Interplay between heme oxygenase-1 and the multifunctional transcription factor yin yang 1 in the inhibition of intimal hyperplasia.

RATIONALE: induction of heme oxygenase (HO)-1 protects against experimental atherosclerotic diseases, and certain pharmacological HO-1 inducers, like probucol, inhibit the proliferation of vascular smooth muscle cells and, at the same time, promote the growth of endothelial cells in vivo and in vitro. OBJECTIVE: because such cell-specific effects are reminiscent of the action of the transcription factor Yin Yang (YY)1, we tested the hypothesis that there is a functional relationship between HO-1 and YY1. METHODS AND RESULTS: we report that probucol increases the number of YY1(+) cells in rat carotid artery following balloon injury at a time coinciding with increased HO-1 expression. The drug also induces the expression of YY1 mRNA and protein in rat aortic smooth muscle cells (RASMCs) in vitro, as do other known HO-1 inducers (tert-butylhydroquinone and hemin) and overexpression of HO-1 using a human HMOX1 cDNA plasmid. Conversely, overexpression of YY1 induces expression of HO-1 in RASMCs. Induction of YY1 expression is dependent on HO-1 enzyme activity and its reaction product CO, because pharmacological inhibition of heme oxygenase activity or CO scavenging block, whereas exposure of RASMCs to a CO-releasing molecule increases, YY1 expression. Furthermore, RNA interference knockdown of YY1 prevents probucol or adeno-HO-1 from inhibiting RASMC proliferation in vitro and neointimal formation in vivo. CONCLUSIONS: our findings show, for the first time, that HO-1 functionally interplays with the multifunctional transcription factor YY1 and that this interplay explains some of the protective activities of HO-1.

Stress-induced premature senescence mediated by a novel gene, SENEX, results in an anti-inflammatory phenotype in endothelial cells.

Cellular senescence is a mechanism to inhibit the growth of mammalian cells after oncogenic activation, or in response to damage or stress. We describe here the identification of a novel gene, SENEX, that regulates stress induced premature senescence pathways in endothelial cells (ECs) involving p16(INK4a) and retinoblastoma protein activation. Endogenous levels of SENEX remain unchanged during replicative senescence but are regulated by H(2)O(2)-mediated stress. In contrast to that previously described for senescence in other cell types, the SENEX induced senescent ECs are profoundly anti-inflammatory. The cells are resistant to tumor necrosis factor (TNF)α-induced apoptosis, adhesion of neutrophils and mononuclear cells, and the surface (but not cytoplasmic) expression of endothelial leukocyte adhesion molecule 1 and vascular cell adhesion molecule 1. Furthermore they are resistant to thrombin induced vascular leak. Senescent ECs such as those lining atherosclerotic lesions may therefore function to limit the inflammatory response. SENEX is also essential for EC survival since depletion either ectopically by siRNA or by high- dose H(2)O(2) treatment causes apoptosis. Together, these findings expand our understanding of the role of senescence in the vasculature and identify SENEX as a fulcrum for driving the resultant phenotype of the endothelium after activation.

Heme oxygenase-1 increases endothelial progenitor cells.

OBJECTIVE: Induction of heme oxygenase-1 (HO-1) protects against atherosclerotic disease in part by promoting reendothelialization. As endothelial progenitor cells (EPCs) contribute to reendothelialization, we examined the role of HO-1 on bone marrow and circulating EPCs. METHODS AND RESULTS: In a rabbit model of aortic balloon injury, pharmacological induction of HO-1 enhanced reendothelialization at sites with and without adjacent blood vessels, the latter indicative of a contribution by EPCs. Coinciding with maximal HO-1 induction in the injured vessel, plasma concentrations of bilirubin and the numbers of circulating progenitor cells were elevated. Both processes were abolished by cotreatment of the animals with an inhibitor of HO-1. Inducers of HO-1 promoted bone marrow cells to form progenitor cell colonies, and Flk1(+)/Sca-1(+)-cells to adhere to the luminal surface of the injured vessel. In noninjured mice, HO-1 inducers also increased bone marrow and circulating EPCs, and the ability of these cells to differentiate and form colonies. Compared to wild-type mice, bone marrow cells from HO-1(-/-) mice generated fewer endothelial colony-forming cells, and HO-1 inducers failed to promote CFU-Hill colony formation. CONCLUSIONS: These findings suggest that HO-1 contributes to vascular repair by increasing circulating EPCs derived from the bone marrow.

Pharmacologic induction of heme oxygenase-1.

Heme oxygenase-1 (HO-1) is a cytoprotective protein whose expression is consistently associated with therapeutic benefits in a number of pathologic conditions such as atherosclerotic vascular disease and inflammation. Although the expression of HO-1 in most tissues is low, a large number of clinical and experimental pharmacologic compounds have been demonstrated to induce HO-1. This induction is suggested to be at least partially responsible for the perceived therapeutic efficacy of these compounds. The increase in HO-1 expression in response to these compounds is the result of a complex regulatory network involving many signaling pathways and transcription factors. Understanding both the pathways by which HO-1 is induced and the mechanism through which the enzyme exerts its beneficial effects may facilitate the development of novel drugs.

Probucol [4,4'-[(1-methylethylidene)bis(thio)]bis-[2,6-bis(1,1-dimethylethyl)phenol]] inhibits compensatory remodeling and promotes lumen loss associated with atherosclerosis in apolipoprotein E-deficient mice.

Probucol [4,4'-[(1-methylethylidene)bis(thio)]bis-[2,6-bis(1,1-dimethylethyl)phenol]] was withdrawn from the United States market because it failed to inhibit atherosclerosis in human femoral arteries, yet the drug was shown subsequently to inhibit atherosclerosis in human carotid arteries, and probucol monosuccinate ester is presently being tested in a phase III clinical trial as an antiatherosclerotic compound based on its anti-inflammatory properties. Inflammatory macrophages are implicated in arterial remodeling associated with atherosclerosis, and probucol inhibits experimental atherosclerosis in part by decreasing macrophages in lesions. However, the impact of probucol on remodeling is unknown, although such knowledge could help explain why the drug's benefit on human atherosclerosis is controversial. We therefore examined the effect of probucol on remodeling of the common carotid artery in apolipoprotein E-deficient mice. We observed that during de novo atherosclerosis, plaque growth was fully compensated by expansive remodeling, such that lumen area was unaffected. Early lesions were composed almost entirely of macrophages, and their contribution to lesion area progressively decreased thereafter. Probucol significantly decreased plaque area, expression of vascular cell adhesion molecule-1, and proliferation of intimal cells, resulting in delayed macrophage accumulation in the vessel. Probucol also decreased the production and activity of matrix metalloproteinases-2 and -9, independent of the plasmin protease system, and this was associated with an inhibition of expansive remodeling, resulting in lumen loss. These studies show that probucol attenuates compensatory remodeling associated with de novo atherosclerosis, probably via its anti-inflammatory properties. Our findings suggest that lumen volume is not a suitable surrogate to assess the antiatherosclerotic activity of probucol and related drugs.

Antioxidants protect from atherosclerosis by a heme oxygenase-1 pathway that is independent of free radical scavenging.

Oxidative stress is implicated in atherogenesis, yet most clinical trials with antioxidants, particularly vitamin E, have failed to protect against atherosclerotic diseases. A striking exception is probucol, which retards atherosclerosis in carotid arteries and restenosis of coronary arteries after angioplasty. Because probucol has in vitro cellular-protective effects independent of inhibiting lipid oxidation, we investigated the mode of action of probucol in vivo. We used three models of vascular disease: apolipoprotein E-deficient mice, a model of atherosclerosis; rabbit aortic balloon injury, a model of restenosis; and carotid injury in obese Zucker rats, a model of type 2 diabetes. Unexpectedly, we observed that the phenol moieties of probucol were insufficient, whereas its sulphur atoms were required for protection. Probucol and its sulphur-containing metabolite, but not a sulphur-free phenolic analogue, protected via cell-specific effects on inhibiting macrophage accumulation, stimulating reendothelialization, and inhibiting vascular smooth muscle cell proliferation. These processes were mediated via induction of heme oxygenase-1 (HO-1), an activity not shared by vitamin E. Our findings identify HO-1 as the molecular target of probucol. They indicate 2-electron rather than radical (1-electron) oxidants as important contributors to atherogenesis, and point to novel lead compounds for therapeutic intervention against atherosclerotic diseases.

Alternate recruitment of signal recognition particle and trigger factor to the signal sequence of a growing nascent polypeptide.

Different from cytoplasmic membrane proteins, presecretory proteins of bacteria usually do not require the signal recognition particle for targeting to the Sec translocon. Nevertheless signal sequences of presecretory proteins have been found in close proximity to signal recognition particle immediately after they have emerged from the ribosome. We show here that at the ribosome, the molecular environment of a signal sequence depends on the nature of downstream sequence elements that can cause an alternate recruitment of signal recognition particle and the ribosome-associated chaperone Trigger factor to a growing nascent chain. While signal recognition particle and Trigger factor might remain bound to the same ribosome, both ligands are clearly able to displace each other from a nascent chain. The data also imply that a signal sequence owes its molecular environment to the fact that it remains closely apposed to the ribosomal exit site during growth of a nascent secretory protein.

Processes involved in the site-specific effect of probucol on atherosclerosis in apolipoprotein E gene knockout mice.

OBJECTIVE: To elucidate processes by which the antioxidant probucol increases lesion size at the aortic sinus and decreases atherosclerosis at more distal sites in apolipoprotein E-deficient (apoE(-/-)) mice. METHODS AND RESULTS: Male apoE(-/-) mice were fed high-fat chow with 1% (w/w) probucol or without (controls) for 6 months, before aortic sinus, arch, and descending aorta were analyzed separately for lesion size and composition. Compared with control, probucol significantly increased lesion size by 33% at the sinus, but it inhibited atherosclerosis at the descending aorta by 94%. Sites where atherosclerosis was inhibited contained substantially fewer macrophages, less lipids (cholesterol and cholesteryl esters), and endogenous antioxidant (alpha-tocopherol), but not oxidized lipids, and the extent to which probucol metabolism occurred was increased. Compared with control, aortic sinus lesions of probucol mice contained a substantially increased content of extracellular matrix, but decreased total cell and macrophage density, comparable levels of lipids and alpha-tocopherol, and decreased concentrations of oxidized lipids (cholesteryl ester hydroperoxides, F2-isoprostanes, and 7-ketocholesterol). CONCLUSIONS: Probucol affects atherosclerosis in apoE(-/-) mice independent of the accumulation of arterial lipid oxidation products, thereby dissociating the 2 processes. Rather, probucol exerts antiinflammatory activity by decreasing accumulation of macrophages in lesions, and it promotes a more stable lesion composition at the aortic sinus.

The tissue distribution of murine Abcc6 (Mrp6) during embryogenesis indicates that the presence of Abcc6 in elastic tissues is not required for elastic fiber assembly.

Mutations in the gene coding for the ABC transporter, ABCC6, in humans cause Pseudoxanthoma elasticum, which is characterized by the deposition of aberrant elastic fibers. To investigate whether the presence of ABCC6 in tissues synthesizing elastin is required for elastin deposition and elastic fiber assembly, we have compared the steady-state levels and tissue distribution of Abcc6 and tropoelastin mRNAs during mouse embryogenesis. Whereas tropoelastin mRNA levels rose during embryogenesis and were the highest in neonatal mice, Abcc6 mRNA levels remained constantly low throughout embryogenesis. In some tissues, both Abcc6 and tropoelastin mRNA were detected. However, Abcc6 mRNA and protein were not detected in neonatal aorta and arteries, which produce large amounts of elastin indicating that the presence of Abcc6 in elastic tissues is not required for elastic fiber assembly.

Analysis of ABCC6 (MRP6) in normal human tissues.

To determine the tissue distribution of the ABC transporter ABCC6 in normal human tissues, we analyzed tissue arrays for the presence of ABCC6 mRNA by in situ hybridization and ABCC6 protein by immunohistochemistry using the polyclonal antibody HB-6. We detected ABCC6 mRNA and protein in various epithelial cells of exocrine and endocrine tissues, such as acinar cells in the pancreas, mucosal cells of the intestine and follicular epithelial cells of the thyroid. We obtained a very strong immunostaining for enteroendocrine G cells in the stomach. In addition, ABCC6 mRNA and protein were present in most neurons of the brain, in alveolar macrophages in the lungs and lymphocytes in the lymph node. Immunohistochemisty using the monoclonal antibody M6II-31 confirmed the widespread tissue distribution of ABCC6. The physiological substrate(s) of ABCC6 are yet unknown, but we suggest that ABCC6 fulfills multiple functions in different tissues. The strong immunostaining for ABCC6 in G cells suggests that it plays an important role in these endocrine cells.

Ligand crowding at a nascent signal sequence.

We have systematically analyzed the molecular environment of the signal sequence of a growing secretory protein from Escherichia coli using a stage- and site-specific cross-linking approach. Immediately after emerging from the ribosome, the signal sequence of pOmpA is accessible to Ffh, the protein component of the bacterial signal recognition particle, and to SecA, but it remains attached to the surface of the ribosome via protein L23. These contacts are lost upon further growth of the nascent chain, which brings the signal sequence into sole proximity to the chaperone Trigger factor (TF). In its absence, nascent pOmpA shows extended contacts with L23, and even long chains interact in these conditions proficiently with Ffh. Our results suggest that upon emergence from the ribosome, the signal sequence of an E. coli secretory protein gradually becomes sequestered by TF. Although TF thereby might control the accessibility of pOmpA's signal sequence to Ffh and SecA, it does not influence interaction of pOmpA with SecB.

Acquired Pseudoxanthoma elasticum-like syndrome in beta-thalassaemia patients.

A significant number of patients diagnosed with beta-thalassaemia develop clinical and histopathological manifestations similar to those of an inherited disorder called Pseudoxanthoma elasticum (PXE). The inherited PXE is caused by mutations in the ATP-binding cassette, subfamily C (CFTR/MRP), member 6 (ABCC6) gene and is characterized by mineralized elastic fibres in dermal, vascular and ocular tissues. As no disease-causing variant was found in the ABCC6 gene of 10 beta-thalassaemia patients with a PXE-like phenotype, the present study suggests that the PXE-like symptoms in these beta-thalassaemic patients are not related to ABCC6 mutations.

The distribution of Abcc6 in normal mouse tissues suggests multiple functions for this ABC transporter.

We have studied the tissue distribution of Abcc6, a member of the ABC transmembrane transporter subfamily C, in normal C57BL/6 mice. RNase protection assays revealed that although almost all tissues studied contained detectable levels of the mRNA encoding Abcc6, the highest levels of Abcc6 mRNA were found in the liver. In situ hybridization (ISH) demonstrated abundant Abcc6 mRNA in epithelial cells from a variety of tissues, including hepatic parenchymal cells, bile duct epithelia, kidney proximal tubules, mucosa and gland cells of the stomach, intestine, and colon, squamous epithelium of the tongue, corneal epithelium of the eye, keratinocytes of the skin, and tracheal and bronchial epithelium. Furthermore, we detected Abcc6 mRNA in arterial endothelial cells, smooth muscle cells of the aorta and myocardium, in circulating leukocytes, lymphocytes in the thymus and lymph nodes, and in neurons of the brain, spinal cord, and the specialized neurons of the retina. Immunohistochemical analysis using a polyclonal Abcc6 rabbit antibody confirmed the tissue distribution of Abcc6 suggested by our ISH studies and revealed the cellular localization of Abcc6 in the basolateral plasma membrane in the epithelial cells of proximal convoluted tubules in the kidney. Although the function of Abcc6 is unknown, mutations in the human ABCC6 gene result in a heritable disorder of connective tissue called pseudoxanthoma elasticum (PXE). Our results demonstrating the presence of Abcc6 in epithelial and endothelial cells in a variety of tissues, including those tissues affected in PXE patients, suggest a possible role for Abcc6 in the normal assembly of extracellular matrix components. However, the presence of Abcc6 in neurons and leukocytes, two cell populations not associated with connective tissue, also suggests a more complex multifunctional role for Abcc6.

Evidence for a founder effect for pseudoxanthoma elasticum in the Afrikaner population of South Africa.

Pseudoxanthoma elasticum (PXE) is a heritable elastic tissue disorder recently shown to be attributable to mutations in the ABCC6 ( MRP6) gene. Whereas PXE has been identified in all ethnic groups studied to date, the prevalence of this disease in various populations is uncertain, although often assumed to be similar. A notable exception however is the prevalence of PXE among South African Afrikaners. A previous report has suggested that a founder effect may explain the higher prevalence of PXE in Afrikaners, a European-derived population that first settled in South Africa in the 17th century. To investigate this hypothesis, we performed haplotype and mutational analysis of DNA from 24 South African families of Afrikaner, British and Indian descent. Among the 17 Afrikaner families studied, three common haplotypes and six different disease-causing variants were identified. Three of these mutant alleles were missense variants, two were nonsense mutations and one was a single base-pair insertion. The most common variant accounted for 53% of the PXE alleles, whereas other mutant alleles appeared at lower frequencies ranging from 3% to 12%. Haplotype analysis of the Afrikaner families showed that the three most frequent mutations were identical-by-descent, indicating a founder origin of PXE in this population.