Spontaneous and cAMP‐dependent induction of a resting phase and neurite formation in cell hybrids between human neuroblastoma cells and thymidine auxotrophs of rat nerve‐like cells

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Abstract
Cell hybrids (BIM) were produced between human neuroblastoma cells (IMR‐32) and thymidine auxotrophs (B3T) of rat nerve‐like cells (B103) in order to obtain cell lines undergoing stable neuronal differentiation. BIM cells exhibited the growth properties of partial transformation: 1) When the cell growth reached a plateau, BIM cells ceased to proliferate and expressed a differentiated phenotype. The shape of the cells changed from flat to round and they extended neurites. 2) When cultured in methylcellulose, BIM cells formed colonies, indicating that BIM cells have the ability of anchorage‐independent growth. By Southern blot analysis, BIM cells had both human and rat types of N‐myc genes. The human N‐myc genes were amplified, but the extent of the amplification was lower in BIM cells than that in the parental cell line IMR‐32. The rat N‐myc gene was detected at a similar level in BIM, B3T, B103, and rat fibroblastic cells 3Y1. Therefore, the decrease in amplification of human N‐myc genes may be involved in the properties of partial reverse‐transformation in BIM cells. When treated with various drugs such as db‐cAMP, forskolin, and cAMP with isobutyl‐methylxanthine, BIM cells expressed a nerve‐like phenotype. These findings indicate that cell hybridization yielded partial normalization of transformed nerve‐like cells.