Stressed Cells Hold On to Their Color: ER Stress Links Senescence to Stubborn Age Spots

Solar lentigines, the flat brown patches that appear on sun-exposed skin as people age, are among the most recognizable signs of photoaging, yet the molecular machinery that keeps them persistently dark has remained only partially understood. A new study published in Cellular and Molecular Life Sciences offers an explanation that ties together several threads of cell biology: chronic stress in the endoplasmic reticulum, the organelle where proteins are folded and processed, appears to simultaneously drive pigment production in melanocytes and block the disposal of that pigment in neighboring keratinocytes. The result, according to the research team led by Shinwon Hwang, Ji Young Kim and corresponding author Sang Ho Oh of Yonsei University College of Medicine in Seoul, is a self-reinforcing pigment-retention state that explains why these spots resist fading.

The researchers began with a straightforward hypothesis grounded in clinical observation. Solar lentigines are characterized by persistent basal hyperpigmentation, meaning that the deepest layer of the epidermis remains loaded with melanin long after the original sun exposure that triggered it. They proposed that a sustained stress response centered on IRE1α, a sensor protein embedded in the endoplasmic reticulum membrane, could act on both sides of the pigment equation at once. IRE1α is best known as a key initiator of the unfolded protein response, a cellular quality-control program that activates when misfolded proteins accumulate. When the stress is brief, the response is protective; when it becomes chronic, it can push cells toward senescence, a state of permanent growth arrest.

To test the idea, the team combined human tissue analysis with mechanistic experiments in cell culture. They examined paired samples of lesional and non-lesional skin from patients with solar lentigines, work approved by the Institutional Review Board of Severance Hospital, and complemented the histology with two laboratory models: MNT-1 melanocytes, the pigment-producing cells of the skin, and HaCaT keratinocytes, the cells that form the bulk of the epidermis and normally receive and degrade transferred melanosomes. Crucially, the keratinocyte model included a doxycycline-inducible system as well as constitutive IRE1α expression, allowing the researchers to switch the stress pathway on at will and observe the consequences in a controlled manner.

The tissue findings set the stage. In lesional epidermis, the investigators observed dense accumulations of stage-IV melanosomes, the fully mature, heavily pigmented organelles that melanocytes manufacture and hand off to keratinocytes. Alongside this pigment load, the stressed skin showed elevated levels of IRE1α and p16, a canonical marker of cellular senescence. This co-occurrence was the first hint that the two phenomena, aging-like growth arrest and pigment retention, might share a common driver rather than being parallel but independent consequences of sun damage.

Experiments in the cell models then dissected the mechanism in detail. When the researchers sustained IRE1α signaling, cell proliferation dropped and the senescence markers p16 and p21 rose, confirming that chronic endoplasmic reticulum stress is sufficient to push these epidermal cells into a senescent state. In the melanocytes, the consequences for pigmentation were direct and measurable: IRE1α activation increased tyrosinase activity, the rate-limiting enzymatic step of melanin synthesis, and raised total melanin content. The cells also produced more melanosomes, and those melanosomes were larger than normal, expanding the raw supply of pigment available for transfer to surrounding keratinocytes.

The second half of the dual-hit mechanism emerged from the keratinocyte experiments. Keratinocytes are not passive pigment containers; they are supposed to degrade the melanosomes they receive through lysosomal pathways, a process sometimes described as melanophagy. Under sustained IRE1α signaling, that degradation slowed markedly. The researchers quantified intracellular melanosomes and found they accumulated because they were being broken down more slowly, not because more were arriving. Probing the lysosomal system, they detected diminished LysoTracker signal, indicating reduced lysosomal acidity, along with reduced maturation of cathepsin-B, a key degradative enzyme that requires an acidic environment to become fully active. Autophagic flux, measured with a mRFP–GFP–LC3 reporter that distinguishes early autophagosomes from mature autolysosomes, was also compromised, with fewer autolysosomes forming.

Together, these results sketch a coherent pathological circuit. Chronic IRE1α signaling in melanocytes ramps up melanogenesis, flooding the epidermis with pigment, while the same stress pathway in keratinocytes weakens the lysosomal machinery responsible for clearing that pigment away. The senescent state that accompanies the stress response likely stabilizes the situation, since senescent cells persist in tissue rather than being replaced, maintaining the altered signaling environment over time. The net effect is that pigment is produced faster and cleared more slowly, exactly the combination needed to explain the dense, persistent basal hyperpigmentation that defines solar lentigines.

The study also points toward intervention. The researchers tested two agents: verapamil, a calcium channel blocker better known as a cardiovascular drug, and STF083010, a selective inhibitor of the IRE1α RNase domain, the enzymatic activity through which IRE1α transmits its stress signal. Both compounds lessened melanosome accumulation in the keratinocyte model and partially restored degradative function. While the restoration was partial, the finding is significant because it demonstrates that the pigment-retention phenotype is not irreversible and that the IRE1α–lysosome axis is a plausible therapeutic target. Existing treatments for solar lentigines, such as laser therapy and topical depigmenting agents, aim primarily at melanin production or destruction; a strategy that instead restores the clearance machinery would represent a fundamentally different approach.

The broader implications extend beyond cosmetically visible age spots. The unfolded protein response has been implicated in a wide range of age-related tissue changes, and this study adds a vivid example of how a single stress sensor can couple senescence to a tissue-specific functional outcome, in this case pigmentation. The work also highlights melanophagy as an underappreciated control point in skin color biology. Most research on hyperpigmentation has focused on melanocytes and their synthetic output, but the fate of melanosomes after transfer is equally decisive, and lysosomal acidification and cathepsin maturation emerge from this study as actionable levers. If the findings hold up in further clinical studies, modulating IRE1α activity or supporting lysosomal function could inform the development of treatments not only for solar lentigines but potentially for other disorders of pigment retention.

The research, funded by the National Research Foundation of Korea, Yonsei University College of Medicine and the Korea Health Technology R&D Project, was published as an open-access article and is citable under DOI 10.1007/s00018-026-06389-6. Its central message is elegant in its economy: one stress pathway, acting chronically, produces pigment faster and disposes of it more slowly, while locking the affected cells into senescence. For the millions of people who develop these stubborn brown patches, the study offers something more concrete than a new description of the problem, namely a defined molecular axis that can, at least in laboratory models, be pharmacologically nudged back toward balance. Translating that laboratory result into safe and effective clinical therapy will require further work, but the identification of the IRE1α and lysosome axis as a coupled driver of senescence and pigment retention gives the field a clear and testable direction.

Subject of Research: Chronic endoplasmic reticulum stress linking cellular senescence to persistent skin hyperpigmentation in solar lentigines

Article Title: Chronic ER stress couples cellular senescence with pigment retention

Article References: Hwang, S., Kim, J. Y., Lee, E. J., Oh, D., Bae, Y. J., Kwon, I. J., Park, S., Seo, H. R., Alqahtani, J., Lee, J., & Oh, S. H. (2026). Chronic ER stress couples cellular senescence with pigment retention. Cellular and Molecular Life Sciences. https://doi.org/10.1007/s00018-026-06389-6

Image Credits: AI Generated

DOI: 10.1007/s00018-026-06389-6

Keywords: solar lentigo, IRE1α, unfolded protein response, ER stress, cellular senescence, melanophagy, melanogenesis, lysosomal acidification, keratinocytes, melanocytes, p16, skin aging