Expression of the pl6 and pl5 Cyclin-dependent Kinase Inhibitors in Lymphocyte Activation and Neuronal Differentiation1

The cyclin-dependcnt kinase inhibitors pl6INK4/MTSl and pl51NK4l>/ MTS2 have been mapped to a region in chromosome 9 (p21 ) that is deleted frequently in acute lymphoblastic leukemias and malignant (¡liornas.To gain insight into the functions of these inhibitors in lymphocytes and neuronal cells, we studied the expression of pl5 and pl6 during lympho cyte mitogenesis and neuronal differentiation. Expression of pl5 was extinguished during lymphocyte activation, concomitant with an increase in retinoblastoma kinase activity. The differentiation of the embryonic teralocarcinoma cell line NT2 into postmitotic neurons (hNT) was asso ciated with enhanced expression of pl5 and pl6 proteins. These findings suggest that pl5 and pl6 play a role in maintaining cell quiescence in lymphocytes and neuronal cells, respectively. Deletions of these genes may thus promote unrestrained growth.