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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.

MicroRNAs (miR)-221 and miR-222, both overexpressed in human thyroid papillary carcinomas, regulate p27Kip1 protein levels and cell cycle