Increased Expression of Ubiquitin During Adenosine 3′,5′-Cyclic Monophosphate-Induced Differentiation of Neuroblastoma Cells in Culture

Authors

  • Francisco G. La Rosa,

    1. Center for Vitamins and Cancer Research, Departments of Radiology and
    2. Pathology, and the
    3. Webb-Waring Institute for Biomedical Research, University of Colorado Health Sciences Center, Denver, Colorado, U.S.A.
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  • Sanjay Kumar,

    1. Center for Vitamins and Cancer Research, Departments of Radiology and
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  • Kedar N. Prasad

    Corresponding author
    1. Center for Vitamins and Cancer Research, Departments of Radiology and
      Address correspondence and reprint requests to Dr. K. N. Prasad at Center for Vitamins and Cancer Research, University of Colorado HSC, 4200 East 9th Ave., Denver, CO 80262-0276, U.S.A.
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Address correspondence and reprint requests to Dr. K. N. Prasad at Center for Vitamins and Cancer Research, University of Colorado HSC, 4200 East 9th Ave., Denver, CO 80262-0276, U.S.A.

Abstract

Abstract: The role of ubiquitin in proliferation and differentiation of nerve cells has not been studied. An elevation of the intracellular level of adenosine 3′,5′-cyclic monophosphate (cAMP) in neuroblastoma cells induces terminal differentiation in these cells. Therefore, in this study we investigated the changes in the level and subcellular distribution of ubiquitin during proliferation and differentiation of neuroblastoma cells. Prostaglandin E1, a stimulator of adenylate cyclase, plus β-carotene, and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro 20-1724), an inhibitor of cyclic nucleotide phosphodiesterase, plus β-carotene were used to induce terminal differentiation in >90% of neuroblastoma cells. Changes in ubiquitin level were studied by immunofluorescent staining using either a mouse monoclonal antibody or a rabbit polyvalent antibody to ubiquitin. Results showed that the dividing neuroblastoma cells contained very low levels of ubiquitin localized primarily in the cytoplasm. The intensity of cytoplasmic staining for ubiquitin markedly increased during cAMP-induced differentiation of neuroblastoma cells, being the highest at 4 days after treatment. The neurites of these differentiated cells were also stained, but the nuclei were not. We propose a hypothesis that higher levels of cytoplasmic ubiquitin are needed during cAMP-induced differentiation of neuroblastoma cells for the removal of proteins responsible for cell proliferation through rapid degradation and/or inhibition of transcription, later leading to terminal differentiation.

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