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¹ Dipartimento di Biologia e Patologia Cellulare e Molecolare c/o Istituto di Endocrinologia ed Oncologia Sperimentale del CNR, Facoltà di Medicina e Chirurgia, Università degli Studi di Napoli ‘Federico II’, via Pansini, 5, 80131 Naples, Italy
² Department of Molecular Virology, Immunology, and Medical Genetics, Comprehensive Cancer Center, Ohio State University, 410 West 12th Avenue, Columbus, Ohio 43210, USA
³ NOGEC (Naples Oncogenomic Center)-CEINGE, Biotecnologie Avanzate-Napoli, and SEMM-European School of Molecular Medicine-Naples Site, via Comunale Margherita, 482, 80145 Naples, Italy

(Correspondence should be addressed to A Fusco, Dipartimento di Biologia e Patologia Cellulare e Molecolare, Facoltà di Medicina e Chirurgia, Università degli Studi di Napoli ‘Federico II’, Via S Pansini 5, 80131 Napoli, Italy; Email: afusco{at}napoli.com)

We have recently reported that MicroRNAs (miR)-221 and miR-222 were up-regulated in human thyroid papillary carcinomas in comparison with the normal thyroid tissue. Bioinformatic analysis proposed the p27^Kip1 protein, a key regulator of cell cycle, as a candidate target for the miR-221/222 cluster. Here, we report that the enforced expression of miR-221 and miR-222 was able to reduce p27^Kip1 protein levels in thyroid carcinoma and HeLa cells in the absence of significant changes in specific p27^Kip1 mRNA levels. This effect is direct as miR-221 and miR-222 negatively regulate the expression of the 3'-untranslated region-based reporter construct from the p27^Kip1 gene, and is dependent on two target sites in this region. Consistent with these results, an enforced expression of the miR-221 and miR-222 induced the thyroid papillary carcinoma cell line (TPC-1) to progress to the S phase of the cell cycle. It is likely that the negative regulation of p27^Kip1 by miR-221 and miR-222 might also have a role in vivo since we report an inverse correlation between miR-221 and miR-222 up-regulation and down-regulation of the p27^Kip1 protein levels in human thyroid papillary carcinomas. Therefore, the data reported here demonstrate that miR-221 and miR-222 are endogenous regulators of p27^Kip1 protein expression, and thereby, the cell cycle.

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