Many processes have been implicated in early atherogenesis. These include endothelial denudation, injury, or activation, including shear stress–related events; local adherence of platelets; lipoprotein oxidation; lipoprotein aggregation; macrophage chemotaxis and foam cell formation; and smooth muscle cell alterations. Which process, if any, could be regarded as the key event in early atherogenesis, ie, absolutely required, yet also sufficient as the sole pathological stimulus in an otherwise normal artery to provoke a cascade of events leading to lesion formation? The work of many investigators, which we summarize here, strongly supports subendothelial retention of atherogenic lipoproteins as the central pathogenic process in atherogenesis (for prior reviews, see References 1 through 61 2 3 4 5 6 ). Our thesis is that other contributory processes are either not individually necessary or are not sufficient. Most often, they are merely normal, expected responses of otherwise-healthy tissue to the presence of retained lipoproteins.

It is instructive to catalog other processes that have been argued to be central to the initiation of atherogenesis. The first is endothelial denudation,7 8 9 injury,10 or activation,11 12 as outlined in the “response-to-injury” hypothesis of Ross, Glomset, and coworkers. Although this important hypothesis has stimulated much of the work that we cite here, there is no definitive evidence in vivo that endothelial injury is either necessary or sufficient for lesion formation.

The response-to-injury hypothesis originally presupposed endothelial desquamation as the key event in atherogenesis.7 8 9 It is now clear, however, that developing atheromata are covered by an intact endothelial layer throughout most stages of lesion progression: lipoprotein retention, fatty streak formation, and formation of advanced lesions.5 6 12 13 14 15 In humans, only the most complicated, ulcerated lesions lose their endothelial layer. Furthermore, in some experimental models, an intact endothelium is required …

The Response-to-Retention Hypothesis of Early Atherogenesis

The macrophage scavenger receptor is a trimeric membrane glycoprotein with unusual ligand-binding properties which has been implicated in the development of atherosclerosis. The trimeric structure of the bovine type I scavenger receptor, deduced by complementary DNA cloning, contains three extracellular C-terminal cysteine-rich domains connected to the transmembrane domain by a long fibrous stalk. This stalk structure, composed of an a-helical coiled coil and a collagen-like triple helix, has not previously been observed in an integral membrane protein.

Type I macrophage scavenger receptor contains α-helical and collagen-like coiled coils

The most significant advance in the medical management of HIV-1 infection has been the treatment of patients with antiviral drugs, which can suppress HIV-1 replication to undetectable levels. The discovery of HIV-1 as the causative agent of AIDS together with an ever-increasing understanding of the virus replication cycle have been instrumental in this effort by providing researchers with the knowledge and tools required to prosecute drug discovery efforts focused on targeted inhibition with specific pharmacological agents. To date, an arsenal of 24 Food and Drug Administration (FDA)-approved drugs are available for treatment of HIV-1 infections. These drugs are distributed into six distinct classes based on their molecular mechanism and resistance profiles: (1) nucleoside-analog reverse transcriptase inhibitors (NNRTIs), (2) non-nucleoside reverse transcriptase inhibitors (NNRTIs), (3) integrase inhibitors, (4) protease inhibitors (PIs), (5) fusion inhibitors, and (6) coreceptor antagonists. In this article, we will review the basic principles of antiretroviral drug therapy, the mode of drug action, and the factors leading to treatment failure (i.e., drug resistance).

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HIV-1 Antiretroviral Drug Therapy

Combinations of antiretroviral drugs have proven remarkably effective in controlling the progression of HIV disease and prolonging survival, but these benefits can be compromised by the development of drug resistance. About 50 percent of patients receiving antiretroviral therapy in the United States are estimated to harbor viruses that express resistance to at least one of the available antiretroviral drugs.

HIV drug resistance.

New information about the benefits and limitations of testing for resistance to human immunodeficiency virus (HIV) type 1 (HIV-1) drugs has emerged. The International AIDS Society-USA convened a panel of physicians and scientists with expertise in antiretroviral drug management, HIV-1 drug resistance, and patient care to provide updated recommendations for HIV-1 resistance testing. Published data and presentations at scientific conferences, as well as strength of the evidence, were considered. Properly used resistance testing can improve virological outcome among HIV-infected individuals. Resistance testing is recommended in cases of acute or recent HIV infection, for certain patients who have been infected as long as 2 years or more prior to initiating therapy, in cases of antiretroviral failure, and during pregnancy. Limitations of resistance testing remain, and more study is needed to refine optimal use and interpretation.

Antiretroviral drug resistance testing in adults infected with human immunodeficiency virus type 1: 2003 recommendations of an International AIDS Society-USA Panel.

The multiple mutations associated with high-level AZT resistance (D67N, K70R, T215F, K219Q) arise in two separate subdomains of the viral reverse transcriptase (RT), suggesting that these mutations may contribute differently to overall resistance. We compared wild-type RT with the D67N/K70R/T215F/K219Q, D67N/K70R, and T215F/K219Q mutant enzymes. The D67N/K70R/T215F/K219Q mutant showed increased DNA polymerase processivity; this resulted from decreased template/primer dissociation from RT, and was due to the T215F/K219Q mutations. The D67N/K70R/T215F/K219Q mutant was less sensitive to AZTTP (IC50 approximately 300 nM) than wt RT (IC50 approximately 100 nM) in the presence of 0.5 mM pyrophosphate. This change in pyrophosphate-mediated sensitivity of the mutant enzyme was selective for AZTTP, since similar Km values for TTP and inhibition by ddCTP and ddGTP were noted with wt and mutant RT in the absence or in the presence of pyrophosphate. The D67N/K70R/T215F/K219Q mutant showed an increased rate of pyrophosphorolysis (the reverse reaction of DNA synthesis) of chain-terminated DNA; this enhanced pyrophosphorolysis was due to the D67N/K70R mutations. However, the processivity of pyrophosphorolysis was similar for the wild-type and mutant enzymes. We propose that HIV-1 resistance to AZT results from the selectively decreased binding of AZTTP and the increased pyrophosphorolytic cleavage of chain-terminated viral DNA by the mutant RT at physiological pyrophosphate levels, resulting in a net decrease in chain termination. The increased processivity of viral DNA synthesis may be important to enable facile HIV replication in the presence of AZT, by compensating for the increased reverse reaction rate.

Phenotypic mechanism of HIV-1 resistance to 3'-azido-3'-deoxythymidine (AZT): increased polymerization processivity and enhanced sensitivity to pyrophosphate of the mutant viral reverse transcriptase.

We have examined the binding of native and cell-modified low density lipoprotein (LDL) to gels of Type I collagen. Diffusion of native 125I-LDL into the collagen gel was slow, reaching equilibrium after 24 to 48 hours, while L-3H-glucose, a low molecular weight marker, equilibrated in 6 hours. Binding of 125I-LDL was measured at 48 hours as the amount associated with the collagen after extensive washing. Binding was saturable with an increasing concentration of LDL. Prior incubation with cell-free culture medium resulted in modest, but progressive, increases in electrophoretic mobility and binding to collagen. Incubation with cells produced a marked increase in electrophoretic mobility and a 5- to 10-fold increase in collagen binding; the presence of butylated hydroxytoluene during incubation prevented both effects. These changes in LDL were induced by porcine aortic endothelial cells, smooth muscle cells, human skin fibroblasts, and a variety of cell lines, as well as by acetylation. There was a curvilinear relationship between the amount of LDL protein bound and the net negative charge of the LDL; increasing net charge was associated with progressively greater increases in binding. These results suggest a potential role for collagen in trapping lipid in the extracellular matrix of arterial intima by slowing the diffusion of and by binding LDL. The data also demonstrate that binding of LDL to collagen is enhanced by modifications that increase its net negative charge.

Interaction of native and cell-modified low density lipoprotein with collagen gel.

Inhibition by serum components of oxidation and collagen-binding of low-density lipoprotein

Previous studies suggested that arterial smooth muscle cells (SMC) may be involved in regulating the growth of capillaries into atherosclerotic plaques. In the present study, we determined the effect of SMC products on porcine aortic endothelial cell (EC) replication in vitro. Quiescent or slowly growing EC in medium without endothelial cell growth factor (ECGF) were stimulated to proliferate in the presence of porcine aortic SMC conditioned medium, while the same conditioned medium inhibited the growth of rapidly dividing EC in high serum concentrations or with ECGF. The magnitude of both activities depended on SMC conditioned medium concentration. The dose-dependent increase in EC number stimulated by ECGF was completely inhibited by SMC conditioned medium. This effect was not due to a direct interaction of conditioned medium with ECGF because SMC conditioned medium inhibited the growth of EC that were rapidly proliferating in 10% serum without ECGF. The inhibitory activity was retained by an ultrafiltration membrane with an exclusion limit of 1000 daltons; the stimulatory activity was recovered in the ultrafiltrate and remained stable after boiling, treatment with acid or base and trypsin, and repeated freezing and thawing, but was removed by activated charcoal. The growth-promoting activity could not be accounted for by release of cell contents from lysed cells or of thymidine into the medium. Conditioned medium from SMC incubated in the presence of serum contained less EC growth-stimulatory activity but more growth-inhibitory activity than that from SMC in serum-free medium.

Effects of porcine aortic smooth muscle cell conditioned medium on endothelial cell replication.

Low density lipoprotein (LDL) increased secretion of glycosaminoglycans (GAG) and the cell cholesterol content of proliferating fibroblasts and smooth muscle cells in culture; with increasing cell density the GAG effect decreased, but the cholesterol effect did not. High density lipoprotein (HDL, d greater than 1.063) decreased GAG secretion by slowly proliferating cells; when cells were actively proliferating, HDL alone did not affect GAG secretion, but it inhibited the increase caused by LDL. Thus HDL appeared to influence GAG secretion by two separate mechanisms, an inhibition which was overcome by rapid proliferation and an anti-LDL effect. HDL2 (d = 1.063-1.100) partially reproduced the latter effect. In addition, HDL, HDL2, and HDL3 increased cell cholesterol; the ability of LDL to increase cholesterol was correspondingly reduced in the presence of HDL and its subfractions, suggesting that they act by common mechanisms.

Suzanne McCormick

Papers

Phenotypic mechanism of HIV-1 resistance to 3'-azido-3'-deoxythymidine (AZT): increased polymerization processivity and enhanced sensitivity to pyrophosphate of the mutant viral reverse transcriptase.

Interaction of native and cell-modified low density lipoprotein with collagen gel.

Inhibition by serum components of oxidation and collagen-binding of low-density lipoprotein

Effects of porcine aortic smooth muscle cell conditioned medium on endothelial cell replication.

Interaction of high and low density lipoproteins on glycosaminoglycan secretion by human vascular smooth muscle cells and fibroblasts