To understand telomere homeostasis, a significant aspect of cancer and growth control, it is important to examine telomerase induction as well as mechanisms of regulated elimination. Makorin-1 (MKRN1) was previously shown to be an E3 ubiquitin ligase that targets the telomerase catalytic subunit (hTERT) for proteasome processing (Kim et al., Genes Dev 19:776-781, 2005). In this study we examined expression and regulation of endogenous MKRN1 during the cell cycle and terminal differentiation. When WI-38 cells transition from active growth into a resting G1 state, basal levels of MKRN1 were found to increase by sixfold. In contrast, cancer cells typically contained low or in some cases undetectable levels of MKRN1 protein. HL-60 cells growing exponentially in culture contain no detectable MKRN1; however, following terminal differentiation, MKRN1 mRNA and protein levels are strongly up-regulated while hTERT mRNA, hTERC, and telomerase are shut down. The initial decrease in telomerase activity is due to a gradual reduction in transcription of the hTERT gene that occurs during the first 12 h of terminal differentiation. MKRN1 protein appears between 12 and 24 h and is attended by a more rapid loss of telomerase activity. As more MKRN1 protein accumulates, significantly less telomerase activity is seen. Addition of the proteasome inhibitor, MG132, reverses the loss of telomerase activity; therefore, reductions in telomerase activity are dynamic, ongoing, and correlated with robust up-regulation of MKRN1 as the cells terminally differentiate. The data are consistent with the idea that MKRN1 represents a telomerase elimination pathway to rapidly draw down the activity during differentiation or cell cycle arrest when telomerase action at chromosome ends is no longer necessary.
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Acknowledgments We thank Dr. W. E. Wright for valuable discussion and input on this study. We wish to acknowledge comments on the article from Dr. Bongyong Lee, Dr. Gun Eui Lee, and Rhea Manjooran. This was supported in part by NIH Grant CA127416.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Clinical Biochemistry
- Cell Biology