Abstract
Lipoprotein(a) (Lp(a)) comprises a low density lipoprotein (LDL) particle with one molecule of apolipoprotein B-100 (apoB-100) and one molecule of apolipoprotein(a) [apo(a)], a protein evolutionary derived from plasminogen. The apo(a) protein consists of two plasminogen kringle domains, kringle IV (KIV) and kringle V (KV). KIV is further divided into 10 subclasses, KIV1-10. Repetition of the KIV2 domain is responsible for the size heterogeneity of apo(a) isoforms. The size of apo(a) isoforms is inversely correlated with plasma Lp(a) levels and elevated levels of Lp(a) (>50 mg/dL) are directly associated with cardiovascular disease risk. Plasma Lp(a) levels and cardiovascular disease risk share a “J” shape relationship where lower plasma Lp(a) levels (<50 mg/dL) reduces cardiovascular disease risk compared to those with no Lp(a). However, a proportional increase in cardiovascular disease risk is observed with increasing plasma Lp(a) levels (>50 mg/dL). The beneficial effects of having low Lp(a) concentrations occur through an unknown mechanism.
A significant positive correlation between Lp(a) and high density lipoprotein cholesterol (HDL-C) is observed in African-American populations and in the multi-ethnic Dallas heart study population, indicative of a biological connection between Lp(a) and HDL-C. Previous studies by our group and by others have also suggested that there is a positive connection between Lp(a) and HDL-C where elevated HDL-C levels were observed in Lp(a) transgenic mice. The first aim of this thesis was to investigate the mechanism responsible for the biological connection between Lp(a) and HDL-C. Plasma HDL-C levels are regulated by the expression of several genes in the liver. The HDL biogenesis pathway is primarily regulated by the ATP-binding cassette A1 (ABCA1), a cholesterol transporter. In this study, the effects of Lp(a) on the HDL biogenesis pathway in a hepatic cellular model (HepG2 cells) were investigated. Interestingly, Lp(a) upregulated ABCA1-mediated cholesterol efflux via the peroxisomal proliferator activated receptor and liver X receptor transcription factors. Further investigations revealed that it was the oxidised phospholipid content of Lp(a) that induced ABCA1 expression. Oxidised phospholipids from Lp(a) were delivered via the scavenger receptor-B1 (SR-B1) to stimulate an ABCA1 response in HepG2 cells.
An interesting observation from these results was the internalisation of apo(a), independent of the SR-B1-mediated Lp(a) lipid internalisation. Multiple studies have implicated several receptors in Lp(a) uptake, however the precise receptor and mechanism responsible for Lp(a) clearance from plasma remains unknown. The second aim of this thesis was to characterise the uptake and intracellular trafficking of Lp(a) in HepG2 cells. This study has determined that the endocytosed Lp(a) follows an early endosome to trans-Golgi/recycling endosome trafficking route. This retrograde trafficking of Lp(a) recycles apo(a) into the media where it appears to re-assemble Lp(a). Interestingly, Lp(a) uptake was dependent on L-type calcium channels, suggesting calcium-dependent endocytosis in HepG2 cells. Notably, Lp(a) uptake was independent of proposed Lp(a) receptors, the asialoglycoprotein receptor and the LDL receptor. A preliminary investigation has revealed that the plasminogen receptor, PlgRKT might be responsible for Lp(a) internalisation.
After establishing the methods to investigate the HDL synthesis pathway, the effects of chemotherapy drugs on lipid metabolism targets were investigated. This work was motivated by the findings of a colleague of my supervisor who had observed a decrease in HDL-C levels in breast cancer patients undergoing chemotherapy. This lead to the third aim: to investigate the effects of chemotherapy drugs (doxorubicin, cyclophosphamide and paclitaxel) on the HDL biogenesis pathway. Surprisingly, doxorubicin, a common chemotherapy agent, lowered HDL-C in HepG2 cells by decreasing ABCA1-mediated cholesterol efflux. Additionally, paclitaxel (another common chemotherapy agent) increased LDL-C by reducing LDLR protein levels and by stimulating the production of the apoB-containing lipoprotein particles in HepG2 cells. Another drug, cyclophosphamide, did not have any effect on lipid metabolism, indicating drug-specific effects on the HDL and LDL production pathway exist.
In conclusion, this PhD thesis has progressed our understanding of Lp(a) metabolism. A potential mechanistic basis for the positive association between Lp(a) and HDL was determined. The intracellular trafficking of Lp(a) in the liver has been shown for the first time to follow a retrograde trafficking and recycling pathway of apo(a). Lp(a) uptake has been shown to be mediated by the plasminogen receptor, PlgRKT. These new findings of the catabolic fate of Lp(a) in the liver may help in tailoring therapeutics to lower plasma Lp(a) levels.
The final chapter of this thesis revealed the effects of chemotherapy drugs in lipid metabolism. These HDL-C lowering and LDL-C elevating effects of chemotherapy drugs are likely to promote cardiovascular disease risk in cancer patients. Although longevity of the lipid alterations needs to be established, lipid alterations should be considered in the longer term management plan for cancer patients in order to reduce their risk of developing cardiovascular disease.