Expression of Ras-GRF in the SK-N-BE neuroblastoma accelerates retinoic-acid-induced neuronal differentiation and increases the functional expressionof the IRK1 potassium channel

Ras-GRF, a neuron-specific Has exchange factor of the central nervous syste m, was transfected in the SK-N-BE neuroblastoma cell line and stable clones were obtained. When exposed to retinoic acid, these clones showed a remark able enhancement of Ras-GRF expression with a concomitant high increase in the level of active (GTP-bound) Has already after 24 h of treatment. In the presence of retinoic acid, the transfected cells stopped growing and acqui red a differentiated neuronal-like phenotype more rapidly than the parental ones. Cells expressing Ras-GRF also exhibited a more hyperpolarized membra ne potential. Moreover, treatment with retinoic acid led to the appearance of an inward rectifying potassium channel with electrophysiological propert ies similar to IRK1. This current was present in a large number of cells ex pressing Ras-GRF, while only a small percentage of parental cells exhibited this current. However, Northern analysis with a murine cDNA probe indicate d that IRK1 mRNA was induced by retinoic acid at a similar level in both ki nds of cells. Brief treatment with a specific inhibitor of the mitogen-acti vated protein kinase (MAPK) pathway reduced the number of transfected cells showing IRK1 activity. These findings suggest that activation of the Has p athway accelerates neuronal differentiation of this cell line. In addition, our results suggest that Ras-GRF and/or Ras-pathway may have a modulatory effect on IRK1 channel activity.