p27^Kip1: a key mediator of retinoic acid induced growth arrest in the SMS–KCNR human neuroblastoma cell line

Abstract

Retinoic acid (RA) treatment of SMS–KCNR neuroblastoma (NB) cells leads to G1 growth arrest and neuronal differentiation. To investigate the molecular mechanisms by which RA alters cell growth, we analysed the expression and activity of components of the cell cycle machinery after culture in RA. Within 2 days of RA treatment and prior to the arrest of NB cells in the G1 phase of the cell cycle, there is a complete downregulation of G1 cyclin/Cdk activities. Protein levels for the G1 cyclin/Cdks were essentially unchanged during this time although there was a decrease in the steady-state levels of p67^N-Myc and hyperphosphorylated Rb proteins. The Cdk inhibitors, p21^Cip1 and p27^Kip1 were constitutively expressed in KCNR while p15^INK4B and p16^INK4A were not detected. RA induced an increase in the expression of p27^Kip1 but not p21^Cip1. Furthermore, coincident with the decrease in kinase activity there was an increase in G1 cyclin/Cdk bound p27^Kip1. These results indicate that changes in the level of p27^Kip1 and its binding to G1 cyclin/Cdks may play a key role in RA induced growth arrest of NB cells.