Complex effects of inhibiting hepatic apolipoprotein B100 synthesis in humans

Mipomersen (Kynamro®) is an antisense oligonucleotide (ASO) that inhibits apolipoprotein B (apoB) synthesis; its LDL lowering effects should, therefore, result from reduced secretion of VLDL. We enrolled 17 healthy volunteers who received placebo injections weekly for 3-wks followed by mipomersen weekly for 7-9 wks. Stable isotopes were used after each treatment to determine fractional catabolic rates (FCRs) and production rates (PRs) of apoB in VLDL, IDL, and LDL, and of TG in VLDL. Mipomersen significantly reduced apoB in VLDL, IDL, and LDL associated with increases in FCRs of VLDL and LDL apoB and reductions in PRs of IDL and LDL apoB. Unexpectedly, the PRs of VLDL apoB and VLDL TG were unaffected. siRNA knockdown of apoB expression in HepG2 cells demonstrated preservation of apoB secretion across a range of apoB synthesis. Titrated ASO knockdown of apoB mRNA in chow-fed mice showed preservation of both apoB and TG secretion. In contrast, titrated ASO knockdown of apoB mRNA in high fat fed mice resulted in stepwise reductions of both apoB and TG secretion. Mipomersen lowered all apoB-lipoproteins without reducing the PR of either VLDL apoB or TG. Our first-in-human data are consistent with longstanding models of post-transcriptional and post-translational regulation of apoB secretion, and are supported by experiments with siRNA in HepG2 cells and ASO in mice. These results indicate that targeting apoB synthesis can lower levels of apoB-lipoproteins without necessarily reducing VLDL secretion, thereby reducing the risk of steatosis associated with this therapeutic strategy

Effects of CETP inhibition with anacetrapib on metabolism of VLDL-TG and plasma apolipoproteins C-II, C-III, and E

Cholesteryl ester transfer protein (CETP) mediates the transfer of HDL cholesteryl esters for triglyceride (TG) in VLDL/LDL. CETP inhibition, with anacetrapib, increases HDL-cholesterol, reduces LDL-cholesterol, and lowers TG levels. This study describes the mechanisms responsible for TG lowering by examining the kinetics of VLDL-TG, apoC-II, apoC-III, and apoE. Mildly hypercholesterolemic subjects were randomized to either placebo (N = 10) or atorvastatin 20 mg/qd (N = 29) for 4 weeks (period 1) followed by 8 weeks of anacetrapib, 100 mg/qd (period 2). Following each period, subjects underwent stable isotope metabolic studies to determine the fractional catabolic rates (FCRs) and production rates (PRs) of VLDL-TG and plasma apoC-II, apoC-III, and apoE. Anacetrapib reduced the VLDL-TG pool on a statin background due to an increased VLDL-TG FCR (29%; P = 0.002). Despite an increased VLDL-TG FCR following anacetrapib monotherapy (41%; P = 0.11), the VLDL-TG pool was unchanged due to an increase in the VLDL-TG PR (39%; P = 0.014). apoC-II, apoC-III, and apoE pool sizes increased following anacetrapib; however, the mechanisms responsible for these changes differed by treatment group. Anacetrapib increased the VLDL-TG FCR by enhancing the lipolytic potential of VLDL, which lowered the VLDL-TG pool on atorvastatin background. There was no change in the VLDL-TG pool in subjects treated with anacetrapib monotherapy due to an accompanying increase in the VLDL-TG PR

Effects of PCSK9 Inhibition With Alirocumab on Lipoprotein Metabolism in Healthy Humans

BACKGROUND: Alirocumab, a monoclonal antibody to proprotein convertase subtilisin/kexin type 9 (PCSK9), lowers plasma low-density lipoprotein (LDL) cholesterol and apolipoprotein B100 (apoB). Although studies in mice and cells have identified increased hepatic LDL receptors as the basis for LDL lowering by PCSK9 inhibitors, there have been no human studies characterizing the effects of PCSK9 inhibitors on lipoprotein metabolism. In particular, it is not known whether inhibition of PCSK9 has any effects on very low-density lipoprotein or intermediate-density lipoprotein (IDL) metabolism. Inhibition of PCSK9 also results in reductions of plasma lipoprotein (a) levels. The regulation of plasma Lp(a) levels, including the role of LDL receptors in the clearance of Lp(a), is poorly defined, and no mechanistic studies of the Lp(a) lowering by alirocumab in humans have been published to date. METHODS: Eighteen (10 F, 8 mol/L) participants completed a placebo-controlled, 2-period study. They received 2 doses of placebo, 2 weeks apart, followed by 5 doses of 150 mg of alirocumab, 2 weeks apart. At the end of each period, fractional clearance rates (FCRs) and production rates (PRs) of apoB and apo(a) were determined. In 10 participants, postprandial triglycerides and apoB48 levels were measured. RESULTS: Alirocumab reduced ultracentrifugally isolated LDL-C by 55.1%, LDL-apoB by 56.3%, and plasma Lp(a) by 18.7%. The fall in LDL-apoB was caused by an 80.4% increase in LDL-apoB FCR and a 23.9% reduction in LDL-apoB PR. The latter was due to a 46.1% increase in IDL-apoB FCR coupled with a 27.2% decrease in conversion of IDL to LDL. The FCR of apo(a) tended to increase (24.6%) without any change in apo(a) PR. Alirocumab had no effects on FCRs or PRs of very low-density lipoproteins-apoB and very low-density lipoproteins triglycerides or on postprandial plasma triglycerides or apoB48 concentrations. CONCLUSIONS: Alirocumab decreased LDL-C and LDL-apoB by increasing IDL- and LDL-apoB FCRs and decreasing LDL-apoB PR. These results are consistent with increases in LDL receptors available to clear IDL and LDL from blood during PCSK9 inhibition. The increase in apo(a) FCR during alirocumab treatment suggests that increased LDL receptors may also play a role in the reduction of plasma Lp(a). CLINICAL TRIAL REGISTRATION: URL: http://www.clinicaltrials.gov. Unique identifier: NCT01959971

Adipose Specific Lipoprotein Lipase Deficiency More Profoundly Affects Brown Than White Fat Biology

Adipose fat storage is thought to require uptake of circulating triglyceride (TG)-derived fatty acids via lipoprotein lipase (LpL). To determine how LpL affects the biology of adipose tissue, we created adipose specific LpL knockout (ATLO) mice, and compared them with whole body LpL knockout mice rescued with muscle LpL expression (MCK/L0) and wild type (WT) mice. ATLO LpL mRNA and activity were reduced, respectively, 75% and 70% in gonadal adipose tissue (GAT), 90% and 80% in subcutaneous (SCAT), and 84% and 85% in brown adipose tissue (BAT). ATLO mice had increased plasma TG levels associated with reduced chylomicron TG uptake into BAT and lung. ATLO BAT, but not GAT, had altered TG composition. GAT from MCK/L0 was smaller and contained less polyunsaturated fatty acids in TG, while GAT from ATLO was normal unless LpL was overexpressed in muscle. High fat diet feeding led to less adipose in MCK/L0 mice but TG acyl composition in SCAT and BAT reverted to that of WT. Therefore, adipocyte LpL in BAT modulates plasma lipoprotein clearance and the greater metabolic activity of this depot makes its lipid composition more dependent on LpL-mediated uptake. Loss of adipose LpL reduces fat accumulation only if accompanied by greater LpL activity in muscle. These data support the role of LpL as the gate-keeper for tissue lipid distributio