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Cellular stress response

The cellular stress response encompasses the suite of adaptive molecular mechanisms that cells activate to detect, counteract, and recover from insults that disrupt homeostasis, including environmental stressors like heat, oxidative damage, hypoxia, and genotoxic agents, as well as intrinsic challenges such as protein misfolding or metabolic imbalances.^[1] These responses aim to repair damaged macromolecules—such as proteins, DNA, RNA, and lipids—or, if damage is irreparable, to initiate programmed cell death pathways like apoptosis to prevent propagation of harm.^[2] Evolutionarily conserved across all organisms, this response forms a universal defense system, often termed the "minimal stress proteome," which integrates sensing, signaling, and effector functions to balance survival and elimination.^[3] Central to these mechanisms are specialized pathways tailored to specific stressors. The heat shock response (HSR), triggered by elevated temperatures or protein denaturation, induces transcription factors like HSF1 to upregulate heat shock proteins (HSPs), such as Hsp70 and Hsp90, which act as molecular chaperones to refold misfolded proteins and inhibit aggregation.^[1] Similarly, the unfolded protein response (UPR) addresses endoplasmic reticulum (ER) stress from accumulated unfolded proteins via three main sensors—IRE1, PERK, and ATF6—that enhance protein folding capacity, reduce translation to alleviate ER load, or activate antioxidants like Nrf2 to combat oxidative stress.^[1] The DNA damage response (DDR) employs kinases such as ATM and ATR to halt the cell cycle, recruit repair enzymes, or activate p53-mediated apoptosis if lesions persist, safeguarding genomic integrity.^[2] Other pathways, including those for oxidative stress (e.g., Nrf2-mediated antioxidant induction) and hypoxia (e.g., HIF signaling), converge on common effectors like autophagy for degrading damaged components or mTOR regulation for metabolic reprogramming.^[2] These responses are pivotal for physiological adaptation and disease prevention but can become dysregulated in pathology. In healthy contexts, they maintain tissue homeostasis, support development, and enhance resilience to transient stresses, such as during exercise or infection.^[3] However, chronic activation or impairment contributes to conditions like cancer, where upregulated HSR and UPR promote tumor survival and therapy resistance; neurodegenerative disorders, such as Parkinson's, where failed protein clearance leads to aggregation; and cardiovascular diseases, including myocardial infarction, via unresolved oxidative damage.^[1] Aging further diminishes response efficiency, exacerbating vulnerability to stress.^[2] Overall, the cellular stress response exemplifies a delicate "balancing act," deciding between repair and death based on stress intensity and duration to optimize organismal fitness.^[2]

Overview

Definition and importance

The cellular stress response refers to a conserved adaptive mechanism by which cells detect and counteract damaging conditions, such as protein misfolding, oxidative damage, or energy depletion, to restore homeostasis and promote survival.^[4] This response encompasses a range of molecular pathways that enable cells to sense macromolecular perturbations and initiate protective measures, including the upregulation of chaperones and repair systems, while conserving resources during adversity.^[1] The importance of the cellular stress response lies in its critical role in preventing apoptosis, enhancing cellular resilience to environmental insults, and maintaining proteostasis—the dynamic regulation of protein folding, trafficking, and degradation.^[4] By inhibiting pro-apoptotic processes like caspase activation and cytochrome c release, these mechanisms allow cells to survive transient stresses that might otherwise lead to programmed cell death.^[1] Furthermore, the response is evolutionarily conserved across prokaryotes and eukaryotes, with a core set of approximately 44 proteins forming a minimal stress proteome that underscores its fundamental necessity for life, from bacteria to humans.^[4] This phenomenon was first observed in 1962 by Ferruccio Ritossa, who noted heat-induced chromosomal puffs in Drosophila melanogaster salivary glands, revealing a novel pattern of gene activity that led to the identification of stress-inducible genes and the broader field of stress responses.^[5] Overall, cellular stress responses integrate multiple signaling pathways to finely balance survival strategies against the risk of irreparable damage, ensuring organismal health and adaptation.^[1]

Types of cellular stress

Cellular stress can be broadly classified into physical, chemical, metabolic, and endogenous categories, each representing distinct inputs that challenge cellular homeostasis and trigger adaptive responses. Physical stressors involve external forces such as elevated temperatures or ionizing radiation, which directly disrupt molecular structures. Chemical stressors encompass exogenous toxins and heavy metals that interfere with biochemical processes. Metabolic stressors arise from imbalances in energy availability or oxygen supply, while endogenous stressors originate from internal cellular dysfunctions like protein misfolding or genetic insults. This classification highlights the diversity of threats cells face, from environmental exposures to intrinsic errors, and underscores the need for tailored protective mechanisms.^[1] Physical stressors, including heat and radiation, primarily affect protein stability and genomic integrity. Heat stress occurs when temperatures rise 3–5°C above physiological norms, leading to protein denaturation where unfolded polypeptides aggregate and impair cellular function. For instance, exposure to temperatures exceeding 42°C rapidly destabilizes proteins, initiating protective pathways to refold or degrade damaged molecules. Radiation, such as UV or ionizing types, induces physical breaks in DNA strands, compromising replication and transcription. These stressors often manifest acutely, like sudden heat exposure, overwhelming cellular thresholds if not resolved promptly.^[1]^[6]^[7] Chemical stressors involve xenobiotics and heavy metals that bind cellular components or generate toxic byproducts. Toxins like pesticides or industrial pollutants disrupt enzymatic activities and membrane integrity, while heavy metals such as arsenic accumulate and catalyze harmful reactions. Arsenic, a common environmental xenobiotic, binds to sulfhydryl groups in proteins, but cells counter this by inducing metallothioneins—cysteine-rich proteins that sequester the metal and mitigate toxicity. Oxidative stress, a frequent outcome of chemical exposure, stems from an imbalance where reactive oxygen species (ROS) production exceeds antioxidant capacity, damaging lipids, proteins, and DNA. Chronic chemical exposure, unlike acute bursts, leads to cumulative bioaccumulation, lowering cellular resilience over time.^[6]^[8]^[9] Metabolic stressors disrupt energy homeostasis through oxygen or nutrient shortages. Hypoxia, or oxygen deprivation, impairs mitochondrial respiration, forcing cells to shift to less efficient anaerobic pathways and accumulate metabolic byproducts. Nutrient deprivation, such as glucose starvation, halts ATP production and triggers catabolic processes to sustain viability. These often overlap with endogenous origins in pathological states like ischemia.^[1] Endogenous stressors emerge from internal perturbations, including endoplasmic reticulum (ER) overload and DNA damage. ER overload occurs when protein synthesis exceeds folding capacity, causing accumulation of misfolded proteins and activating the unfolded protein response. DNA damage, arising from replication errors or repair failures, threatens genomic stability and can propagate if unchecked. Unlike acute physical or chemical insults, chronic endogenous stress, such as persistent ROS imbalance, erodes cellular thresholds progressively, contributing to long-term dysfunction.^[1] A key distinction exists between acute and chronic stressors, influencing their impact on cellular thresholds. Acute stressors, like sudden heat or toxin exposure, elicit rapid, transient responses that restore balance if mild, but severe cases can exceed repair capacity. Chronic stressors, such as ongoing oxidative damage or nutrient scarcity, impose cumulative effects, gradually depleting reserves and heightening vulnerability to further insults. This temporal aspect determines whether cells adapt or succumb.^[10]^[11]

Molecular Mechanisms

Protein quality control systems

Protein quality control systems are essential mechanisms that maintain proteostasis by assisting in the refolding of misfolded proteins and targeting irreparably damaged ones for degradation during cellular stress. These systems primarily involve molecular chaperones and proteolytic pathways, which prevent protein aggregation and ensure cellular homeostasis. Heat shock proteins (HSPs), such as HSP70 and HSP90, play central roles in this process by recognizing and binding to exposed hydrophobic regions of unfolded or denatured polypeptides, thereby inhibiting aggregation and facilitating refolding.^[12]^[13] HSP70 operates in an ATP-dependent manner, where it captures substrate proteins in their ATP-bound state with low affinity and, upon ATP hydrolysis stimulated by co-chaperones like HSP40 (also known as DnaJ), transitions to a high-affinity ADP-bound conformation that holds the substrate for refolding or handover to other chaperones.^[14] HSP40 co-chaperones initiate this cycle by delivering substrates to HSP70 and accelerating ATP hydrolysis, enabling iterative folding attempts.^[14] In contrast, HSP90 primarily stabilizes partially folded proteins and matures client proteins, often in complex with co-chaperones, to prevent aggregation under stress conditions.^[12] The simplified chaperone cycle can be represented as:

\text{Unfolded protein} + \text{HSP70 (ATP-bound)} \xrightarrow{\text{HSP40, ATP hydrolysis}} \text{Folded protein} + \text{HSP70 (ADP-bound)} + \text{P}_\text{i}

This cycle underscores the energy-dependent nature of chaperone-mediated proteostasis.^[14] When refolding fails, degradation pathways clear misfolded proteins to avert toxicity. The ubiquitin-proteasome system (UPS) handles targeted degradation of individual soluble proteins, where E3 ubiquitin ligases tag substrates with ubiquitin chains, marking them for breakdown by the 26S proteasome.^[15] This process is crucial during stress, as it selectively removes damaged proteins to restore proteostasis. Complementing the UPS, the autophagy-lysosomal pathway mediates bulk clearance of protein aggregates and organelles by engulfing them into autophagosomes that fuse with lysosomes for degradation. Selective autophagy, such as aggrephagy, targets aggregates specifically, ensuring efficient removal under proteotoxic stress. Together, these systems provide a robust defense against protein misfolding, with their expression often upregulated transcriptionally during stress to enhance capacity.^[16]

Transcriptional and translational regulation

Cellular stress responses involve intricate transcriptional regulation mediated by key transcription factors that activate protective gene networks. Heat shock factor 1 (HSF1) plays a central role in the heat shock response by undergoing stress-induced trimerization and nuclear translocation, enabling it to bind heat shock elements (HSEs) in promoter regions of target genes such as those encoding molecular chaperones.^[17] This activation is tightly controlled, with HSF1 monomers maintained in an inactive state under normal conditions through interactions with chaperones like HSP90.^[18] Similarly, nuclear factor erythroid 2-related factor 2 (NRF2) regulates antioxidant responses by dissociating from its inhibitor KEAP1 under stress, allowing NRF2 to translocate to the nucleus and bind antioxidant response elements (AREs) to induce genes involved in detoxification and redox balance.^[19] In endoplasmic reticulum (ER) stress, activating transcription factor 4 (ATF4) and X-box binding protein 1 (XBP1) coordinate the unfolded protein response (UPR); ATF4 is transcriptionally upregulated to drive genes for amino acid metabolism and redox control, while XBP1, particularly its spliced isoform (XBP1s), promotes ER expansion and chaperone expression.^[20] Translational regulation during stress is exemplified by the integrated stress response (ISR), a conserved pathway that attenuates global protein synthesis while selectively enhancing translation of specific mRNAs. The ISR is triggered by phosphorylation of eukaryotic initiation factor 2 alpha (eIF2α) by kinases such as PERK, GCN2, PKR, or HRI, which inhibits the formation of the ternary complex required for cap-dependent translation initiation, thereby reducing overall translation by up to 80-90%.^[21] However, this phosphorylation paradoxically boosts translation of ATF4 mRNA through its upstream open reading frames (uORFs), where delayed reinitiation favors scanning to the main ATF4 coding sequence under high eIF2α phosphorylation levels.^[22] This selective mechanism ensures rapid production of ATF4, which then transcriptionally activates UPR target genes, linking translational control directly to downstream transcriptional outputs. Feedback mechanisms prevent overactivation of these pathways, maintaining cellular homeostasis. For instance, newly synthesized heat shock proteins (HSPs), such as HSP70, bind and inhibit HSF1 trimerization and DNA-binding activity, forming a negative feedback loop that attenuates the heat shock response once proteotoxic stress is resolved.^[18] In the ISR, dephosphorylation of eIF2α by phosphatases like PPP1R15A (GADD34), which is itself induced by ATF4, restores global translation and terminates the response.^[21] Recent studies highlight the ISR's role in chronic stress adaptation via the PERK-eIF2α-ATF4 axis, where sustained low-level activation promotes resilience against prolonged proteotoxic or metabolic insults without triggering apoptosis, as seen in models of neurodegeneration and metabolic disorders post-2020.^[23] These regulatory layers ensure that transcriptional and translational adjustments are both protective and reversible, with downstream effectors like chaperones reinforcing protein quality control.^[20]

Responses to Specific Stressors

Heat shock response

The heat shock response (HSR) is a conserved cellular defense mechanism activated primarily by elevated temperatures, resulting in the transcriptional upregulation of heat shock proteins (HSPs) to mitigate protein damage and maintain proteostasis.^[24] This response is orchestrated by heat shock factor 1 (HSF1), a transcription factor that, under normal conditions, exists in an inactive monomeric form bound to HSPs such as HSP70 and HSP90.^[25] Upon thermal stress, misfolded or aggregated proteins accumulate, disrupting these inhibitory complexes and allowing HSF1 to trimerize, translocate to the nucleus, and bind to heat shock elements (HSEs) in the promoters of HSP genes, thereby inducing their expression.^[25] The HSR threshold in mammalian cells typically activates around 42°C, varying slightly by cell type and species, marking the transition from mild to severe proteotoxic stress.^[26] Central to the HSR are molecular chaperones like HSP70 and HSP90, which play critical roles in cytoprotection by refolding denatured proteins, preventing aggregation, and facilitating the degradation of irreparable ones via the ubiquitin-proteasome system.^[27] These HSPs also confer anti-apoptotic effects by inhibiting key executioners of programmed cell death, such as caspase activation and Bax/Bak oligomerization at the mitochondria, thereby preserving cell viability during acute heat exposure.^[28] For instance, HSP70 directly sequesters apoptosis-inducing factor (AIF) and inhibits JNK signaling, while HSP90 stabilizes client proteins involved in survival pathways, collectively enhancing cellular resilience.^[27] The protective outcomes of the HSR extend beyond immediate survival, as demonstrated by preconditioning strategies where mild heat stress (e.g., 42–43°C for short durations) induces HSP expression and confers tolerance to subsequent severe insults like ischemia.^[29] In myocardial and neuronal tissues, this preconditioning reduces ischemic necrosis by bolstering antioxidant defenses and limiting inflammation, independent of HSP induction in some contexts, highlighting the HSR's broader cardioprotective and neuroprotective potential.^[29] Recent studies have expanded the understanding of HSF1's role beyond thermal stress, revealing its activation in non-thermal proteotoxic conditions, such as protein aggregation in neurodegeneration.^[30] In models of Alzheimer's and Parkinson's diseases, impaired HSF1 function exacerbates proteotoxicity by failing to upregulate HSPs, leading to accelerated neuronal loss, whereas enhancing HSF1 activity ameliorates aggregate clearance and synaptic integrity.^[30] This underscores HSF1's therapeutic promise in age-related proteopathies, with post-2020 research emphasizing its dysregulation in exceptional brain aging as a key factor in vulnerability to proteotoxic stress.^[31]

Oxidative and chemical stress responses

The cellular response to oxidative stress is primarily triggered by reactive oxygen species (ROS), which are generated endogenously from sources such as mitochondrial electron transport chain leaks during respiration and NADPH oxidases during immune signaling and vascular function.^[32] These ROS, including superoxide anions and hydrogen peroxide, can accumulate and disrupt cellular integrity if not neutralized, prompting adaptive mechanisms to restore redox balance.^[32] A central pathway in this response is the KEAP1-NRF2 system, where KEAP1 acts as a redox sensor by detecting oxidative insults through oxidation of its cysteine residues, particularly cysteines 151, 273, and 288, leading to conformational changes that release NRF2 from ubiquitination and degradation.^[33] Stabilized NRF2 translocates to the nucleus and binds antioxidant response elements (AREs) to upregulate genes encoding antioxidant enzymes, such as superoxide dismutase (SOD) for superoxide dismutation and glutathione S-transferase (GST) for detoxifying lipid peroxidation products.^[34] This NRF2-mediated induction enhances cellular resilience against ROS-induced damage, with SOD converting superoxide to less harmful hydrogen peroxide and GST conjugating glutathione to xenobiotics and oxidative byproducts.^[34] Chemical stress responses complement oxidative defenses by targeting toxins and heavy metals through detoxification systems. Phase I enzymes, primarily cytochrome P450 (CYP450) monooxygenases, introduce reactive groups via oxidation to make xenobiotics more water-soluble, while phase II enzymes like UDP-glucuronosyltransferases (UGTs) conjugate these intermediates with glucuronic acid for excretion, preventing bioaccumulation and toxicity.^[35]^[36] For heavy metals, metallothioneins (MTs) bind and sequester ions like cadmium and mercury, with their expression induced by the metal-responsive transcription factor 1 (MTF1); for instance, arsenic exposure activates MTF1 via binding to metal response elements, elevating MT levels to mitigate metal-induced ROS and protein mishandling.^[37] Imbalance in redox homeostasis, where antioxidant capacity is overwhelmed, promotes lipid peroxidation—a chain reaction oxidizing polyunsaturated fatty acids in membranes, generating toxic aldehydes like 4-hydroxynonenal that propagate damage and amplify stress signaling.^[38] Chronic oxidative stress from environmental pollution, such as particulate matter exposure, sustains this imbalance by persistently elevating ROS in lung and systemic tissues, contributing to inflammation and long-term cellular dysfunction as evidenced in recent analyses of urban air quality impacts.^[39]

Hypoxic and ER stress responses

The cellular response to hypoxia, or oxygen deprivation, primarily involves the stabilization and activation of hypoxia-inducible factor 1α (HIF-1α), a key transcription factor that orchestrates metabolic and vascular adaptations to low oxygen levels. Under normoxic conditions, HIF-1α is rapidly degraded through hydroxylation at specific proline residues by oxygen-dependent prolyl hydroxylase domain (PHD) enzymes, which serve as cellular oxygen sensors; this modification allows binding to the von Hippel-Lindau (VHL) ubiquitin ligase complex, targeting HIF-1α for proteasomal degradation. During hypoxia, reduced oxygen availability inhibits PHD activity, leading to HIF-1α accumulation, dimerization with HIF-1β (also known as ARNT), nuclear translocation, and binding to hypoxia response elements in target gene promoters. This pathway enables cells to shift toward anaerobic metabolism and enhance oxygen delivery.^[40] A prominent downstream effect of HIF-1α activation is the upregulation of vascular endothelial growth factor (VEGF), which stimulates endothelial cell proliferation and migration to promote angiogenesis, thereby increasing vascular networks in hypoxic tissues. This response is essential for physiological processes like embryonic development and tissue repair but can drive pathological neovascularization in conditions such as tumors, where sustained hypoxia perpetuates a pro-angiogenic environment. HIF-1α also induces glycolytic enzymes like glucose transporter 1 (GLUT1) and lactate dehydrogenase A (LDHA) to support energy production without oxygen-dependent respiration.^[41]^[42] Endoplasmic reticulum (ER) stress occurs when the protein folding capacity of the ER is overwhelmed by unfolded or misfolded proteins, often due to nutrient deprivation, calcium imbalance, or genetic mutations, activating the unfolded protein response (UPR) to restore ER homeostasis. The UPR is coordinated by three ER transmembrane sensors—PERK (protein kinase R-like ER kinase), IRE1 (inositol-requiring enzyme 1), and ATF6 (activating transcription factor 6)—which are sequestered by the chaperone BiP (binding immunoglobulin protein, also GRP78) under basal conditions. ER stress causes unfolded proteins to bind BiP, releasing the sensors and initiating parallel signaling branches: PERK phosphorylates eIF2α to globally attenuate translation while selectively enhancing translation of ATF4, a transcription factor that upregulates genes for redox balance and amino acid transport; IRE1 autophosphorylates, activating its endonuclease to splice XBP1 mRNA into a potent transcription factor that induces ER chaperones, foldases, and ER-associated degradation (ERAD) components; and ATF6 traffics to the Golgi for sequential cleavage by site-1 and site-2 proteases, liberating an N-terminal fragment that translocates to the nucleus to drive expression of BiP and XBP1. These adaptive mechanisms increase protein folding capacity and degrade irreparable proteins.00651-3)^[20]^[43] If ER stress persists unresolved, the UPR transitions from prosurvival to proapoptotic signaling, primarily through upregulation of C/EBP homologous protein (CHOP, also GADD153), a transcription factor induced by ATF4 (via PERK) and ATF6 pathways that sensitizes cells to death by downregulating anti-apoptotic Bcl-2, promoting oxidative stress via GADD34, and enhancing DR5 expression to activate extrinsic apoptosis. This shift prevents prolonged proteotoxic burden and is implicated in diseases like diabetes and neurodegeneration. The integrated stress response (ISR), which converges on eIF2α phosphorylation through PERK among other kinases, overlaps with UPR to fine-tune translation under combined hypoxic and ER stresses. Recent 2025 studies reveal that assemblysomes—dynamic RNA-protein complexes—facilitate stress granule formation during ISR-UPR crosstalk, sequestering mRNAs for selective translation and protecting cells from acute proteotoxic overload.^[44]^[45]^[23]^[46]

Physiological and Pathological Implications

Role in homeostasis and aging

The cellular stress response is integral to maintaining homeostasis during embryonic development, where heat shock proteins (HSPs) serve as early protective mechanisms against proteotoxic stress. HSPs, particularly the 70-kDa family (Hsp70s), are among the first proteins synthesized in mammalian embryos, facilitating proper folding and preventing aggregation of nascent proteins essential for cell differentiation and organogenesis.^[47] For example, Hsp70 expression is dynamically regulated during embryogenesis to support cell survival under fluctuating environmental conditions, such as temperature variations in utero.^[48] This role underscores how stress pathways ensure developmental stability by buffering against intrinsic and extrinsic perturbations.^[49] In mature organisms, stress responses enable physiological adaptations to demands like exercise, promoting tissue resilience without pathological consequences. During intense physical activity, skeletal muscle activates the heat shock response and oxidative stress pathways to mitigate damage from reactive oxygen species (ROS) and mechanical strain, leading to upregulated expression of HSPs and antioxidant enzymes.^[50] Short-term intense exercise training, for instance, enhances protein stability and reduces markers of cellular damage, fostering an adaptive environment that improves endurance and recovery.^[51] These adaptations exemplify how transient stress signaling maintains metabolic homeostasis by optimizing energy production and repair processes in response to routine physiological challenges.^[52] In the context of aging, the hormesis paradigm illustrates how low-level stressors can bolster longevity by priming endogenous defense networks, including sirtuins and FOXO transcription factors, which coordinate antioxidant and repair functions. Mild oxidative or proteotoxic stress induces hormetic effects that activate SIRT1 and FOXO3, enhancing mitochondrial function and DNA repair to counteract age-related decline.^[53] This adaptive strengthening of stress responses has been observed across model organisms, where it promotes healthier aging by amplifying vitagenes like heme oxygenase-1.^[54] Caloric restriction, a classic hormetic intervention, engages the integrated stress response (ISR) via eIF2α phosphorylation, redirecting translation toward stress-protective proteins and extending lifespan by 20-50% in rodents and nematodes through improved proteostasis.^[55]^[56] Despite these benefits, chronic stress response activation imposes trade-offs, often culminating in cellular senescence—a stable proliferative arrest that limits further damage but accumulates with age. Persistent ISR or unfolded protein response signaling can drive senescence by sustaining DNA damage checkpoints and inflammatory secretomes, as seen in models of prolonged oxidative insult. Recent studies highlight how unchecked chronic stress exacerbates senescence in fibroblasts and neurons, contributing to tissue dysfunction.^[57] Age-related decline in NRF2 activity reduces transcription of antioxidant genes like NQO1 and GCLC, impairing redox homeostasis and amplifying vulnerability to cumulative stress, as observed in human and murine tissues.^[58]^[59] This NRF2 attenuation represents a key age-related shift that diminishes the efficacy of protective responses.

Involvement in cancer and neurodegeneration

The cellular stress response plays a dual role in cancer, where dysregulated activation can promote tumorigenesis and therapy resistance while also offering opportunities for selective targeting. Overexpression of heat shock proteins (HSPs), such as HSP70 and HSP90, is frequently observed in various cancers, enabling tumor cells to withstand proteotoxic stress and evade apoptosis induced by chemotherapeutic agents. For instance, elevated HSP levels stabilize oncogenic proteins and inhibit pro-apoptotic pathways, contributing to chemotherapy resistance in breast and lung cancers. Similarly, mutations in the NRF2 pathway, a key regulator of oxidative stress response, drive oncogenesis by conferring constitutive activation that enhances antioxidant defenses and metabolic reprogramming, as seen in lung squamous cell carcinomas where somatic NRF2 mutations impair KEAP1 binding, leading to unchecked proliferation and survival advantages. In neurodegeneration, failure of the unfolded protein response (UPR) and heat shock response (HSR) exacerbates proteotoxicity, particularly in protein aggregation disorders. In Alzheimer's disease, chronic ER stress from tau hyperphosphorylation overwhelms the UPR, promoting tau aggregation into neurofibrillary tangles and synaptic dysfunction, with impaired PERK-eIF2α signaling failing to restore proteostasis. Likewise, in Parkinson's disease, α-synuclein accumulation disrupts UPR activation in dopaminergic neurons, leading to ER stress propagation and Lewy body formation, as evidenced by reduced ATF6 processing and XBP1 splicing in affected brain regions. Genetic evidence underscores this vulnerability: HSF1 knockout in mouse models accelerates amyloid-β oligomerization and pathology, highlighting HSF1's neuroprotective role in clearing misfolded proteins via enhanced chaperone activity. This dysregulation creates a therapeutic window in cancer, where controlled stress induction can overwhelm tumor cells' adaptive capacity without harming normal tissues, contrasting with neuroprotective strategies in neurodegeneration. For example, mitogenic overstimulation combined with stress pathway inhibition exploits cancer cells' reliance on proteostasis networks to induce lethal proteotoxicity. Recent advances post-2020 include modulation of the integrated stress response (ISR) pathway, with inhibitors targeting PERK and GCN2 showing promise in preclinical glioblastoma models by sensitizing tumors to radiotherapy and disrupting adaptive survival under hypoxia. Ongoing efforts, such as those evaluating IRE1 inhibitors, aim to translate these into clinical settings for enhanced therapeutic efficacy.

Therapeutic and Research Applications

Modulating stress pathways for therapy

Modulating cellular stress pathways has emerged as a promising therapeutic strategy to exploit dysregulated stress responses in diseases such as cancer and neurodegeneration, where these pathways contribute to cell survival under adverse conditions. Pharmacological agents targeting key components of stress responses can either inhibit protective mechanisms in malignant cells or enhance adaptive responses in vulnerable neurons, thereby shifting the balance toward disease mitigation. Heat shock protein 90 (HSP90) inhibitors, such as geldanamycin and its derivatives, have been developed to disrupt protein folding and stability in cancer cells, where HSP90 chaperones overexpressed oncoproteins like mutated kinases and steroid receptors.^[60] By binding to the ATP-binding site of HSP90, these inhibitors induce proteotoxic stress, leading to ubiquitination and degradation of client proteins, which halts tumor proliferation and promotes apoptosis.^[61] Clinical applications include geldanamycin analogs like 17-AAG (tanespimycin), which demonstrated antitumor efficacy in phase II trials for multiple myeloma and breast cancer, though hepatotoxicity limits broader use.^[62] Ongoing research focuses on next-generation inhibitors with improved selectivity to minimize off-target effects in non-cancerous tissues.^[63] For neurodegenerative disorders, activators of the nuclear factor erythroid 2-related factor 2 (NRF2) pathway, such as sulforaphane derived from cruciferous vegetables, enhance antioxidant defenses and mitigate oxidative stress.^[64] Sulforaphane modifies Keap1, allowing NRF2 nuclear translocation and upregulation of genes encoding glutathione synthesis enzymes and phase II detoxifiers, which reduces neuronal damage in models of Parkinson's and Alzheimer's diseases.^[65] Preclinical studies show sulforaphane attenuates microglial activation and neuroinflammation, preserving dopaminergic neurons in rotenone-induced models.^[66] These findings support sulforaphane's potential as an adjunct therapy, with phase II trials exploring its efficacy in slowing cognitive decline, though bioavailability challenges persist.^[67] Integrated stress response (ISR) modulators like ISRIB target the phosphorylation of eukaryotic initiation factor 2 alpha (eIF2α) to restore protein synthesis impaired by chronic stress in prion diseases and related neurodegeneration.^[68] ISRIB inhibits the downstream effects of eIF2α phosphorylation without altering its levels, thereby alleviating translational repression and stress granule formation in affected neurons.^[69] In prion-infected mouse models, ISRIB administration extended survival and reduced synaptic loss by partially restoring protein synthesis rates.^[70] This approach highlights ISR modulation's therapeutic window, as low-level ISR inhibition preserves adaptive responses while countering pathological overactivation.^[71] Activation of heat shock factor 1 (HSF1) represents another avenue, particularly for amyotrophic lateral sclerosis (ALS), where impaired heat shock responses exacerbate protein aggregation. Compounds like arimoclomol prolong HSF1 activation under stress, boosting HSP expression to enhance proteostasis in motor neurons.^[72] A phase II trial of arimoclomol in early-stage ALS patients demonstrated safety and trends toward slowed functional decline, though a subsequent phase III study did not meet primary endpoints.^[73] Emerging agents, such as M102—a dual NRF2/HSF1 activator—are advancing to phase I trials in 2025, building on preclinical neuroprotection in ALS models.^[74] Combination therapies further leverage stress pathway modulation by sensitizing tumors to proteotoxic overload, as seen with proteasome inhibitors like bortezomib paired with HSP90 antagonists.^[75] Proteasome inhibition accumulates misfolded proteins, amplifying unfolded protein response (UPR) stress, while HSP90 blockade impairs refolding capacity, synergistically inducing apoptosis in resistant cancers such as multiple myeloma.^[76] This strategy exploits cancer cells' reliance on heightened proteostasis, with clinical data showing enhanced response rates in relapsed patients compared to monotherapy.^[77] Such combinations underscore the value of multi-pathway targeting to overcome adaptive resistance in stress-responsive malignancies.^[78]

Biomarkers and emerging discoveries

Biomarkers of the cellular stress response provide valuable tools for diagnosing and monitoring stress-related pathologies. Circulating levels of heat shock protein 70 (HSP70) have been identified as a promising noninvasive biomarker for cardiovascular risk, with elevated serum concentrations correlating with the severity of atherosclerosis, vascular calcification, and acute myocardial infarction onset.^[79] Similarly, assays measuring NRF2 signaling activity, such as those evaluating nuclear translocation or downstream gene expression like NQO1, enable the assessment of oxidative stress in diseases including chronic obstructive pulmonary disease and neurodegenerative disorders, where NRF2 dysregulation exacerbates inflammation and tissue damage.^[80] Recent discoveries have expanded understanding of stress response dynamics beyond traditional pathways. In 2025, assemblysomes were characterized as novel intracellular RNA-protein complexes that function as dynamic hubs within stress granules, facilitating mRNA sequestration and translational repression during acute cellular insults like oxidative or thermal stress.^[46] These structures integrate RNA-binding proteins and non-coding RNAs to modulate gene expression, offering new insights into stress granule maturation and resolution.^[81] Advancements in the integrated stress response (ISR) have highlighted its therapeutic potential, particularly through potentiation strategies for antiviral applications. Optogenetics-based screening in 2025 identified small-molecule ISR potentiators that enhance eIF2α phosphorylation, leading to broad-spectrum antiviral effects against viruses such as herpes simplex virus type 1 and Zika in vitro and in vivo models, without inducing cytotoxicity.00690-7) This approach exploits ISR-mediated translational shutdown to inhibit viral replication across diverse pathogens.^[82] Emerging research also addresses environmental stressors linked to climate change. A 2024 review synthesized evidence on how prolonged heat exposure and air pollution activate cellular stress responses, impairing immune function and barrier integrity through mechanisms like HSP induction and NRF2 pathway overload, thereby increasing susceptibility to infections and chronic diseases in vulnerable populations.00119-2/fulltext)

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The integrated stress response in neurodegenerative diseases
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Signal Transduction Pathways Leading to Heat Shock Transcription
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The C/EBP Homologous Protein (CHOP) Transcription Factor ...
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Heat Shock Proteins and Their Role in Pregnancy - PMC - NIH
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Biomarkers of NRF2 signalling: Current status and future challenges
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Role of Assemblysomes in Cellular Stress Responses
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