Bortezomib is not a new molecule. The drug, best known for its role in treating multiple myeloma and mantle cell lymphoma, works by blocking the proteasome, the cellular machinery that degrades damaged or unwanted proteins. When proteasomal degradation grinds to a halt, misfolded proteins accumulate, stress pathways ignite and cells that cannot cope are pushed into programmed death. Earlier clinical and laboratory observations had suggested that bortezomib has activity against CTCL, but the molecular wiring behind that activity had never been fully mapped. The research team, led by Kirti S. Prabhu and Shahab Uddin, set out to answer two linked questions: does bortezomib modulate SKP2-associated cell-cycle regulation in CTCL cells, and can it be combined effectively with the platinum chemotherapy drug cisplatin?

To do this, the investigators used two well-characterized human CTCL cell lines, H9 and HH, exposing them to escalating concentrations of bortezomib and measuring viability with cell counting assays, then probing the consequences with fluorescence imaging and flow cytometry. The drug proved remarkably potent. The 24-hour concentration that killed half the cells, the IC50, was just 5.25 nanomolar in H9 cells and an even lower 1.82 nanomolar in HH cells. Values in the low nanomolar range are notable because they sit comfortably within concentrations achievable in clinical practice, lending translational weight to the laboratory findings. The team then asked what kind of death the cells were dying, and the answer was unambiguous: apoptosis, the orderly suicide program that cancer cells often manage to evade.

The apoptotic signature emerged at several levels simultaneously. Flow cytometry revealed a rising sub-G0/G1 population, a classic hallmark of cells with fragmented DNA, alongside an increase in Annexin V-positive cells, which flag the externalization of phosphatidylserine, an early molecular signal of apoptosis. Immunoblotting showed activation of the caspase cascade, the proteolytic machinery that executes cell death: caspase-8, caspase-9 and caspase-3 were all cleaved into their active forms, and PARP, a DNA repair enzyme that caspase-3 slices as a finishing blow, was cleaved as well. The intrinsic, or mitochondrial, arm of apoptosis was clearly engaged, because the ratio of the pro-death protein Bax to the pro-survival protein Bcl-2 increased, and the mitochondrial membrane potential collapsed, a depolarization event that releases apoptogenic factors from the intermembrane space into the cytosol.

To confirm that this death was truly caspase-dependent, the researchers pre-treated cells with z-VAD-FMK, a broad-spectrum inhibitor that shuts down all caspases. The inhibitor significantly blunted bortezomib-induced apoptosis, demonstrating that the caspase cascade is a functional component of the drug’s cytotoxicity rather than a bystander effect. That finding matters because it anchors bortezomib’s action in a defined, druggable death pathway, one that can be monitored and potentially amplified by rational drug combinations.

Perhaps the most mechanistically revealing part of the study concerns reactive oxygen species, or ROS. Bortezomib treatment increased both cellular and mitochondrial ROS and simultaneously depleted intracellular glutathione, the cell’s principal endogenous antioxidant buffer. This double hit, more oxidants and less antioxidant capacity, creates a state of oxidative stress that damages mitochondrial membranes and feeds the apoptotic circuit. The causal contribution of ROS was tested with N-acetyl-L-cysteine, a glutathione-replenishing antioxidant. When NAC was present, bortezomib’s disruption of the cell cycle, its induction of apoptosis and its activation of caspases were all partially reduced. In other words, oxidative stress is not the whole story, but it is a genuine upstream driver of the cytotoxic program, and the partial rescue suggests that bortezomib kills CTCL cells through converging ROS-dependent and ROS-independent routes.

The study also connected bortezomib’s effects to cell-cycle control, an area where CTCL cells are particularly vulnerable. The team observed a marked suppression of SKP2, S-phase kinase-associated protein 2, an E3 ubiquitin ligase component famous for tagging the cyclin-dependent kinase inhibitors p21 and p27 for destruction. When SKP2 falls, its targets accumulate, and that is exactly what the researchers documented: levels of p21 and p27 rose while the cyclin-dependent kinases CDK4 and CDK6, which drive cells through the G1 checkpoint, declined. This SKP2-p21/p27 axis functions as a brake on proliferation, and bortezomib effectively re-engages that brake. Because SKP2 is an oncogenic driver in many lymphoid malignancies, its suppression offers a candidate pharmacodynamic vulnerability, a measurable molecular endpoint that clinicians could track to confirm that the drug is hitting its intended target in patients.

Combination therapy was the final piece of the puzzle. Platinum-based agents such as cisplatin are mainstays of oncology, but their use in CTCL is limited by toxicity and modest durable benefit. Using the Chou-Talalay method, a widely accepted mathematical framework for quantifying drug interactions through the combination index, the researchers tested whether bortezomib and cisplatin act cooperatively. At selected dose pairings, bortezomib enhanced cisplatin-induced loss of viability and apoptosis, and the interaction was strongly synergistic at 5 nanomolar bortezomib combined with 10 micromolar cisplatin. Synergy means the combination kills more cells than the sum of the two drugs acting alone, a property that could, in principle, allow lower doses of each agent and a narrower toxicity profile. The mechanistic logic is plausible: bortezomib’s ROS generation and caspase priming may lower the apoptotic threshold that cisplatin-induced DNA damage must cross to trigger cell death.

The authors are careful to frame these results as a foundation rather than a clinical verdict. All of the experiments were performed in established cell lines, and the study’s own conclusions call for validation in primary CTCL samples from patients and in in vivo models before the strategy can move toward the clinic. That caveat is standard and appropriate, yet the work still carries real significance. It converts a clinical observation, that bortezomib has activity in CTCL, into a mechanistic account involving ROS accumulation, glutathione depletion, mitochondrial depolarization, caspase activation and SKP2 pathway suppression, and it nominates a specific synergistic drug pairing for further testing. For a disease with limited durable options in its advanced stages, that combination of mechanistic depth and translational direction is exactly the kind of progress that translational medicine aims to deliver, and it gives CTCL researchers a clear set of molecular signposts to follow in the next phase of preclinical and clinical evaluation.

Subject of Research: Proteasome inhibitor bortezomib-induced ROS-dependent mitochondrial apoptosis and SKP2 signaling suppression in cutaneous T-cell lymphoma cells

Article Title: Bortezomib induces ROS-dependent mitochondrial apoptosis and suppresses SKP2 signaling in cutaneous T-cell lymphoma cells

Article References: Bortezomib induces ROS-dependent mitochondrial apoptosis and suppresses SKP2 signaling in cutaneous T-cell lymphoma cells. (n.d.). https://doi.org/10.1186/s12967-026-09039-4

Image Credits: AI Generated

DOI: 10.1186/s12967-026-09039-4

Keywords: cutaneous T-cell lymphoma, bortezomib, SKP2, reactive oxygen species, mitochondrial apoptosis, caspases, p21, p27, cisplatin, proteasome inhibitor, glutathione, drug synergy