Glucocorticoids engage different signal transduction pathways to induce apoptosis in thymocytes and mature T cells

Area of Science:

• Cell Biology
• Molecular Biology
• Immunology

Background:

• Glucocorticoids (GC) are known to induce apoptosis, a programmed cell death, in various cell types.
• The precise molecular mechanisms underlying GC-induced apoptosis are not fully understood and appear to be cell-type specific.
• While GC receptor (GR) activation and gene expression are initial steps, the effector phase varies significantly.

Purpose of the Study:

• To elucidate the specific molecular pathways involved in glucocorticoid-induced apoptosis.
• To investigate the role of lysosomal enzymes in GC-induced cell death.
• To compare the apoptotic mechanisms in different immune cell types and leukemia cells.

Main Methods:

• Live imaging using confocal microscopy to observe cellular events in real-time.
• Assays to detect the activity of proteasomes, caspases (caspase-3, -8, -9), and lysosomal cathepsin B.
• Comparative analysis of GC-induced apoptosis in murine thymocytes, splenic T cells, and acute T lymphoblastic leukemia cells.

Main Results:

• Proteasomal degradation and caspase activity are crucial for GC-induced apoptosis in thymocytes but not splenic T cells.
• Lysosomal cathepsin B is rapidly activated in thymocytes upon GC exposure, subsequently leaking into the cytosol.
• Cathepsin B activation precedes nuclear condensation and caspase-8 and -3 processing, suggesting a lysosomal amplification loop for caspase-3 activation.
• Acute T lymphoblastic leukemia cells, like thymocytes, require caspase activity for GC-induced cell death.

Conclusions:

• Glucocorticoid-induced apoptosis involves cell type-specific pathways, with a novel lysosomal amplification loop identified in thymocytes.
• Cathepsin B activation is a key, previously unrecognized, component in the apoptotic cascade initiated by glucocorticoids in thymocytes.
• The findings highlight both commonalities and differences in GC-induced apoptosis, with implications for understanding T-cell development and leukemia treatment.