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The interaction of Cyclosporin A with Lipoproteins

University of British Columbia

The success of transplantation has been largely attributed to the introduction of the immunosuppressant, Cyclosporin A (CsA). However, patients receiving CsA frequently become dyslipidemic and this is thought to augment the nephrotoxic and hepatotoxic effects by interfering with the distribution and pharmacokinetics of CsA in plasma. In addition, CsA alone can cause hypercholesterolemia, which is believed to contribute to the pathogenesis of post-transplant atherosclerosis and coronary artery disease. The mechanism of CsA-induced hypercholesterolemia is unknown but it has been suggested that CsA affects the uptake of LDL at the level of the LDL-receptor. The objectives of this thesis were two-fold: 1) to test the hypothesis that CsA contributes to dyslipidemia by decreasing the cellular uptake of LDL via the LDL-receptor and 2) to determine how lipoproteins affect the binding and distribution of CsA in human plasma. To investigate the effect(s) of CsA on LDL uptake via the LDL-receptor, the binding, internalization, and degradation of ¹²⁵I-LDL In vitro was measured in human skin fibroblasts. The results show that CsA does not decrease LDL binding. Further, CsA did not decrease the Bm a x or Kd of the LDL for its receptor, nor did it decrease LDL-receptor mRNA levels. Contrary to my expectations, CsA significantly increased LDL degradation. To determine whether the association of CsA with LDL had any effect on the binding of LDL to its receptor, CsA was equilibrated with ¹²⁵I-LDL prior to its incubation with human skin fibroblasts. These results demonstrate that the association of CsA with LDL did not affect the binding or Kd of LDL to its receptor. Collectively, these data show that CsA does not reduce L D L uptake by decreasing the binding, internalization, or degradation of L D L and this suggests that decreasing LDL uptake via the LDL-receptor is not a mechanism by which CsA contributes to hyperlipidemia in patients receiving this drug. To investigate CsA's distribution in dyslipidemic plasma, CsA was added to plasma from the following groups: normolipidentic, hypercholesterolemic, hypertriglyceridemic, hypoalphalipoproteinemic, and a combination of hypercholesterolemic and hypertriglyceridemic. By using the phosphotungstic acid precipitation method to fractionate plasma, it was shown that the distribution of CsA in all of the dyslipidemic groups was significantly different from the normolipidentic control. In addition, the amount of CsA associated with the VLDL/LDL and HDL fractions was quite variable between the groups but the amount with the LPDP fraction remained relatively constant. These data suggest that factors other than the amount of lipid, such as the composition of the lipoprotein, play a role in the distribution of CsA.

Thesis/Dissertation

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2009-02-18

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http://hdl.handle.net/2429/4779

Pathology

University of British Columbia

UBCV

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