injury, rena...

Lecture 76: Pharmacology - Metabolic Energetics & Drug Metabolism in Kidneys Substrate supplies & respiratory rates in different kidney regions: Why kidney energetics are important: kidneys have high O2 consumption (2nd to heart), high blood flow, are particularly susceptible to hypoxic injury, renal mitochondrial supply lots of ATP Dependence on ATP: renal Na+ pump (“pacemaker” for cellular respiration), tissue-specific synthesis of key proteins (erythropoietin, vitamin D receptor), kidney can be gluconeogenic Role of kidneys in drug metabolism: many drugs (especially ionic ones) undergo renal excretion (impacts efficacy & toxicity), significant drug metabolism activity (especially in proximal tubules) Key processes: glomerular filtration, reabsorption, secretion (all depend on membrane transport) Transporter competition: b/w drugs can lead to drug-drug interactions, b/w drugs & endogenous metabolites can lead to acute kidney injury (AKI) Substrate supply in different kidney regions: PCT doesn’t metabolize glucose, TALH & DCT have high capacity for glucose (& lactate) oxidation, collecting tubules have highest capacity for aerobic glycolysis Drug metabolism & metabolic heterogeneity in different nephron cell types: Gluconeogenesis: enzymes found in PCT (most) & PST, but none in distal nephron segments Na+/K+ ATPase: highest in DCT, TALH, & PCT; lowest in thin limb; intermediate in collecting duct & PST Mitochondrial density: highest in PCT & mTALH (proximal segments = most transport activity) Drug metabolism: mostly in proximal tubules (includes all major enzymes of phase I & phase II metabolism), but selective localization in other nephron segments HETEs & EETs: derived from action of CYPs on arachidonic acid (AA) released by phospholipase A2 (cPLA2) acton on PM lipids 20-HETE: ↑TGF, ↑PKD, ↓IR injury, ↑natriuresis, ↓HTN, ↓glomerular injury, ↑RBF autoregulation EETs: ↑natriuresis, ↓inflammation, ↓HTN, ↓glomerular injury CYPs: important in drug metabolism but also in metabolism of endogenous chemicals Redox balance & renal mitochondrial function: ROS → breaks in mitochondrial DNA (mtDNA) → aberrant mitochondrial proteins → impaired mitochondrial function → further ↑ROS Renal mitochondria: targets for & sources of reactive oxygen species (ROS) Insults: substances (eg. toxic chemicals, reactive metabolites of drugs) that can increase the production of ROS in cytoplasm & mitochondria NADPH oxidase (NOX2 & NOX4): can contribute to the production of ROS Nuclear factor erythroid 2-related factor 2 (NRF2): activated by ROS → binds antioxidant-responsive elements (AREs) in nucleus → activate transcription of genes encoding antioxidant enzymes Mitochondrial superoxide dismutase 2 (SOD2): reduces superoxide anions to H2O2 & H2O Glutathione peroxidase (GPX): oxidizes glutathione (GSH) → results in glutathione disulfide (GSSG) as a byproduct of reducing H2O2 to H2O → mitochondrial GSSG (mGSSG) converted back to GSH by glutathione reductase (GR), requires NADPH Mitochondrial uncoupling protein 2 (UCP2): activity increased in response to ROS breakdown, dissipates proton motive force & decreases ROS production Mitochondrial biogenesis as a pharmacological target: Mitochondrial homeostasis: mitochondria adapt to different metabolic conditions by regulation of mechanistic/mammalian target of rapamycin (mTOR) & AMP-activated protein kinase (AMPK) pathways Fission: the splitting of mitochondria in 2; isolates damaged mitochondria from the mitochondrial network, allowing damaged mitochondria to be targeted for mitophagy Fusion: the combining of 2 mitochondria; leads to the elongation of mitochondrial under physiological conditions, helping to maintain oxidative phosphorylation (Oxphos) Mitophagy: a process by which mitochondria are degraded Mitochondrial biogenesis: activation of peroxisome proliferator-activated receptor γ-coactivator 1⍺ (PGC-1⍺) → translocation to nucleus initiates translation of various genes, including that for mitochondrial transcription factor A (TFAM) → protein products for Oxphos, TCA cycle, & mitochondrial biogenesis Activation of GPCRs (eg. β2-adrenergic receptors, β2AR & 5-hydroxytryptamine receptor 1F, 5-HT1F) → dissociation of G protein subunits → activation of PKA & endothelial nitric oxide synthase (eNOS) → eNOS stimulates soluble guanylate cyclase (sGC) to form cGMP (can be converted to GMP by 3’,5’-cyclic phosphodiesterase, PDE5)→ cGMP activates PGC-1⍺ Chemicals: can activate nuclear receptors to induce mitochondrial biogenesis; eg. peroxisome- proliferator-activated receptors (PPARs) & estrogen-related receptors (ERRs) Other transcription factors: can directly act on PGC-1⍺ to induce mitochondrial biogenesis; eg. nuclear respiratory factor 1 (NRF1) & NRF2 Other stimuli: can activate eNOS → ↑cGMP → activates PGC-1⍺; eg. caloric restriction mTOR: serine-threonine kinase complexes, inhibited by hypoxia & AMPK mTORC1: acts as a nutrient sensor (activated by growth factors, nutrients, & oxidative stress) regulating cell growth & proliferation (↑protein, lipid, & nucleotide synthesis & mitochondrial biogenesis) & inhibiting autophagy by stimulating anabolic processes via activation of PGC-1⍺ mTORC2: believed to regulate K+ & Na+ levels in the kidneys AMPK: acts as a nutrient sensor in the kidneys & stimulates catabolic (degradative) processes via phosphorylation of several proteins (leads to production of antioxidant enzymes, induction of mitochondrial biogenesis via phosphorylation of PGC-1⍺, and ↑glycolysis, FA oxidation, & glucose transport); activated by high AMP:ATP & low [O2], also acts by inhibiting mTORC1 Sirtuins: a family of 7 NAD+-dependent deacetylases (stimulated by ↑NAD+:NADH) that regulate histone proteins at specific lysine residues, promoting post-translational modification that results in chromatin silencing & transcriptional repression SIRT1: acts as a positive transcriptional regulator of PGC-1⍺ (promotes mitochondrial biogenesis & Oxphos) through promoter deacetylation; activated by resveratrol (a stilbenoid found in red wine); inhibits TNF⍺-dependent transactivation of NF-kB, limiting the expression of proinflammatory genes; responds to DNA damage/oxidative stress by deacetylation of p53 & FoxO transcription factors → ↓cell apoptosis & senescence; regulates expression of hypoxia-inducible factor-2⍺ (responsible for induction of erythropoietin by renal cells); represses genes regulated by PPAR-γ after food withdrawal SIRT3: major regulator of the organelle acetylome; binds ETC complexes to boost ATP; major regulator of mitochondrial antioxidant pathways by activation of SOD2, isocitrate DH 2 & deacetylation of acetyl-CoA synthase 2, & glutamate DH (fuels urea cycle); also promotes β-oxidation (by driving long-chain acyl CoA DH activity) & ketone body generation (by promoting deacetylation of 3-hydroxy-3-methylglutaryl CoA synthase 2) SIRT4: predominantly an ADP-ribosylase that has opposite effects of SIRT 3 (inactivates enzymes of urea cycle & β-oxidation) but induces lipogenesis through malonyl CoA decarboxylase deacetylation; governs cellular response to DNA damage via glutamine metabolism inhibition (potential role as tumor suppressor) SIRT5: mitochondrial deacetylase that regulates the urea cycle through direct activation of carbamoyl phosphate synthetase 1; also exhibits demalonylase & desuccinylase activities (controls ketogenesis); induces energetic flux via glycolysis Aging & kidney function: Nephron loss: kidney loses nephrons & mass → remaining nephrons undergo hypertrophic adaptation Cellular senescence: permanent cell cycle arrest after cumulative damage (telomere shortening, DNA damage, ROS) derived through the p53 or p16INK4a pathway Cellular hypertrophy: associated w/ interstitial fibrosis, tubular atrophy, glomerulosclerosis (GS), & microvascular rarefaction; podocyte hypertrophy is believed to drive GS Immunological & inflammatory responses: senescence-associated secretory phenotype (SASP) associate w/ inflammatory factors which act in a paracrine fashion, recruiting immune cells which destroy the microenvironment & altering the function of neighboring cells