XIAP is a potent suppressor of apoptosis that directly inhibits specific members of the caspase family of cysteine proteases. Here we demonstrate a novel role for XIAP in the control of intracellular copper levels. XIAP was found to interact with MURR1, a factor recently implicated in copper homeostasis. XIAP binds to MURR1 in a manner that is distinct from that utilized by XIAP to bind caspases, and consistent with this, MURR1 did not affect the antiapoptotic properties of XIAP. However, cells and tissues derived from Xiap-deficient mice were found to contain reduced copper levels, while suppression of MURR1 resulted in increased intracellular copper in cultured cells. Consistent with these opposing effects, XIAP was observed to negatively regulate MURR1 protein levels by the formation of K48 polyubiquitin chains on MURR1 that promote its degradation. These findings represent the first described phenotypic alteration in Xiap-deficient mice and demonstrate that XIAP can function through MURR1 to regulate copper homeostasis.

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We have utilized an experimental model of human zinc deficiency for study of cytokines production by TH1 and TH2 cells. Additionally, we determined ratios of CD4+ to CD8+ and CD4+ CD45RA+ to CD4+CD45RO+ cells and percentages of CD73+ T cytolytic cells in the CD8+ subset. The data were collected during baseline, at the end of the zinc-restricted period, and following zinc repletion. Our results showed that functions of TH1 cells, as evidenced by production of interferon-gamma, interleukin-2 (IL-2), and tumor necrosis factor-alpha, were decreased, whereas functions of TH2 cells (production of IL-4, IL-6, and IL-10) were unaffected by zinc deficiency. Thus an imbalance between TH1 and TH2 cells resulted because of zinc deficiency in humans. Our studies also showed that zinc may be required for regeneration of new CD4+ T lymphocytes and maintenance of T cytolytic cells. We conclude that an imbalance between TH1 and TH2 cells, decreased recruitment of T naive cells, and decreased percentage of T cytolytic cells may account for decreased cell-mediated immune functions in zinc-deficient subjects.

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The activity of thymulin (a thymic hormone) is dependent on the presence of zinc in the molecule. We assayed serum thymulin activity in three models of mildly zinc-deficient (ZD) human subjects before and after zinc supplementation: (a) two human volunteers in whom a specific and mild zinc deficiency was induced by dietary means; (b) six mildly ZD adult sickle cell anemia (SCA) subjects; and (c) six mildly ZD adult non-SCA subjects. Their plasma zinc levels were normal and they showed no overt clinical manifestations of zinc deficiency. The diagnosis of mild zinc deficiency was based on the assay of zinc in lymphocytes, granulocytes, and platelets. Serum thymulin activity was decreased as a result of mild zinc deficiency and was corrected by in vivo and in vitro zinc supplementation, suggesting that this parameter was a sensitive indicator of zinc deficiency in humans. An increase in T101-, sIg-cells, decrease in T4+/T8+ ratio, and decreased IL 2 activity were observed in the experimental human model during the zinc depletion phase, all of which were corrected after repletion with zinc. Similar changes in lymphocyte subpopulation, correctable with zinc supplementation, were also observed in mildly ZD SCA subjects. Inasmuch as thymulin is known to induce intra-and extrathymic T cell differentiation, our studies provide a possible mechanism for the role of zinc on T cell functions.