Increased expression of p27Kip1 arrests neuroblastoma cell growth.

Abstract

To investigate the molecular mechanisms by which retinoic acid (RA) alters cell growth, the expression and activity of components of the cell cycle machinery were analyzed. Within 2 days of RA treatment, and prior to the arrest of NB cells in the G1 phase of the cell cycle, there was a complete downregulation of GI cyclin/cdk activities. Protein levels for the G1 cyclin/cdk were essentially unchanged during this time, although there was a decrease in the steady state levels of hyperphosphorylated Rb and p60N-MYC proteins. The cdk inhibitors, p21Cip1 and p27Kip1 were constitutively expressed in KCNR, while p15 INK4B and p16 INK4A mRNA were undetected. Within 24 hr of RA treatment, there was a 4-fold increase in the expression of p27Kip1, although p27 mRNA levels were unchanged. Levels of p21Cip1 were unaltered. Coincident with the decrease in kinase activity there was an increase in p27 bound to G1 cyclin/cdk. The increase in p27 was not due to an increase in transcription. In other cell systems, increased expression of c-MYC has been shown to lead to a decrease in p27 levels that is regulated at the post-transcriptional level (sequestration). To determine whether increased levels of N-MYC could affect the level of p27, we evaluated the expression of p27 in a series of N-MYC transfected cells and found that constitutive overexpression of N-MYC led to a decrease in the steady-state levels of p27 and in p27 bound to G1 cyclin/cdk complexes. Using adenoviral vectors expressing p27, we found that infection leads to increased p27 expression, which causes a decrease in cdk activity and an accumulation of cells in G1.