Data from The Inhibitor of Cyclin-Dependent Kinase 4a/Alternative Reading Frame (<b><i>INK4a/ARF</i></b>) Locus Encoded Proteins p16<sup>INK4a</sup> and p19<sup>ARF</sup> Repress Cyclin D1 Transcription through Distinct <b><i>cis</i></b> Elements

The Ink4a/Arf locus encodes two structurally unrelated tumor suppressor proteins, p16^INK4a and p14^ARF (murine p19^ARF). Invariant inactivation of either the p16^INK4a-cyclin D/CDK-pRb pathway and/or p53-p14^ARF pathway occurs in most human tumors. Cyclin D1 is frequently overexpressed in breast cancer cells contributing an alternate mechanism inactivating the p16^INK4a/pRb pathway. Targeted overexpression of cyclin D1 to the mammary gland is sufficient for tumorigenesis, and cyclin D1−/− mice are resistant to Ras-induced mammary tumors. Recent studies suggest cyclin D1 and p16^INK4a expression are reciprocal in human breast cancers. Herein, reciprocal regulation of cyclin D1 and p16^INK4a was observed in tissues of mice mutant for the Ink4a/Arf locus. p16^INK4a and p19^ARF inhibited DNA synthesis in MCF7 cells. p16^INK4a repressed cyclin D1 expression and transcription. Repression of cyclin D1 by p16^INK4a occurred independently of the p16^INK4a-cdk4-binding function and required a cAMP-response element/activating transcription factor-2-binding site. p19^ARF repressed cyclin D1 through a novel distal cis-element at −1137, which bound p53 in chromatin-immunoprecipitation assays. Transcriptional repression of the cyclin D1 gene through distinct DNA sequences may contribute to the tumor suppressor function of the Ink4a/Arf locus.