Statins and Protein Prenylation in the Retina

  • Author / Creator
    Ling, Jennifer YM
  • Protein prenylation is the post-translational addition of isoprenoid lipid moieties to proteins, which regulates their subcellular localization and function. Farnesyl or geranylgeranyl isoprenoids are covalently linked to cysteines residues in a C-terminal prenylation recognition sequence in selected proteins. A host of inherited retinal diseases, such as choroideremia (CHM), results from prenylation defects suggesting that prenylation is of heightened importance in the eye. Yet, in many cases, it is unclear what drives retinal degeneration. Small GTPases represent the most important group of prenylated proteins and among them Rabs GTPases (Rabs) fulfill crucial cellular functions. Rabs regulate many steps of membrane traffic, including vesicle formation, vesicle movement along actin and tubulin networks, and membrane fusion. These processes are essential for endocytosis, autophagy and phagocytosis, among others cellular activities. Phagocytosis of the photoreceptor outer segments (OS) is one of the main functions of the retinal pigment epithelium (RPE). This process requires the activity of several Rabs, at different stages, e.g. Rab5 and Rab7 participate in phagosome maturation and Rab7 is required for the fusion of phagosomes and lysosomes to allow degradation of the phagosome cargo. Previous work in our laboratory has demonstrated that inhibition of Rab prenylation impairs the endocytic-autophagic pathway. Here, we hypothesize that inhibiting protein prenylation in the RPE will lead to decreased degradation of OS following phagocytosis. The isoprenoids attached to proteins during prenylation are synthesized by the mevalonate pathway that also generates cholesterol. The mevalonate pathway can be inhibited by statins, which are competitive inhibitors of the rate limiting enzyme Hydroxy-methyl glutaryl CoA Reductase. Statins may inhibit synthesis of both cholesterol and isoprenoids or just cholesterol. Therefore, some statins inhibit protein prenylation. We use two type of statins to regulate protein prenylation in retina-derived cells and in fibroblasts from patients with CHM. First, we demonstrate that simvastatin, but not pravastatin, inhibits protein prenylation in ARPE- 19 cells, a model of the RPE. This inhibititon can be prevented by geranylgeranyl pyrophosphate (GGPP), suggesting that prenylation inhibition is due to a shortage of isoprenoids. Second, we examined the effect of statins on OS degradation. Simvastatin, but not pravastatin, inhibited the degradation of OS in ARPE- 19, though this effect was not fully prevented by the addition of GGPP. Third, we extend our work to CHM, a retinal degeneration that results from the absence of Rab Escort Protein-1 (REP-1). REP-1 is an essential chaperone for prenylation of newly synthesized Rabs. We examined the effects of statins on Rab prenylation in dermal fibroblasts from age-matched choroideremia patients and healthy participants. Protein prenylation in fibroblasts from CHM patients was significantly more sensitive to inhibition by simvastatin than fibroblasts from healthy individuals. Our studies suggest that inhibition of protein prenylation in the RPE may lead to decreased OS degradation, and that CHM patients might be more sensitive treatment with statins.

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  • Degree
    Master of Science
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