Retinoic acid induces neuronal differentiation of embryonal carcinoma cells by reducing proteasome-dependent proteolysis of the cyclin-dependent inhibitor p27

Retinoic acid (RA) treatment of embryonal carcinoma cell line NTERA-2 clone D1 (NT2/D1) induces growth arrest and terminal differentiation along the n euronal pathway, In the present study, we provide a functional link between RA and p27 function in the control of neuronal differentiation in NT2/D1 c ells. We report that RA enhances p27 expression, which results in increased association with cyclin E/cyclin-dependent kinase 2 complexes and suppress ion of their activity; however, antisense clones, which have greatly reduce d RA-dependent p27 inducibility (NT2-p27AS), continue to synthesize DNA and are unable to differentiate properly in response to RA as determined by la ck of neurite outgrowth and by the failure to modify surface antigens. As t o the mechanism involved in RA-dependent p27 upregulation, our data support the concept that RA reduces p27 protein degradation through the ubiquitin/ proteasome-dependent pathway. Taken together, these findings demonstrate th at in embryonal carcinoma cells, p27 expression is required for growth arre st and proper neuronal differentiation.