Increased expression of p27Kip1 arrests neuroblastoma cell growth

Background and Procedure. To investigate the molecular mechanisms by which retinoic acid (RA) alters cell growth, the expression and activity of compo nents of the cell cycle machinery were analyzed. Results and Conclusions. W ithin 2 days of RA treatment, and prior to the arrest of NE cells in the G( 1) phase of the cell cycle, there was a complete downregulation of Cl cycli n/cdk activities. Protein levels for the G(1) cyclin/cdk were essentially u nchanged during this time, although there was a decrease in the steady-stat e levels of hyperphosphorylated Rb and p60N-MYC proteins. The cdk inhibitor s, p21Cip1 and p27Kip1 were constitutively expressed in KCNR, while p15 INK 4B 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 level s were unchanged. Levels of p21Cip1 were unaltered. Coincident with the dec reasein kinase activity there was an increase in p27 bound to G(1) cyclin/c dk. The increase in p21 was not due to an increase in transcription. In oth er 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 affec t 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 G(1) cy clin/cdk complexes. Using adenoviral vectors expressing p27 we found that i nfection leads to increased p27 expression, which causes a decrease in cdk activity and an accumulation of cells in G(1). Med. Pediatr. Oncol. 36: 97- 99, 2001. Published 2001 Wiley-Liss, Inc.