Apolipoprotein E and Cholesterol Lowering Drugs in Age-Related Macular Degeneration

The specific aims outlined in this proposal will provide a cell biological understanding of the role of a protein called apolipoprotein E and the regulation of cholesterol levels in the retinal pigment epithelial (RPE) tissue of the eye). We will also investigate the potential of two promising drugs, statins and PPARgamma agonists, which are currently being used to treat atherosclerosis and diabetes. These drugs will be tested for their ability to maintain cholesterol balance in the RPE.

Grantee institution at the time of this grant: University of Wisconsin-Madison

It is now becoming increasingly apparent that many chronic age-related diseases, such as atherosclerosis, Alzheimer’s disease, and macular degeneration, share common pathological characteristics. One such characteristic is the abnormal metabolism of cholesterol as a result of the defective functioning of proteins involved in this process. For example, genetic variations in apolipoprotein E (ApoE), which transports cholesterol into and out of the cell, have been associated with Alzheimer’s disease and age-related macular degeneration (AMD). Another protein called sterol regulatory element binding protein (SREBP) controls the levels of the low-density lipoprotein (LDL) receptor on the cell surface and the synthesis of new cholesterol. Thus, SREBP helps control the amount of cholesterol in the cell.

Our goal is to understand the functioning of ApoE and SREBP in cells of the retinal pigment epithelium because previous data from our lab shows that these cells lose the ability to handle cholesterol under conditions that mimic those found in AMD. Our results have the potential to ascertain whether drugs like statins, which activate SREBP and are widely used to treat atherosclerosis, can also help maintain proper cholesterol metabolism in the retinal pigment epithelial (RPE), which may, in turn, help treat or prevent AMD.

In this project, we aim to:

1. Investigate the role of ApoE in the RPE, especially with regard to how genetic variations in this protein affect its function and
2. Investigate the activation of SREBP and regulation of the LDL receptor and statin function in the RPE.

Project Updates

Dr. Lakkaraju’s team aims to clarify the role of apolipoprotein E in cholesterol transport in the retinal pigment epithelium (RPE), the cell layer that nourishes the light-detecting cells. Previous data from their laboratory shows that these cells lose the ability to handle cholesterol under conditions that mimic those found in macular degeneration. The team’s previously published results also showed that the major component of the age-pigment lipofuscin, called A2E (that is deposited in the retina of individuals with macular degeneration), causes cholesterol accumulation in RPE cells. The team is now investigating the roles of different isoforms (different forms of the same protein created by either genetic or post-expression processing) of apolipoprotein E (ApoE) in subcellular localization, in cholesterol transport in the RPE, and in how lipid-metabolizing enzymes are expressed. In the past year, they have examined the sub-cellular localization of different ApoE isoforms in pig RPE cells by confocal microscopy (a technique that reconstructs 3D-images of the landscape inside and outside of cells). The team has also examined the effect of the different ApoE isoforms on the amounts of cholesterol and fat-processing proteins within the cells. Finally, they used confocal time-lapse microscopy to examine how the different apolipoprotein E isoforms move around the cell. Surprisingly, they found that there is significant difference between different isoforms in how the fat is deposited and the levels of two fat-processing proteins in the cells. The team also found that one of the isoforms appears to block the sensitivity of the cells to the cholesterol lowering effect of the drug simvastatin. Since there is strong evidence that the different ApoE isoforms are related to the risk of macular degeneration, this evidence may point to underlying cellular mechanisms related to cholesterol transport that could contribute to RPE dysfunction in macular degeneration.