β1B subunit of voltage-dependent Ca 2+ channels is predominant isoform expressed in human neuroblastoma cell line IMR32

Abstract

Human neuroblastoma cells (IMR32) respond to treatment with either dibutyryl-cAMP or nerve factor by acquiring a neuronal phenotype which is accompanied by a marked increase in the density of neuronal (N-type) VDCC currents. Using IMR32 cells as a model for neuronal differentiation, we were interested in examining possible changes in the level of expression of the α1B subunit of N-type calcium channels as well as beta subunit isoforms. Upon differentiation with dibutyryl-cAMP and 5-bromo-2-deoxyuridine for 16 days, we observed a dramatic increase in α1B protein which initiated between day 8 and 10. Day 10 evidenced maximal expression of α1B protein, which was followed by an interval of relatively constant expression of α1B (day 12 to day 16). Monitoring beta subunit expression using a pan specific anti-beta antibody (Ab CW20), we observed an increase in expression of a single 82 kDa beta subunit. The predominant 82 kDa beta subunit expressed throughout the course of differentiation was identified as the β1b isoform using a panel of beta subunit specific antibodies. Of significance, neither the β2 nor β3 isoforms were detected in full differentiated IMR32 cells. Contrary to a previous report on the absence of neurotypic expression of VDCC beta subunits in a second model for in vitro differentiation, NGF-treated rat pheochromocytoma cells (PC12 cells) [1], we report the regulated expression of the β1b protein in differentiated IMR32 cells suggesting a cell specific function for this beta subunit which parallels the acquisition of the neuronal phenotype. The restrictive expression of the β1b in IMR32 cells may reflect a cell-type specific function that extends beyond its role as an auxiliary subunit of VDCC complexes.