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
beta-carotene, and
4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro 20-1724), an inhibitor of
cyclic nucleotide phosphodiesterase, plus
beta-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.