Bilirubin, the principal tetrapyrrole, bile pigment and catabolite of haem, is an emerging biomarker of disease resistance, which may be related to several recently documented biological functions. Initially believed to be toxic in infants, the perception of bilirubin has undergone a transformation: it is now considered to be a molecule that may promote health in adults. Data from the last decade demonstrate that mildly elevated serum bilirubin levels are strongly associated with reduced prevalence of chronic diseases, particularly cardiovascular diseases (CVDs), as well as CVD-related mortality and risk factors. Recent data also link bilirubin to other chronic diseases, including cancer and Type 2 diabetes mellitus, and to all-cause mortality. Therefore, there is evidence to suggest that bilirubin is a biomarker for reduced chronic disease prevalence and a predictor of all-cause mortality, which is of important clinical significance. In the present review, detailed information on the association between bilirubin and all-cause mortality, as well as the pathological conditions of CVD, cancer, diabetes and neurodegenerative diseases, is provided. The mechanistic background concerning how bilirubin and its metabolism may influence disease prevention and its clinical relevance is also discussed. Given that the search for novel biomarkers of these diseases, as well as for novel therapeutic modalities, is a key research objective for the near future, bilirubin represents a promising candidate, meeting the criteria of a biomarker, and should be considered more carefully in clinical practice as a molecule that might provide insights into disease resistance. Clearly, however, greater molecular insight is warranted to support and strengthen the conclusion that bilirubin can prevent disease, with future research directions also proposed.

A positive relationship between unconjugated bilirubin and free plasma haem, iron and carboxy haemoglobin indicated a positive feedback loop of haem oxygenase induction possibly mediated by unconjugated bilirubin. Furthermore, reduced body mass index in Gilbert's syndrome individuals was linked to reduced inflammation status, which could be influenced by circulating haem oxygenase catabolites and contribute to reduced risk of noncommunicable diseases in this population.

Energy metabolism, involving the ATP-dependent AMPK-PgC-Ppar pathway impacts metabolic health immensely, in that its impairment can lead to obesity, giving rise to disease. Based on observations that individuals with Gilbert’s syndrome (GS; UGT1A1*28 promoter mutation) are generally lighter, leaner and healthier than controls, specific inter-group differences in the AMPK pathway regulation were explored. Therefore, a case-control study involving 120 fasted, healthy, age- and gender matched subjects with/without GS, was conducted. By utilising intra-cellular flow cytometry (next to assessing AMPKα1 gene expression), levels of functioning proteins (phospho-AMPK α1/α2, PgC 1 α, Ppar α and γ) were measured in PBMCs (peripheral blood mononucleated cells). In GS individuals, rates of phospho-AMPK α1/α2, -Ppar α/γ and of PgC 1α were significantly higher, attesting to a boosted fasting response in this condition. In line with this finding, AMPKα1 gene expression was equal between the groups, possibly stressing the post-translational importance of boosted fasting effects in GS. In reflection of an apparently improved health status, GS individuals had significantly lower BMI, glucose, insulin, C-peptide and triglyceride levels. Herewith, we propose a new theory to explain why individuals having GS are leaner and healthier, and are therefore less likely to contract metabolic diseases or die prematurely thereof.

Immune privilege (IP), a term introduced to explain the unpredicted acceptance of allogeneic grafts by the eye and the brain, is considered a unique property of these tissues. However, immune responses are modified by the tissue in which they occur, most of which possess IP to some degree. The eye therefore displays a spectrum of IP because it comprises several tissues. IP as originally conceived can only apply to the retina as it contains few tissue-resident bone-marrow derived myeloid cells and is immunologically shielded by a sophisticated barrier – an inner vascular and an outer epithelial barrier at the retinal pigment epithelium. The vascular barrier comprises the vascular endothelium and the glia limitans. Immune cells do not cross the blood-retinal barrier (BRB) despite two-way transport of interstitial fluid, governed by tissue oncotic pressure. The BRB, and the blood-brain barrier (BBB) mature in the neonatal period under signals from the expanding microbiome and by 18 months are fully established. However, the adult eye is susceptible to intraocular inflammation (uveitis; frequency ~200/100,000 population). Uveitis involving the retinal parenchyma (posterior uveitis, PU) breaches IP, while IP is essentially irrelevant in inflammation involving the ocular chambers, uveal tract and ocular coats (anterior/intermediate uveitis/sclerouveitis, AU). Infections cause ~50% cases of AU and PU but infection may also underlie the pathogenesis of immune-mediated “non-infectious” uveitis. Dysbiosis accompanies the commonest form, HLA-B27–associated AU, while latent infections underlie BRB breakdown in PU. This review considers the pathogenesis of uveitis in the context of IP, infection, environment, and the microbiome.

The antioxidant properties of protoporphyrin IX and related tetrapyrroles are poorly characterized. Therefore, eight tetrapyrroles, five of which are produced in vivo, were tested to assess their antioxidant capacities in the Salmonella reverse mutation, TEAC, FRAP and ORAC assays. Tertiary-butyl hydroperoxide (tert-BOOH) in the presence or absence of metabolic activation (±S9) was added to Salmonella strain TA102 together with the test compounds. In the absence of metabolic activation, the order of effectiveness was protoporphyrin > biliverdin > bilirubin ditaurate > bilirubin > biliverdin dimethyl ester > stercobilin > bilirubin dimethyl ester > urobilin. In the presence of S9, the effectiveness was reversed, with urobilin > biliverdin dimethyl ester > bilirubin dimethyl ester > stercobilin > biliverdin > bilirubin > bilirubin ditaurate > protoporphyrin. In the antioxidant capacity assays FRAP, TEAC and ORAC, mainly bilirubin, bilirubin ditaurate, biliverdin and protoporphyrin showed antioxidant activity. This study reports that previously untested tetrapyrroles of related structure prevent oxidatively induced genotoxicity, and for some, novel underlying mechanisms of antioxidant action were revealed. These results support the physiological importance and biological relevance of tetrapyrroles including protoporphyrin that might act as antioxidants, protecting from oxidatively induced DNA damage, particularly in the tissues/organs where they accumulate.