Jesús M. Paramio ^{1, @}, Carmen Segrelles ¹, Sergio Ruiz ¹, Juan Martín-Caballero¶, Angustias Page ¹, Jesús Martínez ¹, Manuel Serrano¶, and José L. Jorcano ¹

¹ From the  Cell, Molecular Biology, and Gene Therapy Project, CIEMAT, Avenida Complutense 22, E-28040 Madrid, Spain and the
¶ Department of Immunology and Oncology, Centro Nacional de Biotecnología, Campus de Cantoblanco, E-28049 Madrid, Spain

^@ To whom correspondence should be addressed. Tel.: 34-91-3466438;    Fax: 34-91-3466393;
E-mail:   jesusm.paramio@ciemat.es

In mammalian cells, cell cycle withdrawal is a prerequisite for terminal differentiation. Accordingly, in most tissues, including epidermis, the expression of the cyclin-dependent kinase inhibitors increases during differentiation. However, the actual role of cyclin-dependent kinase inhibitors is unclear. Different aspects of epidermal growth and differentiation in ink4a^{delta 2, 3}-null, p21-null, and ink4a^{delta 2, 3}/p21-doubly deficient mice were studied. Altered differentiation and decreased age-related senescence were found in the epidermis of ink4a^{delta 2, 3}/p21-null mice and, to a lesser extent, in ink4a^{delta 2, 3}- and p21-null mice. ink4a^{delta 2, 3}/p21-null primary keratinocytes underwent cell cycle arrest upon calcium or transforming growth factor- treatment, but failed to differentiate. This differentiation deficiency was not observed in p21- or ink4a^{delta 2, 3}-deficient keratinocytes. Upon infection with a v-Ha-ras-coding retrovirus, wild-type keratinocytes displayed features indicative of premature cell senescence. In p21- or ink4a^{delta 2, 3}-deficient keratinocytes, only a partial response was observed. ink4a^{delta 2, 3}/p21-deficient keratinocytes did not display senescent features, but showed increased tumorigenic potential upon injection into nude mice. These results indicate that ink4a/arf and cip1/waf genes cooperate to allow normal keratinocyte differentiation and that the absence of both favors malignant transformation.

* This work was supported in part by Grants SAF98-0047 and PB94-1230 from the Spanish Dirección General de Investigación Científica y Técnica.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section1734 solely to indicate this fact.

1. Inoue K, Zindy F, Randle DH, Rehg JE, and Sherr CJ, "Dmp1 is Haplo-Insufficient for Tumor Suppression and Modifies the Frequencies of Arf and p53 Mutations in Myc-Induced Lymphomas", Genes & Dev. vol. 15, no. 22, pp. 2934-2939 (November 15, 2001).

2. Frenster JH, "Nuclear Polyanions as De-repressors of Synthesis of Ribonucleic Acid", Nature 206: 680-683 (May 15, 1965).

3. Frenster JH,  "Activation of DNA Transcription Within Repressed Chromatin", 14th John Innes Symposium, Sept. 5-8, 2001.