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Control of cyclin-dependent kinase inhibitor p27 expression by cap-independent translation
W K Miskimins1, G Wang, M Hawkinson
1Division of Basic Biomedical Sciences, University of South Dakota School of Medicine, Vermillion, South Dakota 57069, USA. kmiskimi@usd.edu
Abstract:
p27 is a key regulator of cell proliferation through inhibition of G(1) cyclin-dependent kinase (CDK) activity. Translation of the p27 mRNA is an important control mechanism for determining cellular levels of the inhibitor. Nearly all eukaryotic mRNAs are translated through a mechanism involving recognition of the 5' cap by eukaryotic initiation factor 4E (eIF4E). In quiescent cells eIF4E activity is repressed, leading to a global decline in translation rates. In contrast, p27 translation is highest during quiescence, suggesting that it escapes the general repression of translational initiation. We show that the 5' untranslated region (5'-UTR) of the p27 mRNA mediates cap-independent translation. This activity is unaffected by conditions in which eIF4E is inhibited. In D6P2T cells, elevated cyclic AMP levels cause a rapid withdrawal from the cell cycle that is correlated with a striking increase in p27. Under these same conditions, cap-independent translation from the p27 5'-UTR is enhanced. These results indicate that regulation of internal initiation of translation is an important determinant of p27 protein levels.
Insights
The p27 protein, a cell proliferation regulator, is translated independently of the 5' cap mechanism. This cap-independent translation, mediated by the p27 mRNA's 5' untranslated region, is crucial for maintaining p27 levels during cell quiescence.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- p27 is a critical regulator of cell proliferation by inhibiting G(1) cyclin-dependent kinase (CDK) activity.
- Cellular p27 protein levels are controlled at the translational level.
- Most eukaryotic messenger RNAs (mRNAs) rely on cap-dependent translation initiation, involving eukaryotic initiation factor 4E (eIF4E).
Purpose of the Study:
- To investigate the mechanism of p27 mRNA translation during cell quiescence.
- To determine if p27 translation circumvents the general repression of cap-dependent translation.
- To identify the role of the p27 5' untranslated region (5'-UTR) in regulating its translation.
Main Methods:
- Analysis of p27 mRNA translation in quiescent cells.
- Investigating the effect of eIF4E inhibition on p27 translation.
- Studying p27 translation in D6P2T cells treated with cyclic AMP to induce cell cycle withdrawal.
Main Results:
- p27 mRNA translation is significantly elevated during cell quiescence, a state of general translational repression.
- The 5'-UTR of p27 mRNA confers cap-independent translation.
- This cap-independent translation mechanism is active even when eIF4E is inhibited.
- Elevated cyclic AMP levels, which induce cell cycle withdrawal and increase p27 levels, also enhance cap-independent translation from the p27 5'-UTR.
Conclusions:
- p27 protein levels are regulated by internal translation initiation, bypassing the canonical cap-dependent pathway.
- The p27 5'-UTR plays a key role in mediating cap-independent translation, allowing for increased p27 expression during cell quiescence.
- Regulation of internal translation initiation is a significant mechanism for controlling p27 protein levels in response to cellular signals.