Different pathways mediate cytochrome c release after photodynamic therapy with hypericin

Area of Science:

• Cell Biology
• Biochemistry
• Oncology

Background:

• Photodynamic therapy (PDT) induces apoptosis via various cellular mechanisms.
• Bcl-2 is a key regulator of apoptosis, often overexpressed in cancer cells.
• Understanding the role of Bcl-2 in PDT response is crucial for cancer treatment strategies.

Purpose of the Study:

• To investigate the mechanism of cytochrome c release during hypericin-induced PDT in cells overexpressing Bcl-2.
• To elucidate the role of caspases and mitochondrial membrane potential in PDT-induced apoptosis in the presence of Bcl-2.
• To determine if Bcl-2 overexpression alters the kinetics or pathways of apoptosis induction.

Main Methods:

• Utilized PC60R1R2 and PC60R1R2/Bcl-2 cell lines.
• Administered hypericin-based photodynamic therapy (PDT).
• Assessed cytochrome c release, procaspase-3 activation, poly(ADP-ribose)polymerase cleavage, and mitochondrial membrane potential.
• Employed caspase inhibitors (zVAD-fmk) and permeability transition pore inhibitors (cyclosporin A).

Main Results:

• Bcl-2 overexpression significantly delayed cytochrome c release, procaspase-3 activation, and PARP cleavage during PDT-induced apoptosis.
• Bcl-2 did not protect against PDT-induced necrosis.
• Mitochondrial membrane potential reduction occurred similarly in both cell types and was unaffected by inhibitors.
• Caspase inhibition partially protected cells from apoptosis in non-Bcl-2 overexpressing cells, but suppressed cytochrome c release and apoptosis in Bcl-2 overexpressing cells.

Conclusions:

• Bcl-2 overexpression reveals a caspase-dependent mechanism for cytochrome c release in PDT.
• A caspase-dependent feed-forward amplification loop for cytochrome c release is implicated in PDT-induced apoptosis in Bcl-2-overexpressing cells.
• The findings highlight distinct apoptotic pathways modulated by Bcl-2 during PDT.