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Isoprenoids influence expression of Ras and Ras-related proteins
Sarah A Holstein1, Christine L Wohlford-Lenane, Raymond J Hohl
1Department of Pharmacology, University of Iowa, Iowa City, Iowa 52242, USA.
Abstract:
Mevalonate depletion by inhibition of hydroxymethylglutaryl coenzyme A reductase impairs post-translational processing of Ras and Ras-related proteins. We have previously shown that this mevalonate depletion also leads to the upregulation of Ras, Rap1a, RhoA, and RhoB. This upregulation may result from global inhibition of isoprenylation or depletion of key regulatory isoprenoid species. Studies utilizing specific isoprenoid pyrophosphates in mevalonate-depleted cells reveal that farnesyl pyrophosphate (FPP) restores Ras processing and prevents RhoB upregulation while geranylgeranyl pyrophosphate (GGPP) restores Rap1a processing and prevents RhoA and RhoB upregulation. Either FPP or GGPP completely prevents lovastatin-induced upregulation of RhoB mRNA. Inhibition of FPP or squalene synthase allowed for the further identification of the putative regulatory species. Studies involving the specific isoprenyl transferase inhibitors FTI-277 and GGTI-286 demonstrate that selective inhibition of protein isoprenylation does not mimic lovastatin's ability to increase Ras and RhoA synthesis, decrease Ras and RhoA degradation, increase RhoB mRNA, or increase total levels of Ras, Rap1a, RhoA, and RhoB. In aggregate, these findings reveal a novel role and mechanism for isoprenoids to influence levels of Ras and Ras-related proteins.
Insights
Mevalonate depletion impacts Ras protein processing and levels. Specific isoprenoids like farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP) can restore processing and regulate protein levels.
Area of Science:
- Biochemistry
- Molecular Biology
- Cell Biology
Background:
- Mevalonate depletion inhibits hydroxymethylglutaryl coenzyme A reductase, affecting Ras and related protein processing.
- Previous studies indicated mevalonate depletion upregulates Ras, Rap1a, RhoA, and RhoB.
- Upregulation may stem from isoprenylation inhibition or depletion of regulatory isoprenoids.
Purpose of the Study:
- To investigate the role of specific isoprenoid pyrophosphates in regulating Ras and Ras-related protein levels during mevalonate depletion.
- To elucidate the mechanism by which isoprenoids influence the synthesis, degradation, and mRNA levels of these proteins.
Main Methods:
- Utilizing mevalonate-depleted cells and specific isoprenoid pyrophosphates (FPP, GGPP).
- Employing isoprenyl transferase inhibitors (FTI-277, GGTI-286).
- Analyzing protein processing, upregulation, degradation, and mRNA levels of Ras and related proteins.
Main Results:
- Farnesyl pyrophosphate (FPP) restored Ras processing and prevented RhoB upregulation.
- Geranylgeranyl pyrophosphate (GGPP) restored Rap1a processing and prevented RhoA and RhoB upregulation.
- Neither FPP nor GGPP alone mimicked all effects of lovastatin on Ras and Rho protein synthesis, degradation, or mRNA levels, suggesting complex regulatory roles.
Conclusions:
- Isoprenoids play a critical role in regulating the post-translational processing and cellular levels of Ras and Ras-related proteins.
- Specific isoprenoids differentially affect the processing and abundance of various Ras family members.
- These findings reveal a novel mechanism for isoprenoid-mediated control over small GTPase signaling pathways.