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Updated: Oct 3, 2026
Measuring Caspase Activity Using a Fluorometric Assay or Flow Cytometry
Published on: March 24, 2023
Lack of correlation between caspase activation and caspase activity assays in paclitaxel-treated MCF-7 breast cancer
Timothy J Kottke1, April L Blajeski, X Wei Meng
1Division of Oncology Research, Mayo Clinic, Mayo Graduate School, Rochester, Minnesota 55905, USA.
Abstract:
MCF-7 human breast cancer cells are widely utilized to study apoptotic processes. Recent studies demonstrated that these cells lack procaspase-3. In the present study, caspase activation and activity were examined in this cell line after treatment with the microtubule poison paclitaxel. When cells were harvested 72 h after the start of a 24-h treatment with 100 nm paclitaxel, 37 +/- 5% of the cells were nonadherent and displayed apoptotic morphological changes. Although mitochondrial cytochrome c release and caspase-9 cleavage were detectable by immunoblotting, assays of cytosol and nuclei prepared from the apoptotic cells failed to demonstrate the presence of activity that cleaved the synthetic caspase substrates LEHD-7-amino-4-trifluoromethylcoumarin (LEHD-AFC), DEVD-AFC, and VEID-AFC. Likewise, the paclitaxel-treated MCF-7 cells failed to cleave a variety of caspase substrates, including lamin A, beta-catenin, gelsolin, protein kinase Cdelta, topoisomerase I, and procaspases-6, -8, and -10. Transfection of MCF-7 cells with wild type procaspase-3 partially restored cleavage of these polypeptides but did not result in detectable activities that could cleave the synthetic caspase substrates. Immunoblotting revealed that caspase-9, and -3, which were proteolytically cleaved in paclitaxel-treated MCF-7/caspase-3 cells, were sequestered in a salt-resistant sedimentable fraction rather than released to the cytosol. Immunofluorescence indicated large cytoplasmic aggregates containing cleaved caspase-3 in these apoptotic cells. These observations suggest that sequestration of caspases can occur in some model systems, causing tetrapeptide-based activity assays to underestimate the amount of caspase activation that has occurred in situ.
Insights
Paclitaxel treatment induced apoptosis in MCF-7 breast cancer cells, but active caspases were sequestered in aggregates, not detected by standard assays. This suggests assays may underestimate caspase activation in some apoptotic models.
Area of Science:
- Cell Biology
- Biochemistry
- Cancer Research
Background:
- MCF-7 human breast cancer cells are a common model for studying apoptosis.
- These cells are known to lack procaspase-3, a key enzyme in apoptosis.
- Paclitaxel is a microtubule poison that can induce apoptosis.
Purpose of the Study:
- To investigate caspase activation and activity in MCF-7 cells treated with paclitaxel.
- To determine if caspase cleavage activity can be detected in these cells despite the absence of procaspase-3.
- To explore the localization and potential sequestration of activated caspases.
Main Methods:
- MCF-7 cells were treated with paclitaxel (100 nm for 24 h).
- Apoptotic morphological changes, cytochrome c release, and caspase cleavage were assessed via immunoblotting.
- Caspase activity was measured using synthetic substrates (LEHD-AFC, DEVD-AFC, VEID-AFC) and endogenous substrates.
- Cells were transfected with procaspase-3 to assess its role in restoring activity.
- Caspase localization was examined using salt extraction and immunofluorescence microscopy.
Main Results:
- Paclitaxel treatment induced apoptosis in 37% of MCF-7 cells, with detectable cytochrome c release and caspase-9 cleavage.
- Despite cleavage, no significant caspase activity was detected using synthetic or endogenous substrates in cytosol or nuclei.
- Transfection with procaspase-3 partially restored substrate cleavage but not synthetic substrate activity.
- Cleaved caspases-9 and -3 were found sequestered in salt-resistant aggregates, not released into the cytosol.
- Immunofluorescence revealed large cytoplasmic aggregates of cleaved caspase-3.
Conclusions:
- Paclitaxel treatment triggers caspase activation in MCF-7 cells, but the activity is masked by sequestration.
- Standard in vitro caspase activity assays may underestimate caspase activation in certain cellular contexts.
- Caspase sequestration in cytoplasmic aggregates is a potential mechanism regulating apoptosis in some cancer cell models.
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