Mitochondrial Oxidative Stress Is the General Reason for Apoptosis Induced by Different-Valence Heavy Metals in Cells and Mitochondria

The review addresses why and how different-valence heavy metals induce apoptosis, focusing on mitochondrial oxidative stress as a unifying mechanism. It summarizes early and modern studies and explains the shift from research on isolated mitochondria to broader in vitro and in vivo cellular systems. The purpose is to integrate evidence that metals such as Ag+, Tl+, Hg2+, Cd2+, Pb2+, Al3+, Ga3+, In3+, As3+, Sb3+, Cr6+, and U6+ converge on mitochondrial pathways—particularly ROS/H2O2 production, loss of ΔΨmito, MPTP opening, calcium dysregulation, and inhibition or oxidative damage to respiratory complexes—ultimately activating caspase-dependent apoptosis. The review also highlights distinctions for Tl+, whose toxicity is not mitigated by metallothioneins and may involve reversible oxidation to Tl3+ near ROS-generation sites.

The review compiles in vitro and in vivo studies across metals: - Ag(I)/AgNPs: In isolated mitochondria, Ag+ increases state 4 respiration at low doses and collapses ΔΨmito and state 3/4 respiration at higher doses by inhibiting CI/CII, increasing proton/K+ permeability, swelling, ROS, lipid peroxidation, and cytochrome c release; DTT blocks many effects, CsA often does not. AgNPs induce MPTP opening, oxidative stress, ΔΨmito loss, ATP depletion, and reduced Ca2+ retention; thiols and Ca2+ chelators can attenuate. In cells, Ag+ and AgNPs trigger mitochondrial apoptosis (caspase-3/9), Bax/Bcl-2 modulation, ATP depletion, DNA fragmentation, increased ROS/lipid peroxidation, ΔΨmito loss, and CI/CII inhibition. - Tl(I): IMM is permeable to Tl+; Tl+ increases passive cation permeability and state 4 respiration but does not inhibit mitochondrial dehydrogenases or strongly react with thiols. Tl+-induced MPTP opening requires Ca2+ loading and is inhibited by MPTP blockers (ADP, CsA, BKA), MCU blockers (RR, La3+, etc.), or Ca2+ chelation. In cells (hepatocytes, neurons, glia, lymphocytes), Tl+ causes apoptosis, ΔΨmito decline, ROS/H2O2 rise, lipid peroxidation, GSH depletion, cytochrome c release, and Na+/K+-ATPase inhibition; MAPK (ERK/JNK/p38) and p53 pathways implicated. - Hg(II)/MeHg: In mitochondria, Hg2+ induces ΔΨmito decline, inhibits CI/CIII (not CIV), increases state 4, swelling, Ca2+ efflux, ROS/H2O2, lipid peroxidation, and depletes thiols/GSH; effects mitigated by DTT, antioxidants, and MPTP/Ca2+ transport blockers. In cells, Hg2+/MeHg cause apoptosis (caspase-3), ΔΨmito loss, ATP depletion, GSH/TrxR/GPx decline, and ER–mitochondria Ca2+ dysregulation; NAC and antioxidants partly protective. - Cd(II): In mitochondria, Cd2+ increases IMM passive H+/K+ permeability, induces swelling, state 4 rise, ΔΨmito decline, inhibits CI/CII (not CIV), promotes H2O2, cytochrome c release, and MPTP opening (CsA/ADP-sensitive; ANT involvement). Effects modulated by Ca2+ and MCU/MPTP inhibitors and thiol reducers (DTT). In cells, Cd2+ enters via multiple transporters/endocytosis, increases cytosolic and mitochondrial Ca2+, triggers ROS, lipid peroxidation, ΔΨmito loss, CI–CIII dysfunction, ATP depletion, caspase-9/3 activation, MAPKs (ERK/JNK/p38, p53), DNA damage, and cytoskeletal changes; mitigated by antioxidants, thiols, Ca2+ chelators/MCU blockers, Zn2+/Mn2+ competition, and MPTP inhibitors. - Pb(II): In mitochondria, Pb2+ inhibits Ca2+ uptake, increases passive K+ permeability, induces swelling, ΔΨmito decline, MPTP opening, ROS/lipid peroxidation, and decreases CI–CIV activity and antioxidant enzymes. In cells, Pb2+ causes ΔΨmito loss, ATP/GSH depletion, increased ROS, cytochrome c release, caspase activation, and Ca2+ dysregulation; MPTP (CsA, BKA, DIDS) and MCU blockers (RR) are protective. Pb2+ competes with Ca2+/Fe2+/Zn2+ in key pathways. - Al(III), Ga(III), In(III): These often enter via Fe3+ transport systems, disrupt iron homeostasis, promote ROS and lipid/protein oxidation, and reduce GSH. Al3+ and AlNPs induce ΔΨmito decline, CIII inhibition, swelling, cytochrome c release, and MPTP opening via thiol oxidation; effects mitigated by CsA/DTT/NEM and antioxidants. Ga3+ disrupts iron uptake, triggers Bax/p53-mediated apoptosis, ΔΨmito loss, and ROS. In3+ induces swelling, ΔΨmito loss, proton leak changes, ROS, and MPTP opening (blocked by ADP/CsA/RR/chelators, not DTT); in cells, In compounds elevate ROS/lipid peroxidation, deplete GSH/SOD, alter mitochondrial morphology, and upregulate apoptotic genes. - As(III): In mitochondria, As2O3/AsO2− inhibit CI/CII (context dependent), decrease ΔΨmito and GSH, increase ROS/H2O2, lipid peroxidation, swelling, cytochrome c release, and MPTP opening (Ca2+-dependent; inhibited by CsA/NEM/RR). RLM reduce As(V) to As(III) via TrxR/GSH. In cells, As(III) increases ER-derived Ca2+, induces ΔΨmito loss, ATP and GSH changes, ROS, MPTP opening, caspase activation, and pathways including Nrf2, ERK, ferroptosis, and mitophagy; mitigated by NAC, ascorbate, selenite (dose-dependent), metallothionein, and antioxidants. - Sb(III): In mitochondria/cells, Sb3+ decreases CI/CIII and antioxidant enzymes, lowers GSH, ΔΨmito, ATP, and increases ROS and apoptosis; potassium antimonyl tartrate induces ROS-dependent apoptosis; effects enhanced by GSH depletion. - Cr(VI): In mitochondria, dichromate decreases state 3/3U respiration, ΔΨmito, increases ROS; glutathione attenuates. In cells, Cr(VI) causes ROS-mediated ΔΨmito loss, MPTP opening, Ca2+ overload (mitochondrial and ER stress), CI–CII inhibition, ATP decrease, swelling, caspase-3 and p53 signaling, mitophagy, and ERK/AMPK signaling; antioxidants/Ca2+ chelation mitigate. - U(VI) (uranyl): In mitochondria/cells, UO2 2+ inhibits CII/CIII, decreases ΔΨmito and ATP, increases ROS/H2O2 and lipid peroxidation, induces swelling and cytochrome c release; MPTP inhibitors, antioxidants, ROS scavengers, and Zn2+ provide protection. Uranyl disrupts Ca2+ handling and can form complexes with cytochromes.

Narrative review of experimental studies on isolated mitochondria and diverse cell types in vitro and in vivo. The author synthesizes findings on mitochondrial respiration (states 3, 4, uncoupled), inner membrane potential (ΔΨmito), MPTP opening/swelling, ROS/H2O2 generation, lipid peroxidation, cytochrome c release, calcium transport/overload (including ER release and MCU uptake), glutathione/thiol status, respiratory complex (CI–CIV) activities, and apoptosis signaling (caspases, Bax/Bcl-2, MAPKs/p53). Comparisons across metals and valence states identify shared and distinct mechanisms, and the modulatory effects of inhibitors, chelators, antioxidants, and channel blockers are summarized.

- Heavy metals broadly induce mitochondrial oxidative stress leading to apoptosis characterized by decreased cell viability, caspase-3/-9 activation, Bax/Bcl-2 modulation, and MAPK/p53 signaling. - Hallmarks include MPTP opening, mitochondrial swelling, ΔΨmito decline, ATP depletion, increased ROS/H2O2 and lipid peroxidation, cytochrome c release, reduced oxygen consumption, and Ca2+ overload from ER and via MCU. - Mechanistic bifurcation: metals with high thiol affinity (Ag+, Hg2+, Cd2+, Pb2+, As3+, Sb3+) directly interact with critical mitochondrial/cellular thiols in respiratory complexes and ANT to block CI–CIII and induce MPTP; others (Tl+, Al3+, Ga3+, In3+, Cr6+, U6+) more prominently drive ROS/H2O2 production and thiol oxidation indirectly, damaging complexes. - Complex-specific effects: CI/CIII frequently inhibited (Hg2+, Cd2+, Pb2+, As3+, Cr6+), CII variably inhibited (Pb2+, As3+, UO2 2+), CIV generally spared in several models. - Modulators: MPTP inhibitors (ADP, CsA, BKA, tamoxifen) and MCU blockers (RR/Ru360) often attenuate toxicity; thiol reducers (DTT, NAC, GSH) and antioxidants (catalase, quercetin) mitigate ROS-driven injury; chelators (EGTA/EDTA) and competing cations (Zn2+, Mn2+, Sr2+) reduce some metal effects. - Thallium is distinct: minimal direct thiol binding and no respiratory enzyme inhibition in isolated mitochondria; requires Ca2+ loading to induce MPTP; toxicity not reduced by metallothioneins; possible reversible oxidation to Tl3+ near ROS sites may deplete mitochondrial GSH; nonetheless, in cells Tl+ produces oxidative stress and apoptotic phenotypes similar to other metals. - Nanoparticles (AgNPs, AlNPs, indium oxide) often show greater cellular uptake and toxicity than ionic forms, with pronounced mitochondrial damage, ΔΨmito loss, and oxidative stress. - Biological consequences span organ systems (nervous system, kidney, liver, pancreas), with implications for diseases including cancer, diabetes, neurodegeneration, and toxicogenomic disorders.

Synthesizing decades of mitochondrial and cellular studies, the review argues that mitochondrial oxidative stress is the convergent mechanism underlying apoptosis induced by diverse heavy metals. Metals either directly bind and inactivate thiol-dependent components (respiratory complexes, ANT) to trigger MPTP, or indirectly oxidize critical thiols via enhanced ROS/H2O2 production, leading to ΔΨmito collapse, ATP loss, Ca2+ dysregulation, and activation of caspase-dependent pathways. The analysis explains why inhibitors of MPTP and MCU, antioxidants/thiol reducers, and chelators can interrupt these cascades. It reconciles similarities and differences among metals—highlighting Tl+ as an outlier in isolated mitochondria yet producing similar cellular outcomes through calcium overload and oxidative mechanisms—and links metal-induced mitochondrial dysfunction to broader pathophysiology (e.g., iron metabolism disruption by Al3+/Ga3+/In3+, Pb2+ competition with Ca2+/Fe2+/Zn2+, Cr(VI) strong oxidizing activity). The findings support targeting mitochondrial permeability transition, redox balance, and calcium handling to mitigate heavy metal toxicity.

Heavy metals of varying valences (Ag+, Tl+, Hg2+, Cd2+, Pb2+, Al3+, Ga3+, In3+, As3+, Sb3+, Cr6+, U6+) converge on mitochondria to induce oxidative stress and apoptosis. Common features include MPTP opening, ROS/H2O2 elevation, ΔΨmito decline, ATP depletion, lipid peroxidation, cytochrome c release, and Ca2+ overload, alongside CI–CIII dysfunction via direct thiol interactions (Ag+, Hg2+, Cd2+, Pb2+, As3+, Sb3+) or indirect ROS-mediated oxidation (Tl+, Al3+, Ga3+, In3+, Cr6+, U6+). Some metals also disrupt Fe2+ metabolism and iron–sulfur centers. Tl+ differs by lacking strong thiol reactivity and respiratory inhibition in isolated mitochondria and requiring Ca2+ loading to open MPTP; its human toxicity may be elevated due to poor metallothionein buffering and potential Tl+→Tl3+ redox cycling near ROS sites that deplete matrix GSH. The synthesis underscores mitochondria as central therapeutic targets; interventions that inhibit MPTP/MCU, bolster antioxidant/thiol systems, and chelate metals show protective potential. Future work should identify effective Tl+-binding reagents to facilitate elimination and mitigate neuro-, cardio-, and nephrotoxicity.