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  • Phosphorylation of Bcl-2 in G2/M phase-arrested cells following photodynamic therapy with hypericin involves a CDK1-mediated signal and delays the onset of apoptosis.

Phosphorylation of Bcl-2 in G2/M phase-arrested cells following photodynamic therapy with hypericin involves a CDK1-mediated signal and delays the onset of apoptosis.

The Journal of biological chemistry (2002-07-09)
Annelies Vantieghem, Yan Xu, Zerihun Assefa, Jacques Piette, Jackie R Vandenheede, Wilfried Merlevede, Peter A M De Witte, Patrizia Agostinis
ABSTRACT

The role of Bcl-2 in photodynamic therapy (PDT) is controversial, and some photosensitizers have been shown to induce Bcl-2 degradation with loss of its protective function. Hypericin is a naturally occurring photosensitizer with promising properties for the PDT of cancer. Here we show that, in HeLa cells, photoactivated hypericin does not cause Bcl-2 degradation but induces Bcl-2 phosphorylation in a dose- and time-dependent manner. Bcl-2 phosphorylation is induced by sublethal PDT doses; increasing the photodynamic stress promptly leads to apoptosis, during which Bcl-2 is neither phosphorylated nor degraded. Bcl-2 phosphorylation involves mitochondrial Bcl-2 and correlates with the kinetics of a G(2)/M cell cycle arrest, preceding apoptosis. The co-localization of hypericin with alpha-tubulin and the aberrant mitotic spindles observed following sublethal PDT doses suggest that photodamage to the microtubule network provokes the G(2)/M phase arrest. PDT-induced Bcl-2 phosphorylation is not altered by either the overexpression or inhibition of p38 mitogen-activated protein kinase (p38 MAPK) and c-Jun NH(2)-terminal protein kinase 1 (JNK1) nor by inhibiting the extracellular signal-regulated kinases (ERKs) or protein kinase C. By contrast, Bcl-2 phosphorylation is selectively suppressed by the cyclin-dependent protein kinase (CDK)-inhibitor roscovitine, completely blocked by the protein synthesis inhibitor cycloheximide and enhanced by the overexpression of CDK1, suggesting a role for this pathway. However, in an in vitro kinase assay, active CDK1/cyclin B1 complex failed to phosphorylate immunoprecipitated Bcl-2, suggesting that this protein kinase may not directly modify Bcl-2. Mutation of serine-70 to alanine in Bcl-2 abolishes PDT-induced phosphorylation and restores the caspase-3 activation to the same levels of the vector-transfected cells, indicating that Bcl-2 phosphorylation may be a signal to delay apoptosis in G(2)/M phase-arrested cells.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
CDK1/Cyclin A2, active, GST tagged human, PRECISIO® Kinase, recombinant, expressed in baculovirus infected Sf9 cells, ≥70% (SDS-PAGE), buffered aqueous glycerol solution
Sigma-Aldrich
CDK1/CyclinB1, active, GST tagged human, PRECISIO® Kinase, recombinant, expressed in baculovirus infected Sf9 cells, ≥70% (SDS-PAGE), buffered aqueous glycerol solution