Total synthesis and detection of the bilirubin oxidation product (Z)-2-(3-ethenyl-4-methyl-5-oxo-1,5-dihydro-2H-pyrrol-2-ylidene)ethanamide (Z-BOX A)

All Three Supersystems-Nervous, Vascular, and Immune-Contribute to the Cortical Infarcts After Subarachnoid Hemorrhage

The recently published DISCHARGE-1 trial supports the observations of earlier autopsy and neuroimaging studies that almost 70% of all focal brain damage after aneurysmal subarachnoid hemorrhage are anemic infarcts of the cortex, often also affecting the white matter immediately below. The infarcts are not limited by the usual vascular territories. About two-fifths of the ischemic damage occurs within ~ 48 h; the remaining three-fifths are delayed (within ~ 3 weeks). Using neuromonitoring technology in combination with longitudinal neuroimaging, the entire sequence of both early and delayed cortical infarct development after subarachnoid hemorrhage has recently been recorded in patients. Characteristically, cortical infarcts are caused by acute severe vasospastic events, so-called spreading ischemia, triggered by spontaneously occurring spreading depolarization. In locations where a spreading depolarization passes through, cerebral blood flow can drastically drop within a few seconds and remain suppressed for minutes or even hours, often followed by high-amplitude, sustained hyperemia. In spreading depolarization, neurons lead the event, and the other cells of the neurovascular unit (endothelium, vascular smooth muscle, pericytes, astrocytes, microglia, oligodendrocytes) follow. However, dysregulation in cells of all three supersystems-nervous, vascular, and immune-is very likely involved in the dysfunction of the neurovascular unit underlying spreading ischemia. It is assumed that subarachnoid blood, which lies directly on the cortex and enters the parenchyma via glymphatic channels, triggers these dysregulations. This review discusses the neuroglial, neurovascular, and neuroimmunological dysregulations in the context of spreading depolarization and spreading ischemia as critical elements in the pathogenesis of cortical infarcts after subarachnoid hemorrhage.

A Platform for the Synthesis of Oxidation Products of Bilirubin

Bilirubin is the principal product of heme catabolism. High concentrations of the pigment are neurotoxic, yet slightly elevated levels are beneficial. Being a potent antioxidant, oxidative transformations of bilirubin occur in vivo and lead to various oxidized fragments. The mechanisms of their formation, intrinsic biological activities, and potential roles in human pathophysiology are poorly understood. Degradation methods have been used to obtain samples of bilirubin oxidation products for research. Here, we report a complementary, fully synthetic method of preparation. Our strategy leverages repeating substitution patterns in the parent tetracyclic pigment. Functionalized ready-to-couple γ-lactone, γ-lactam, and pyrrole monocyclic building blocks were designed and efficiently synthesized. Subsequent modular combinations, supported by metal-catalyzed borylation and cross-coupling chemistries, translated into the concise assembly of the structurally diverse bilirubin oxidation products (BOXes, propentdyopents, and biopyrrins). The discovery of a new photoisomer of biopyrrin A named lumipyrrin is reported. Synthetic bilirubin oxidation products made available in sufficient purity and quantity will support future in vitro and in vivo investigations.

Bilirubin oxidation end product B prevents CoCl2-induced primary cortical neuron apoptosis by promoting cell survival Akt/mTOR/p70S6K signaling pathway

Bilirubin oxidation end products (BOXes) are associated with the late-developing neurological deficits after subarachnoid hemorrhage (SAH) possibly by direct constricting the cerebral arteries, but their specific impacts on neurons especially in the state of hypoxia, a prominent feature during the late stage of SAH, remain unclear. Here, we explored the effects of BOXes on the primary cortical neurons subjected to CoCl₂-induced hypoxia by evaluating the morphological and apoptotic changes of neurons. The present study showed that Z-BOX B but not Z-BOX A greatly alleviated CoCl₂-induced neuronal cell deterioration and apoptosis. Immunocytochemical staining assay showed Z-BOX B significantly increased neurite length, the numbers of both secondary and tertiary branches, and the protein level of Synaptophysin. Caspase 3/7 apoptosis assay and DAPI staining showed that Z-BOX B markedly reduced primary cortical neurons apoptosis. The expression of cleaved Caspase-3 was suppressed by Z-BOX B treatment, while the expression of Bcl-xL was upregulated. To further discover the mechanism of the neuroprotective effect observed in Z-BOX B, we found Z-BOX B increased the expression of p-mTOR, p-Akt, and p-p70S6K. In general, our results implicated Z-BOX B may prevent CoCl₂-induced primary cortical neurons apoptosis by activating sAkt/mTOR/p70S6K signaling pathway. Hence, the present data may provide new insights into the pathophysiological mechanism of delayed neurological dysfunction after SAH and novel targets for treating SAH.

Bilirubin Oxidation End Products (BOXes) Induce Neuronal Oxidative Stress Involving the Nrf2 Pathway

Delayed ischemic neurological deficit (DIND) is a severe complication after subarachnoid hemorrhage (SAH). Previous studies have suggested that bilirubin oxidation end products (BOXes) are probably associated with the DIND after SAH, but there is a lack of direct evidence yet even on cellular levels. In the present study, we aim to explore the potential role of BOXes and the involved mechanisms in neuronal function. We synthesized high-purity (>97%) BOX A and BOX B isomers. The pharmacokinetics showed they are permeable to the blood-brain barrier. Exposure of a moderate concentration (10 or 30 μM) of BOX A or BOX B to isolated primary cortical neurons increased the production of reactive oxygen species. In the human neuroblastoma SH-SY5Y cells, BOX A and BOX B decreased the mitochondrial membrane potential and enhanced nuclear accumulation of the protein Nrf2 implicated in oxidative injury repair. In addition, both chemicals increased the mRNA and protein expression levels of multiple antioxidant response genes including Hmox1, Gsta3, Blvrb, Gclm, and Srxn1, indicating that the antioxidant response element (ARE) transcriptional cascade driven by Nrf2 is activated. In conclusion, we demonstrated that primary cortical neurons and neuroblastoma cells undergo an adaptive response against BOX A- and BOX B-mediated oxidative stress by activation of multiple antioxidant responses, in part through the Nrf2 pathway, which provides in-depth insights into the pathophysiological mechanism of DIND after SAH or other neurological dysfunctions related to cerebral hemorrhage.

The Effects of Bilirubin and Lumirubin on Metabolic and Oxidative Stress Markers

For severe unconjugated hyperbilirubinemia the gold standard treatment is phototherapy with blue-green light, producing more polar photo-oxidation products, believed to be non-toxic. The aim of the present study was to compare the effects of bilirubin (BR) and lumirubin (LR), the major BR photo-oxidation product, on metabolic and oxidative stress markers. The biological activities of these pigments were investigated on several human and murine cell lines, with the focus on mitochondrial respiration, substrate metabolism, reactive oxygen species production, and the overall effects on cell viability. Compared to BR, LR was found to be much less toxic, while still maintaining a similar antioxidant capacity in the serum as well as suppressing activity leading to mitochondrial superoxide production. Nevertheless, due to its lower lipophilicity, LR was less efficient in preventing lipoperoxidation. The cytotoxicity of BR was affected by the cellular glycolytic reserve, most compromised in human hepatoblastoma HepG2 cells. The observed effects were correlated with changes in the production of tricarboxylic acid cycle metabolites. Both BR and LR modulated expression of PPARα downstream effectors involved in lipid and glucose metabolism. Proinflammatory effects of BR, evidenced by increased expression of TNFα upon exposure to bacterial lipopolysaccharide, were observed in murine macrophage-like RAW 264.7 cells. Collectively, these data point to the biological effects of BR and its photo-oxidation products, which might have clinical relevance in phototherapy-treated hyperbilirubinemic neonates and adult patients.

Photoisomerization Neutralizes Vasoconstrictive Activity of a Heme Degradation Product

Delayed cerebral ischemia (DCI) caused by cerebral vasospasm is the leading determinant of poor outcome and mortality in subarachnoid hemorrhage (SAH) patients, but current treatment options lack effective prevention and therapy. Two substance families of heme degradation products (HDPs), bilirubin oxidation end products (BOXes) and propentdyopents (PDPs), are elicitors of pathologic cerebral hypoperfusion after SAH. Z-configured HDPs can be photoconverted into the corresponding E-isomers. We hypothesize that photoconversion is a detoxification mechanism to prevent and treat DCI. We irradiated purified Z-BOXes and Z-PDPs with UV/Vis light and documented the Z-E photoconversion. E-BOX A slowly reisomerizes to the thermodynamically favored Z-configuration in protein-containing media. In contrast to vasoconstrictive Z-BOX A, E-BOX A does not cause vasoconstriction in cerebral arterioles in vitro and in vivo in wild-type mice. Our results enable a critical assessment of light-induced intrathecal photoconversion and suggest the use of phototherapy to prevent and cure HDP-mediated cerebral vasospasms.

Pyrrolic and Dipyrrolic Chlorophyll Degradation Products in Plants and Herbivores

The degradation of chlorophyll, the omnipresent green pigment, has been investigated intensively over the last 30 years resulting in many elucidated tetrapyrrolic degradation products. With a comparison to the degradation of the structurally similar heme, we hereby propose a novel additional chlorophyll degradation mechanism to mono- and dipyrrolic products. This is the first proof of the occurrence of a family of mono- and dipyrrols in leaves that are previously only known as heme degradation products. This product family is also found in spit and feces of herbivores with specific metabolomic patterns reflecting the origin of the samples. Based on chromatographic and mass spectrometric evidence as well as on mechanistic considerations we also suggest several tentative new degradation products. One of them, dihydro BOX A, was fully confirmed as a novel natural product by synthesis and comparison of its spectroscopic data.

Pseudosymmetric layers in high-Z′ and P1 structures of organic molecules

Layers having obvious approximate symmetry higher than that of the overall 3-D crystal are present in 20–25% of the Z′ > 4 and P1 organic structures archived in the Cambridge Structural Database. In some structures different types of layers alternate.

BOX A-type monopyrrolic heterocycles modified via the Suzuki-Miyaura cross-coupling reaction

The in vivo oxidation of heme yields bilirubin which is further degraded to the bilirubin oxidation end products (BOXes) that are biologically highly active. To study the mode of action and fate of (Z)-2-(4-methyl-5-oxo-3-vinyl-1,5-dihydro-2H-pyrrol-2-ylidene)acetamide (BOX A), the Suzuki-Miyaura cross-coupling reaction allows to introduce various alkenyl- and aryl-substituents in 3-position of the (Z)-2-(4-methyl-5-oxo-1,5-dihydro-2H-pyrrol-2-ylidene)acetamides (BOX A-type monopyrroles). The influence of these groups on structural and NMR-spectroscopic parameters of the central monopyrrolic system is negligible. Special focus has been given to derivatives with 3-positioned aryl substituents carrying trifluoromethyl groups for future in vivo 19F NMR studies.

Total synthesis, structural revision and biological evaluation of γ-elemene-type sesquiterpenes

Total synthesis and absolute configuration confirmation of γ-elemene-type sesquiterpenes, which possess vast potential for biological activities, was investigated based on a convergent synthetic strategy. A key intermediate with all functional groups of this family of natural products was accessed by an intermolecular aldol reaction and then an acetylation of a known ketone (12) derived from commercially available verbenone. The versatile intermediate can be easily transformed into structurally different γ-elemene-type sesquiterpenes based on control of base-promoted cyclization manipulation in different solvents. The utility of this robust approach is illustrated by the first syntheses of elema-1,3,7(11),8-tetraen-8,12-lactam (4') and 8β-methoxy-isogermafurenolide (6a), as well as the syntheses of elem-1,3,7,8-tetraen-8,12-olide (3) and hydroxyisogermafurenolide (5) in only 6 or 7 steps. In addition, the structure of the reported 5βH-elem-1,3,7,8-tetraen-8,12-olide (1) was revised as elem-1,3,7,8-tetraen-8,12-olide (3) by comparison of their identified datum, and the absolute configuration of elema-1,3,7(11),8-tetraen-8,12-lactam was confirmed as 4'. Furthermore, the inhibitory effect of all synthesized natural compounds and their natural analogues on cancer cell proliferation was evaluated. Among them compounds 3, 4 and 4' were found to possess potent inhibitory activity against Kasumi-1 and Pfeiffer. Meanwhile, preliminary structure-activity relationships for these compounds are discussed.

Total syntheses of the bilirubin oxidation end product Z-BOX C and its isomeric form Z-BOX D

Oxidative degradation products of bilirubin (BOXes) are biologically highly active and certain BOXes cause long-lasting narrowing of cerebral blood vessels presumably with a significant role in subarachnoid hemorrhage. Due to the fact that mode of action as well as fate of these BOXes is widely unknown, larger amounts of these bilirubin degradation end products are required. The total synthesis of colorless (Z)-3-(5-(2-amino-2-oxoethylidene)-4-methyl-2-oxo-2,5-dihydro-1H-pyrrol-3-yl)propanoic acid (BOX C) succeeds via a seven-step procedure with a total yield of 20%. Its isomeric form (Z)-3-(2-(2-amino-2-oxoethylidene)-4-methyl-5-oxo-2,5-dihydro-1H-pyrrol-3-yl)propanoic acid (BOX D) can be prepared via a five-step protocol with a yield of 30%. NMR and crystallographic studies reveal that charge delocalization within the conjugated π-systems of BOXes C and D is negligible. Exposure of solutions of Z-BOX C and Z-BOX D to bright sunlight leads to Z/E-isomerization and mixtures of the respective E/Z-BOXes C and D.

UV light absorption parameters of the pathobiologically implicated bilirubin oxidation products, MVM, BOX A, and BOX B

The formation of the bilirubin oxidation products (BOXes), BOX A ([4-methyl-5-oxo-3-vinyl-(1,5-dihydropyrrol-2-ylidene)acetamide]) and BOX B (3-methyl-5-oxo-4-vinyl-(1,5-dihydropyrrol-2-ylidene)acetamide), as well as MVM (4-methyl-3-vinylmaleimide) were synthesized by oxidation of bilirubin with H₂O₂ without and with FeCl₃, respectively. Compound identity was confirmed with NMR and mass spectrometry (MS; less than 1 ppm, tandem MS up to MS⁴). UV absorption profiles, including λ_max, and extinction coefficient (ε; estimated using NMR) for BOX A, BOX B, and MVM in H₂O, 15% CH₃CN plus 10 mM CF₃CO₂H, CH₃CN, CHCl₃, CH₂Cl₂, and 0.9% NaCl were determined. At longer wavelengths, λ_max's for 1) BOX A were little affected by the solvent, ranging from 295-297 nm; 2) BOX B, less polar solvent yielded λ_max's of lower wavelength, with values ranging from 308-313 nm, and 3) MVM, less polar solvent yielded λ_max's of higher wavelength, with values ranging from 318-327 nm. Estimated ε's for BOX A and BOX B were approximately 5- to 10-fold greater than for MVM.

Neuro-inflammatory effects of photodegradative products of bilirubin

Phototherapy was introduced in the early 1950’s, and is the primary treatment of severe neonatal jaundice or Crigler-Najjar syndrome. Nevertheless, the potential biological effects of the products generated from the photodegradation of bilirubin during phototherapy remain unknown. This is very relevant in light of recent clinical observations demonstrating that the use of aggressive phototherapy can increase morbidity or even mortality, in extremely low birthweight (ELBW) infants. The aim of our study was to investigate the effects of bilirubin, lumirubin (LR, its major photo-oxidative product), and BOX A and B (its monopyrrolic oxidative products) on the central nervous system (CNS) using in vitro and ex vivo experimental models. The effects of bilirubin photoproducts on cell viability and expression of selected genes were tested in human fibroblasts, three human CNS cell lines (neuroblastoma SH-SY5Y, microglial HMC3, and glioblastoma U-87 cell lines), and organotypic rat hippocampal slices. Neither bilirubin nor its photo-oxidative products affected cell viability in any of our models. In contrast, LR in biologically-relevant concentrations (25 μM) significantly increased gene expression of several pro-inflammatory genes as well as production of TNF-α in organotypic rat hippocampal slices. These findings might underlie the adverse outcomes observed in ELBW infants undergoing aggressive phototherapy.

Bilirubin Oxidation Products and Cerebral Vasoconstriction

Key evidence in support of the hypothesis that bilirubin oxidation products (BOXes) contribute to the vasoconstriction associated with subarachnoid hemorrhage (SAH) are the (1) presence of BOXes in cerebral spinal fluid from SAH patients and (2) ability of one or more BOXes to elicit vasoconstriction. We critically evaluate this key evidence, detail where gaps remain, and describe recent approaches that will address these gaps.

Impact of higher-order heme degradation products on hepatic function and hemodynamics

BACKGROUND & AIMS

Biliverdin and bilirubin were previously considered end products of heme catabolism; now, however, there is evidence for further degradation to diverse bioactive products. Z-BOX A and Z-BOX B arise upon oxidation with unknown implications for hepatocellular function and integrity. We studied the impact of Z-BOX A and B on hepatic functions and explored their alterations in health and cholestatic conditions.

METHODS

Functional implications and mechanisms were investigated in rats, hepatocytic HepG2 and HepaRG cells, human immortalized hepatocytes, and isolated perfused livers. Z-BOX A and B were determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS) in acute and acute-on-chronic liver failure and hereditary unconjugated hyperbilirubinemia.

RESULTS

Z-BOX A and B are found in similar amounts in humans and rodents under physiological conditions. Serum concentrations increased ∼20-fold during cholestatic liver failure in humans (p<0.001) and in hereditary deficiency of bilirubin glucuronidation in rats (p<0.001). Pharmacokinetic studies revealed shorter serum half-life of Z-BOX A compared to its regio-isomer Z-BOX B (p=0.035). While both compounds were taken up by hepatocytes, Z-BOX A was enriched ∼100-fold and excreted in bile. Despite their reported vasoconstrictive properties in the brain vasculature, BOXes did not affect portal hemodynamics. Both Z-BOX A and B showed dose-dependent cytotoxicity, affected the glutathione redox state, and differentially modulated activity of Rev-erbα and Rev-erbβ. Moreover, BOXes-triggered remodeling of the hepatocellular cytoskeleton.

CONCLUSIONS

Our data provide evidence that higher-order heme degradation products, namely Z-BOX A and B, impair hepatocellular integrity and might mediate intra- and extrahepatic cytotoxic effects previously attributed to hyperbilirubinemia.

LAY SUMMARY

Degradation of the blood pigment heme yields the bile pigment bilirubin and the oxidation products Z-BOX A and Z-BOX B. Serum concentrations of these bioactive molecules increase in jaundice and can impair liver function and integrity. Amounts of Z-BOX A and Z-BOX B that are observed during liver failure in humans have profound effects on hepatic function when added to cultured liver cells or infused into healthy rats.

Simultaneous determination of the bilirubin oxidation end products Z-BOX A and Z-BOX B in human serum using liquid chromatography coupled to tandem mass spectrometry

Bilirubin oxidation end products (BOXes) appear upon endogenous heme degradation and can be found in the cerebrospinal fluid after hemorrhagic stroke. BOXes are assumed to contribute to delayed cerebral vasospasm and secondary loss of brain tissue. Here, we present a validated LC-ESI-MS/MS method for the sensitive determination of the regio-isomers Z-BOX A and Z-BOX B in human serum. We found that Z-BOX A and Z-BOX B appear in serum of healthy volunteers. The sample preparation includes the addition of 5-bromonicotinamide as internal standard and protein precipitation with acetonitrile. Baseline-separation was achieved on a C-18 column with a binary solvent gradient of formic acid in water/acetonitrile at 1 mL/min within a total analysis time of 17 min. Using single reaction monitoring in the positive ion mode, the linear working ranges were 2.74-163 pg/μL (Z-BOX A) and 2.12-162.4 pg/μL (Z-BOX B) with R(2)>0.995. Intra- and inter-day precisions were <10%. The inherent analyte concentrations of Z-BOX A (14.4 ± 5.1 nM) and Z-BOX B (10.9 ± 3.1 nM) in pooled human serum were determined by standard addition. The photolability of both analytes was demonstrated. This method enables to monitor Z-BOX A and Z-BOX B as a prerequisite to systematically study the biological significance of higher order metabolites of heme degradation.

Impact of heme and heme degradation products on vascular diameter in mouse visual cortex

BACKGROUND

Delayed cerebral vasospasm is the most common cause of mortality and severe neurological impairment in patients who survive subarachnoid hemorrhage. Despite improvements in the field of diagnostic imaging, options for prevention and medical treatment-primarily with the calcium channel antagonist nimodipine or hemodynamic manipulations-are insufficient. Previous studies have suggested that heme and bilirubin oxidation end products, originating from degraded hemoglobin around ruptured blood vessels, are involved in the development of vasospasm by inhibiting large conductance BKC a potassium channels in vascular smooth muscle cells. In this study, we identify individual heme degradation products regulating arteriolar diameter in dependence of BKC a channel activity.

METHODS AND RESULTS

Using differential interference contrast video microscopy in acute brain slices, we determined diameter changes of intracerebral arterioles in mouse visual cortex. In preconstricted vessels, the specific BKC a channel blockers paxilline and iberiotoxin as well as iron-containing hemin caused vasoconstriction. In addition, the bilirubin oxidation end product Z-BOX A showed a stronger vasoconstrictive potency than its regio-isomer Z-BOX B. Importantly, Z-BOX A had the same vasoconstrictive effect, independent of its origin from oxidative degradation or chemical synthesis. Finally, in slices of Slo1-deficient knockout mice, paxilline and Z-BOX A remained ineffective in changing arteriole diameter.

CONCLUSIONS

We identified individual components of the oxidative bilirubin degradation that led to vasoconstriction of cerebral arterioles. The vasoconstrictive effect of Z-BOX A and Z-BOX B was mediated by BKC a channel activity that might represent a signaling pathway in the occurrence of delayed cerebral vasospasm in subarachnoid hemorrhage patients.