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Bryostatin 1 induces ubiquitination and proteasome degradation of Bcl-2 in the human acute lymphoblastic leukemia cell line, Reh.

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The ubiquitin-mediated proteolytic system has been implicated in the turnover of a number of intracellular proteins. In the present study, we investigated the novelty and potential role of bryostatin 1, a macrocyclic lactone isolated from the marine bryozoan, Bugula neritina, in inducing the ubiquitin-mediated proteolysis of the oncoprotein Bcl-2. Immunoprecipitation and immunoblotting analyses revealed that Bcl-2 is ubiquitinated following exposure of the acute lymphoblastic leukemia (ALL) cell line Reh to 1 nM bryostatin 1. Bcl-2 protein rapidly decreases to 50% of that recorded in the control after 24 h of bryostatin 1 treatment. In the subsequent 24 h, Bcl-2 protein again rapidly decreases to 6% of its pre-bryostatin 1 level at which time a plateau is reached and maintained for another 72 h. Furthermore, ubiquitin-Bcl-2 conjugates are detected in untreated as well as bryostatin 1 treated cells, indicating that ubiquitin-dependent proteolysis plays a role in the normal turnover of Bcl-2. However, ubiquitin-Bcl-2 conjugates increase in a time-dependent manner following bryostatin 1 treatment. Lactacystin, which inhibits the proteinase activities of the proteasome, inhibited the bryostatin 1-induced decrease of Bcl-2 protein. The effect of bryostatin 1 on the proteolytic efficiency of the 26S proteasome in Reh cell extracts was also investigated and shown to increase following 1 h of bryostatin 1 treatment. Proteolytic activity reached its highest point by 3 h, and subsequently returned to control levels by 12 h, post-bryostatin 1 treatment. In addition, bryostatin 1 treatment of the Reh cell line decreased expression of bcl-2 mRNA within 3 h. However, bcl-2 mRNA expression returned after 24 h. We speculate that this decrease in mRNA together with increased 26S proteolytic activity accounts for the initial rapid decrease recorded in Bcl-2 protein. These findings indicate that bryostatin 1 treatment of Reh ALL cells decreases Bcl-2 expression through two processes: a) enhanced Bcl-2 protein degradation through the activation of the ubiquitin-proteasome pathway and b) decreased bcl-2 mRNA expression.
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Document Type: Research Article

Affiliations: Division of Hematology and Oncology, Department of Internal Medicine, Karmanos Cancer Institute at Wayne State University School of Medicine, Detroit, MI 48201, USA.

Publication date: February 1, 2000

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  • The International Journal of Molecular Medicine is a monthly, peer-reviewed journal devoted to the publication of high quality studies related to the molecular mechanisms of human disease. The journal welcomes research on all aspects of molecular and clinical research, ranging from biochemistry to immunology, pathology, genetics, human genomics, microbiology, molecular pathogenesis, molecular cardiology, molecular surgery and molecular psychology.

    The International Journal of Molecular Medicine aims to provide an insight for researchers within the community in regard to developing molecular tools and identifying molecular targets for the diagnosis and treatment of a diverse number of human diseases.
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