Background Proprotein convertase subtilisin/kexin type 9 (PCSK9) modulates low-density lipoprotein (LDL) receptor (LDLR) degradation, thus influencing serum cholesterol levels. However, dysfunctional LDLR causes hypercholesterolemia without affecting PCSK9 clearance from the circulation. Methods and Results To study the reciprocal effects of PCSK9 and LDLR and the resultant effects on serum cholesterol, we produced transgenic mice expressing human (h) PCSK9. Although hPCSK9 was mainly expressed in the kidney, LDLR degradation was more evident in the liver. Adrenal LDLR levels were not affected, likely due to impaired PCSK9 retention in this tissue. In addition, hPCSK9 expression increased hepatic secretion of apoB-containing lipoproteins in an LDLR-independent fashion. Expression of hPCSK9 raised serum murine (m) PCSK9 levels by 4.3-fold in wild-type (WT) mice and not at all in LDLR−/− ice, where mPCSK9 levels were already 10-fold higher than in WT mice. In addition, LDLR+/− mice had 2.7-fold elevation in mPCSK9 levels and no elevation in cholesterol levels. Conversely, acute expression of hLDLR in transgenic mice caused a 70% decrease in serum mPCSK9 levels. Turnover studies using physiological levels of hPCSK9 showed rapid clearance in WT (half-life 5.2 min), faster in hLDLR transgenics (2.9 min), and much slower in LDLR−/− recipients (50.5 min). Supportive results were obtained using an in vitro system. Finally, up to 30% of serum hPCSK9 was associated with LDL regardless of LDLR expression. Conclusions Our results support a scenario where LDLR represents the main route of elimination of PCSK9, and a reciprocal regulation between these two proteins controls serum PCSK9 levels, hepatic LDLR expression, and serum LDL levels.

Objective The balance between apoptosis susceptibility and efferocytosis of macrophages is central to plaque remodeling and inflammation. LRP1 and its ligand, apoE, have been implicated in efferocytosis and apoptosis in some cell types. We investigated the involvement of the macrophage LRP1/apoE axis in controlling plaque apoptosis and efferocytosis. Method and Results LRP1-/- macrophages displayed nearly 2-fold more TUNEL positivity compared to WT cells in the presence of DMEM alone or with either LPS or oxidized LDL. The survival kinase, pAkt, was barely detectable in LRP1-/- cells, causing decreased pBad and increased cleaved caspase-3. Regardless of the apoptotic stimulation and degree of cell death, LRP1-/- macrophages displayed enhanced inflammation with increased IL-1β, IL-6, and TNFα expression. Efferocytosis of apoptotic macrophages was reduced by 60% in LRP1-/- versus WT macrophages despite increased apoE expression by both LRP1-/- phagocytes and WT apoptotic cells. Compared to WT macrophage lesions, LRP1-/- lesions had 5.7-fold more necrotic core with more dead cells not associated with macrophages. Conclusion Macrophage LRP1 deficiency increases cell death and inflammation by impairing pAkt activation and efferocytosis. Increased apoE expression in LRP1-/- macrophages suggests that the LRP1/apoE axis regulates the balance between apoptosis and efferocytosis thereby preventing necrotic core formation.

Macrophage apoptosis and efferocytosis are key determinants of atherosclerotic plaque inflammation and necrosis. Bone marrow transplantation studies in ApoE- and LDLR-deficient mice revealed that hematopoietic scavenger receptor class B type I (SR-BI) deficiency results in severely defective efferocytosis in mouse atherosclerotic lesions, resulting in a 17-fold higher ratio of free to macrophage-associated dead cells in lesions containing SR-BI−/− cells, 5-fold more necrosis, 65.2% less lesional collagen content, nearly 7-fold higher dead cell accumulation, and 2-fold larger lesion area. Hematopoietic SR-BI deletion elicited a maladaptive inflammatory response [higher interleukin (IL)-1β, IL-6, and TNF-α lower IL-10 and transforming growth factor β]. Efferocytosis of apoptotic thymocytes was reduced by 64% in SR-BI−/− versus WT macrophages, both in vitro and in vivo. In response to apoptotic cells, macrophage SR-BI bound with phosphatidylserine and induced Src phosphorylation and cell membrane recruitment, which led to downstream activation of phosphoinositide 3-kinase (PI3K) and Ras-related C3 botulinum toxin substrate 1 (Rac1) for engulfment and clearance of apoptotic cells, as inhibition of Src decreased PI3K, Rac1-GTP, and efferocytosis in WT cells. Pharmacological inhibition of Rac1 reduced macrophage efferocytosis in a SR-BI-dependent fashion, and activation of Rac1 corrected the defective efferocytosis in SR-BI−/− macrophages. Thus, deficiency of macrophage SR-BI promotes defective efferocytosis signaling via the Src/PI3K/Rac1 pathway, resulting in increased plaque size, necrosis, and inflammation.

Nef proteins from human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) have been found to associate with an active cellular serine/threonine kinase designated Nef-associated kinase (Nak). The exact identity of Nak remains controversial, with two recent studies indicating that Nak may be either Pak1 or Pak2. In this study, we investigated the hypothesis that such discrepancies arise from the use of different Nef alleles or different cell types by individual investigators. We first confirm that Pak2 but not Pak1 is cleaved by caspase 3 in vitro and then demonstrate that Nak is caspase 3 sensitive, regardless of Nef allele or cell type used. We tested nef alleles from three lentiviruses (HIV-1 SF2, HIV-1 NL4-3, and SIVmac239) and used multiple cell lines of myeloid, lymphoid, and nonhematopoietic origin to evaluate the identity of Nak. We demonstrate that ectopically expressed Pak2 can substitute for Nak, while ectopically expressed Pak1 cannot. We then show that Nef specifically mediates the robust activation of ectopically expressed Pak2, directly demonstrating that Nef regulates Pak2 activity and does not merely associate with activated Pak2. We report that most of the active Pak2 is found bound to Nef, although a fraction is not. In contrast, only a small amount of Nef is found associated with Pak2. We conclude that Nak is Pak2 and that Nef specifically mediates Pak2 activation in a low-abundance complex. These results will facilitate both the elucidation of the role of Nef in pathogenesis and the development of specific inhibitors of this highly conserved function of Nef.

Lipid peroxidation generates reactive dicarbonyls including isolevuglandins (IsoLGs) and malondialdehyde (MDA) that covalently modify proteins. Humans with familial hypercholesterolemia (FH) have increased lipoprotein dicarbonyl adducts and dysfunctional HDL. We investigate the impact of the dicarbonyl scavenger, 2-hydroxybenzylamine (2-HOBA) on HDL function and atherosclerosis in Ldlr −/− mice, a model of FH. Compared to hypercholesterolemic Ldlr −/− mice treated with vehicle or 4-HOBA, a nonreactive analogue, 2-HOBA decreases atherosclerosis by 60% in en face aortas, without changing plasma cholesterol. Ldlr −/− mice treated with 2-HOBA have reduced MDA-LDL and MDA-HDL levels, and their HDL display increased capacity to reduce macrophage cholesterol. Importantly, 2-HOBA reduces the MDA-and IsoLG-lysyl content in atherosclerotic aortas versus 4-HOBA. Furthermore, 2-HOBA reduces inflammation and plaque apoptotic cells and promotes efferocytosis and features of stable plaques. Dicarbonyl scavenging with 2-HOBA has multiple atheroprotective effects in a murine FH model, supporting its potential as a therapeutic approach for atherosclerotic cardiovascular disease.