Eugenio Mocchegiani

Zinc and immunoresistance to infection in aging: new biological tools

Infections can cause mortality when the immune system is damaged. The catalytic, structural (in zinc-finger proteins) and regulatory roles of zinc mean that this ion is involved in the maintenance of an effective immune response. Both zinc deficiency and impaired cell-mediated immunity combine during aging to result in increased susceptibility to infection. Dietary supplementation with the recommended daily allowance of zinc for between one and two months decreases the incidence of infection and increases the survival rate following infection in the elderly. This article reviews the biochemical pathways through which zinc might act to increase immunoresistance to infection in the elderly.

Benefit of oral zinc supplementation as an adjunct to zidovudine (AZT) therapy against opportunistic infections in aids

Abstract Zinc is perhaps the most important trace element for immune function. Congenital or acquired zinc deficiencies are associated with immune abnormalities and increased susceptibility to infectious diseases. AIDS subjects suffer from reduced zinc bioavailability, more severe in stage IV than in stage III. Such zinc deficiency causes, among other effects, a profound reduction in the biological activity of one of the thymic hormones, thymulin (zinc-facteur-timique-serique, ZnFTS). With these premises, zinc sulphate was administered orally at a daily dose of 200 mg for 30 days to AZT-treated stage III subjects with generalized lymphadenopathy (17 subjects) and stage IV subgroup C1 (12 subjects) AIDS patients. 18 stage III subjects with generalized lymphoadenopathy and 10 stage IV subgroup C1 subjects treated only with AZT served as controls. Zinc sulphate supplementation of stage III and in stage IV CI patients was followed by an increase or a stabilization in the body weight and an increase of the number of CD4+ cells and the plasma level of active zinc-bound thymulin. The frequency of opportunistic infectious episodes in the 24 months following entry into the study was reduced after zinc supplementation in stage IV C1 subjects (11 infections vs 25 in controls) and delayed in stage III zinc-treated subjects (1 infection/24 months vs 13 infections/24 months in controls). The effect of zinc on opportunistic infections is restricted to infections due to Pneumocystis carinii and Candida, whereas no variations have been observed in the frequencies of cytomegalovirus and toxoplasma infections. These data may support the benefit of zinc as an adjunct to AZT therapy in AIDS pathology.

Zinc, human diseases and aging

Zinc is one of the most important trace elements in the body for many biological functions; it is required as a catalytic component for more than 200 enzymes, and as a structural constituent of many proteins, hormones, neuropeptides, hormone receptors, and probably polynucleotides. Due to its role in cell division and differentiation, programmed cell death, gene transcription, biomembrane functioning and obviously many enzymatic activities, zinc is considered a major element in assuring the correct functioning of an organism, from the very first embryonic stages to the last periods of life. This biological role together with the many factors that modulate zinc turnover explains on one hand, the variety of clinical and laboratory signs resulting from its reduced bioavailability, and on the other, the high number of human pathologies characterized by alterations in the zinc pool. As zinc supplementation is efficacious in most of these conditions, it is regarded more as an oriented therapeutical support, than a simple dietary integrator. Furthermore, the relevance of zinc status to many age- associated diseases and, according to experimental studies, the aging itself of the major homeostatic mechanisms of the body, i.e., the nervous, neuroendocrine and immune systems, places zinc in a pivotal position in the economy of the aging organism. (Aging Clin. Exp. Res. 7: 77–93, 1995)

Zinc, metallothioneins, immune responses, survival and ageing.

Zinc is required as a catalytic, structural (zincfingers) and regulatory ion. In this capacity, it isinvolved in many homeostatic mechanisms, includingimmune responses. Metallothioneins (MTs) may play keyroles because of their preferential binding to zincespecially in ageing. MTs protect from oxidativedamage during transient stress conditions atyoung-adult age. This protection no longer exists inageing and in age-related diseases (cancer andinfections) because the stress condition is constant.As such, MTs may constantly deplete zinc from plasmaand tissues. This phenomenon causes increased MTslevels on the one hand, but on the other hand induceslow zinc ion bioavailability for normal immuneresponses. This may be particularly relevant forthymic functions and natural killer activity.Therefore MTs which are protective in young-adults maybecome dangerous in immune responses during ageing.Physiological supplementation of zinc in ageingcorrects central and peripheral immune defects,resulting prolonged survival and decreased mortality (50%) frominfections and tumours, especially during middle age.Because of increased MT gene expression and proteinlevels in the liver and atrophic thymus of old mice,MTs are proposed as genetic markers of immunosenescence.

Zinc dyshomeostasis: a key modulator of neuronal injury.

Zn2+ is a potently toxic cation involved in the neuronal injury observed in cerebral ischemia, epilepsy, and brain trauma. Toxic Zn2+ accumulation may result from either trans-synaptic Zn2+movement and/or cation mobilization from intracellular sites. To gain entry to the cytosol, Zn2+ can flux through glutamate receptor-associated channels, voltage-sensitive calcium channels, or Zn2+-sensitive membrane transporters, while metallothioneins and mitochondria provide sites of intracellular Zn2+ release. Intracellular Zn2+ homeostasis is sensitive to patho-physiological environmental changes, such as acidosis, inflammation and oxidative stress. The mechanisms by which Zn2+ exerts its neurotoxicity include mitochondrial and extra-mitochondrial production of reactive oxygen species and disruption of metabolic enzymatic activity, ultimately leading to activation of apoptotic and/or necrotic processes. Beside acute neuronal injury, an exciting new area of investigation is offered by the role of Zn2+ dysmetabolism in Alzheimers disease as the cation acts as a potent trigger for Abeta aggregation and plaque formation. Finally, recent findings suggest that alteration of Zn2+ homeostasis might also be a critical contributor to aging-related neurodegenerative processes. Thus, multiple evidence suggest that modulation of intracellular and extracellular Zn2+ might be an important therapeutical target for the treatment of a vast array of neurological conditions ranging from stroke to Alzheimers disease.

Correlation between zinc status and immune function in the elderly

Zinc is essential for the immune system and elderly people have an increased probability for zinc deficiency, documented by a decline of serum or plasma zinc levels with age. Although most healthy elderly are not classified as clinically zinc deficient, even marginal zinc deprivation can affect immune function. Several striking similarities in the immunological changes during aging and zinc deficiency, including a reduction in the activity of the thymus and thymic hormones, a shift of the T helper cell balance towards TH2, decreased response to vaccination, and impaired functions of innae immune cells indicate that a wide prevalence of marginal zinc deficiency in elderly people may contribute to immunosenescence. Studies with oral zinc supplementation show the potential to improve the immune response of elderly people by restoration of the zinc levels, showing that balancing the zinc status may be a way to healthy aging. This review summarizes the current literature about zinc supplementation in the elderly and thereby defines the rationale for the immunological part of the ZINCAGE project.

Plasticity of neuroendocrine-thymus interactions during aging.

Thymic regrowth and reactivation of thymic endocrine activity may be achieved even in old animals by different endocrinological or nutritional manipulations such as, (a) intrathymic transplantation of pineal gland or treatment with melatonin, (b) implantation of a growth hormone (GH) secreting tumor cell line or treatment with exogenous GH, (c) castration or treatment with exogenous luteinizing hormone-releasing hormone (LH-RH), (d) treatment with exogenous thyroxine or triiodothyronine, and (e) nutritional interventions such as arginine or zinc supplementation. These data strongly suggest that thymic, involution is a phenomenon secondary to age-related alterations in neuroendocrine-thymus interactions and that it is the disruption of such interactions in old age that is responsible for age-associated dysfunction. With regard to the mechanisms involved in hormone-induced thymic reconstitution, it is at present, difficult to draw any definitive conclusions. The effect of GH, thyroid hormones, and LH-RH may be due to the presence on thymic epithelial cells supposed to produce thymic peptides, of the specific hormone receptors. Melatonin or other pineal factors may also act through specific receptors, but experimental evidence is still lacking. The role of zinc, whose turnover is usually reduced in old age, is diverse. The effects range from the reactivation of zinc-dependent enzymes, required for both cell proliferation and apoptosis, to the reactivation of thymulin, a zinc-dependent thymic hormone. The role of zinc may even be more crucial. According to recent preliminary data obtained both in animal and human studies, it appears that the above reported endocrinological manipulations capable of restoring thymic activity in old age, may act also by normalizing the altered zinc pool.

Zinc, infections and immunosenescence

Abstract Infections may cause mortality in old age due to damaged immune responses. As zinc is required as a catalyst, structural (zinc fingers) and regulatory ion, it is involved in many biological functions, including immune responses. Low zinc ion bioavailability and impaired cell-mediated immunity are common in ageing and may be restored by physiological supplementation with zinc for 1–2 months, impacting upon morbidity and survival. This article reviews the role of zinc in immune efficacy during ageing, and also describes the main biochemical pathways involved in the role of zinc in resistance to infections in ageing in order to better understand the possible causes of immunosenescence.

Zinc-bound metallothioneins as potential biological markers of ageing

Metallothioneins (MTs) (I+II) play pivotal roles in metal-related cell homeostasis because of their high affinity for metals forming clusters. The main functional role of MTs is to sequester and/or dispense zinc participating in zinc homeostasis. Consistent with this role, MT gene expression is transcriptionally induced by a variety of stressing agents to protect cells from reactive oxygen species. In order to accomplish this task, MTs induce the secretion of pro-inflammatory cytokines by immune and brain cells, such as astrocytes, for a prompt response against oxidative stress. These cytokines are in turn involved in new synthesis of MTs in the liver and brain. Such protective mechanism occurs in the young-adult age, when stresses are transient. Stress-like condition is instead constant in the old age, and this causes continuous stealing of intracellular zinc by MTs and consequent low bioavailability of zinc ions for immune, endocrine, and cerebral functions. Therefore, a protective role of zinc-bound MTs (I+II) during ageing can be questioned. Because free zinc ions are required for optimal efficiency of the immune-endocrine-nervous network, zinc-bound MTs (I+II) may play a different role during ageing, switching from a protective to a deleterious one in immune, endocrine, and cerebral activities. Physiological zinc supply, performed cautiously, can correct deficiencies in the immune-neuroendocrine network and can improve cognitive performances during ageing and accelerated ageing. Altogether these data indicate that zinc-bound MTs (I+II) can be considered as novel potential markers of ageing.

Melatonin administration in tumor-bearing mice (intact and pinealectomized) in relation to stress, zinc, thymulin and IL-2.

Melatonin (MEL) may counteract tumors through a direct oncostatic role. MEL is also an antistress agent with immunoenhancing properties against tumors due to a suppressive role of MEL on corticosterone release. Rotational stress (RS) (spatial disorientation) facilitates metastasis progression in mice. Also, MEL counteracts tumors because of its influence on immune responses via the metabolic zinc pool, which, is reduced in tumors and stress. Zinc is required for normal thymic endocrine activity (i.e. thymulin) and interleukin-2 (IL-2) production. Because in vivo data is still controversial, exogenous MEL treatment (22 days in drinking water) in both intact and pinealectomized (px) mice bearing Lewis lung carcinoma leads to significant decrements of metastasis volume, restoration of the negative crude zinc balance, recovery of thymulin activity and increment of IL-2 exclusively in intact and px tumor bearing mice subjected to RS. Significant inverse correlations are found in both stressed intact and px tumor bearing mice after MEL treatment between zinc and corticosterone (r = 0.78, P < 0.01; r = 0.80, P < 0.01, respectively). Positive correlations between zinc and IL-2 (r = 0.75, P < 0.01; r = 0.73, P < 0.01, respectively) or thymulin (r = 0.75, P < 0.01; r = 0.82, P < 0.01, respectively) are observed in same models of mice. These findings suggest a MEL action to decrease metastasis mediated by a possible interplay between zinc and MEL, via corticosterone, with consequent restoration of thymic efficiency and IL-2 production. MEL as an antistress agent with immunoenhancing properties in cancer deserves further consideration.nuclear factor-kb; POMC, proopiomelanocortin; Px, pinealectomized mice; RIA, radioimmunoassay; RS, rotational stress; SDI, stressed intact mice; SDPx, stressed pinealectomized mice; TNF-alpha, tumor necrosis factor-alpha; ZnFTS, active zinc-bound thymulin; ZnFTS + FTS, total thymulin.