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Human glioblastoma cell cultures were established and the expression of glial fibrillary acidic protein (GFAP) and the gap-junction protein connexin 43 (Cx43) was confirmed by Western blot. Following treatment with 4-phenylbutyrate (4-PB), increased concentrations of non-phosphorylated GFAP were seen, while phosphorylated isoforms remained intact. Immunocytochemical staining of glioblastoma cells revealed an intracellular redistribution of GFAP. In addition to cytoplasmic immunostaining, GFAP immunoreactivity was also associated with the nucleus and/or the nuclear membrane. Phosphorylated and non-phosphorylated Cx43 proteins were increased 2- to 5-fold following 4-PB treatment, and were redistributed to areas of the cell surface, participating in cell-to-cell contacts. In addition, functional gap-junction coupling was amplified, as indicated by increased fluorescent dye transfer, and elevated levels of Cx43 protein were detected in parallel with enhanced gap-junction communication. Induced cell differentiation, with improved functional coupling of tumour cells, may be of importance for therapeutic strategies involving intercellular transport of low molecular-weight compounds. [Copyright &y& Elsevier]