The cyclin-dependent kinase inhibitor p27(Kip1) is involved in thyroid hormone-mediated neuronal differentiation

The thyroid hormone (triiodothyronine, T3) is essential for normal brain ma turation. To determine the mechanisms by which T3 controls neuronal prolife ration and differentiation, we have analyzed the effect of this hormone on the expression and activity of cell cycle-regulating molecules in neuroblas toma N2a-beta cells that overexpress the beta 1 isoform of the T3 receptor. Our results show that incubation of N2a-beta cells with T3 leads to a rapi d down-regulation of the c-myc gene and to a decrease of cyclin D1 levels. T3 also causes a strong and sustained increase of the levels of the cyclin kinase inhibitor p27(Kip1). This increase is secondary, to the augmented le vels of p27(Kip1) transcripts as well as to stabilization of the p27(Kip1) protein. The increased levels of p27(Kip1) lead to a significant increase i n the amount of p27(Kip1) associated with cyclin-dependent kinase 2 (CDK2), and to a marked inhibition of the kinase activity of the cyclin . CDK2 com plexes. As a consequence, the retinoblastoma protein (pRb) and the retinobl astoma protein-related protein p130 are hypophosphorylated in T3-treated N2 a-beta cells. This study shows for the first time that T3-mediated growth a rrest and neuronal differentiation are associated with an increase in the l evels of a cyclin kinase inhibitor, which does not allow the inactivation o f retinoblastoma proteins required for progression through the restriction point in the cell cycle.