Lipoprotein(a) accelerates atherosclerosis in uremic mice

Abstract

Uremic patients have increased plasma lipoprotein(a) [Lp(a)] levels and elevated risk of cardiovascular disease. Lp(a) is a subfraction of LDL, where apolipoprotein(a) [apo(a)] is disulfide bound to apolipoprotein B-100 (apoB). Lp(a) binds oxidized phospholipids (OxPL), and uremia increases lipoprotein-associated OxPL. Thus, Lp(a) may be particularly atherogenic in a uremic setting. We therefore investigated whether transgenic (Tg) expression of human Lp(a) increases atherosclerosis in uremic mice. Moderate uremia was induced by 5/6 nephrectomy (NX) in Tg mice with expression of human apo(a) (n = 19), human apoB-100 (n = 20), or human apo(a) + human apoB [Lp(a)] (n = 15), and in wild-type (WT) controls (n = 21). The uremic mice received a high-fat diet, and aortic atherosclerosis was examined 35 weeks later. LDL-cholesterol was increased in apoB-Tg and Lp(a)-Tg mice, but it was normal in apo(a)-Tg and WT mice. Uremia did not result in increased plasma apo(a) or Lp(a). Mean atherosclerotic plaque area in the aortic root was increased 1.8-fold in apo(a)-Tg (P = 0.025) and 3.3-fold (P = 0.0001) in Lp(a)-Tg mice compared with WT mice. Plasma OxPL, as detected with the E06 antibody, was associated with both apo(a) and Lp(a). In conclusion, expression of apo(a) or Lp(a) increased uremia-induced atherosclerosis. Binding of OxPL on apo(a) and Lp(a) may contribute to the atherogenicity of Lp(a) in uremia.

Fig. 1.

Plasma apo(a) is not affected by moderate uremia in apo(a)-Tg mice. Plasma levels of apo(a) in uremic (5/6 NX) (open symbols) and nonuremic (CTRL) (closed symbols) mice. Mice were transgenic for cDNA-apo(a) [diamonds, n = 11 (5 NX, 6 CTRL)], both cDNA-apo(a) and human apoB (Lp(a)-Tg mice, circles, n = 4), or YAC-apo(a) [triangles, n = 14 (7 NX, 7 CTRL)]. Apo, apolipoprotein; Lp(a), lipoprotein(a); Tg, transgenic.

Fig. 2.

Expression of Lp(a) increases uremic atherosclerosis. Atherosclerotic plaque areas in the aortic root in female (open symbols) and male (closed symbols) WT (n = 18), cDNA-apo(a)-Tg (n = 18), apoB-Tg (n = 19), and Lp(a)-Tg (n = 14) mice 35 weeks after induction of uremia. Black lines represent mean values. P values are from Mann-Whitney tests. Apo, apolipoprotein; Lp(a), lipoprotein(a); Tg, transgenic; WT, wild-type.

Fig. 3.

Plasma cholesterol levels and profiles in uremic mice. A: Plasma cholesterol levels were measured at baseline (1–2 weeks before induction of uremia) (open bars) and two months after induction of uremia and ingestion of a high-fat, high-cholesterol Western-type diet (WD) (closed bars) in female and male mice. For a given genotype and gender, post uremia and WD levels were higher than baseline levels. *P < 0.05 compared with WT and cDNA-apo(a)-Tg mice at the given time point (one way ANOVA). B: Cholesterol gel filtration profiles in female (left) and male (right) nontransgenic WT (solid line), cDNA-apo(a)-Tg (circles), apoB-Tg (squares), and Lp(a)-Tg (crosses) uremic mice two months after induction of uremia. The elution of VLDL (V), LDL (L1 and L2), and HDL (H1 and H2) sized lipoproteins is indicated in the left panel. C: Apo(a) Western blot analyses of gel filtration fractions V, L1, L2, H1, and H2 [as specified in (B)] was performed under denaturing, nonreducing conditions to separate noncovalently bound “free” apo(a) from apoB-bound apo(a) in Lp(a). Apo, apolipoprotein; Lp(a), lipoprotein(a); Tg, transgenic; WT, wild-type.

Fig. 4.

Plasma levels of apo(a) and apoB in uremic mice. A: Plasma apo(a) in female (left) and male (right) cDNA-apo(a)- and Lp(a)-Tg uremic mice. *P < 0.05 (one-way ANOVA). B: Representative apo(a) Western blot of plasma from cDNA-apo(a)-Tg and Lp(a)-Tg uremic mice. Each lane shows data from one mouse. The gel was run under denaturing, reducing conditions. C: Plasma human apoB in female (left) and male (right) apoB-Tg and Lp(a)-Tg uremic mice. Apo, apolipoprotein; Lp(a), lipoprotein(a); Tg, transgenic.

Fig. 5.

OxPL associate with apo(a) and apoB in uremic transgenic mice. Plasma E06-reactive OxPL associated with apo(a) (A) or mouse apoB (B) in the various models. Note that apo(a) associates noncovalently with mouse apoB, and in the format of this assay, most of the OxPL on mouse apoB is actually on apo(a), as shown in panel C and as previously shown in Ref. . OxPL associated with human apoB particles (D) in WT mice and mice expressing human apoB, cDNA-apo(a), and Lp(a). The data in panels A (OxPL/apo(a)), B (OxPL/mapoB), and D (OxPL/hapoB) are presented as a ratio, and the data in panel C as relative light units (RLU). P < 0.05 as determined by one-way ANOVA (*) or Mann-Whitney tests (**). Apo, apolipoprotein; Lp(a), lipoprotein(a); OxPL, oxidized phospholipid; WT, wild-type.