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Translational initiation factor expression and ribosomal protein gene expression are repressed coordinately but by
1Department of Biological Chemistry, School of Medicine, University of California, Davis 95616.
Abstract:
P1798 murine lymphosarcoma cells cease to proliferate upon exposure to 10(-7) M dexamethasone and exhibit a dramatic inhibition of rRNA and ribosomal protein synthesis (O. Meyuhas, E. Thompson, Jr., and R. P. Perry, Mol. Cell Biol. 7:2691-2699, 1987). These workers demonstrated that ribosomal protein synthesis is regulated primarily at the level of translation, since dexamethasone did not alter mRNA levels but shifted the mRNAs from active polysomes into inactive messenger ribonucleoproteins. We have examined the effects of dexamethasone on the biosynthesis of initiation factor proteins in the same cell line. The relative protein synthesis rates of eIF-4A and eIF-2 alpha were inhibited by about 70% by the hormone, a reduction comparable to that for ribosomal proteins. The mRNA levels of eIF-4A, eIF-4D, and eIF-2 alpha also were reduced by 60 to 70%, indicating that synthesis rates are proportional to mRNA concentrations. Analysis of polysome profiles showed that the average number of ribosomes per initiation factor polysome was only slightly reduced by dexamethasone, and little or no mRNA was present in messenger ribonucleoproteins. The results indicate that initiation factor gene expression is coordinately regulated with ribosomal protein synthesis but is controlled primarily by modulating mRNA levels rather than mRNA efficiency.
Insights
Dexamethasone inhibits protein synthesis in lymphosarcoma cells. Initiation factor gene expression is regulated by mRNA levels, similar to ribosomal protein synthesis.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- Dexamethasone inhibits proliferation and rRNA/ribosomal protein synthesis in P1798 lymphosarcoma cells.
- Previous studies indicated translational regulation of ribosomal protein synthesis by dexamethasone, involving mRNA shifts to inactive ribonucleoproteins.
Purpose of the Study:
- To investigate the effects of dexamethasone on the biosynthesis of initiation factor proteins in P1798 murine lymphosarcoma cells.
- To determine if initiation factor synthesis is regulated at the translational or transcriptional level.
Main Methods:
- Cell culture of P1798 murine lymphosarcoma cells.
- Treatment with 10(-7) M dexamethasone.
- Analysis of protein synthesis rates using radiolabeling.
- Quantification of mRNA levels via Northern blotting or similar techniques.
- Polysome profiling to assess mRNA localization and translation efficiency.
Main Results:
- Dexamethasone significantly inhibited the synthesis of eukaryotic initiation factor 4A (eIF-4A) and eukaryotic initiation factor 2 alpha (eIF-2 alpha) by approximately 70%.
- mRNA levels for eIF-4A, eIF-4D, and eIF-2 alpha were reduced by 60-70%, correlating with synthesis rates.
- Polysome analysis showed minimal changes in ribosome occupancy on initiation factor mRNAs, with little mRNA found in inactive messenger ribonucleoproteins.
Conclusions:
- Initiation factor gene expression is coordinately regulated with ribosomal protein synthesis in response to dexamethasone.
- Regulation occurs primarily through modulation of mRNA levels, not translational efficiency, contrasting with ribosomal protein regulation.