Cleavage of MCM2 licensing protein fosters senescence in human keratinocytes

In eukaryotic cells, MCM, the minichromosome maintenance proteins, form a heterohexamer during G(1) phase in the cell cycle and constitute a DNA helicase activity at the onset of replication. MCM proteins are downregulated and dissociated from chromatin when cells exit the cell cycle. MCM proteins are upregulated frequently in a variety of dysplastic and cancer cells. To delineate the role of MCM in esophageal epithelial biology, we determined the MCM family gene expression during cellular senescence, immortalization, differentiation and apoptosis. All of the MCM2-7 proteins appeared to be downregulated in primary human esophageal keratinocytes upon replicative senescence. Their expression was restored by ectopic expression of a catalytic subunit of human telomerase, resulting in immortalization. Interestingly, we found a reciprocal induction of a novel MCM2-related protein fragment upon cell growth inhibition associated with senescence, contact inhibition or terminal differentiation, but not apoptosis. Epitope mapping of this MCM2-related fragment suggested the lack of amino- and carboxyl-terminal regions, including one of the putative nuclear localization signals and the ATPase domain, the MCM box. The absence of multiple MCM2 transcripts implied a possible posttranslational molecular cleavage in generation of the MCM2-related fragment, and a potential functional role in the regulation of the activity of the MCM protein complex.