Could Nickel Become a Novel Erythropoiesis-Stimulating Compound for Cheating Athletes?

The erythropoietin gene (EPO) is controlled by the hypoxia-inducible transcription factors (HIFs). HIFs are heterodimeric proteins composed of α- and β-subunits. There are different HIF-αsubunits. HIF-1α is rather ubiquitously expressed, with HIF-1 (HIF-1α/HIF-1β) playing important roles in metabolic processes, such as glucose metabolism (15). In contrast, HIF-2α is restricted to specific cell types, including renal and extra-renal cells producing EPO, and endothelial cells producing vascular endothelial growth factor (VEGF) (9). Hence, HIF-2 (HIF-2α/HIF-1β) promotes erythropoiesis and angiogenesis. Distinct prolyl residues of the HIF-αsubunits are hydroxylated in an O₂-pressure-dependent manner by specific enzymes, the prolyl-4-hydroxylase domain proteins (PHD-1, -2 and -3). PHDs are iron- and 2-oxoglutarate-dependent dioxygenases. Prolyl hydroxylated HIF-αis immediately ubiquitinated by the so-called SCF complex (S-phase kinase-associated protein 1, Cullin, F-box containing complex) to undergo immediate proteasomal degradation. Salnikow et al. were the first to demonstrate that nickel exposure activates HIF-1 (13). Subsequent studies have shown that Ni²⁺ does not only stabilize HIF-1α but also HIF-2α (11). Nickel is less potent than cobalt as a HIF-stabilizer (4).
Several hypotheses have been proposed to explain the stabilization of HIF-α by nickel. Ni²⁺ can enter cells via the divalent metal transporter-1 (DMT1) and thereby compete with Fe²⁺ for entry into cells (2). Of note, however, simple competitive inhibition of the HIF-αPHDs has been considered unlikely (4). It has been suggested that divalent metals such as Ni²⁺ or Co²⁺ may stabilize HIF-α by binding to Cullin-2 (8).

Almost 40 years ago the group of Hopfer discovered that the intra-renal injection of nickel compounds (αNi₃S₂, NiO, NiS₂, βNiS, Ni dust) produces an increase in circulating EPO and erythrocytosis in rats (6). The intra-renal injection of Ni₃S₂ was also effective in guinea pigs but not in in hamsters or gerbils, indicating significant species-specificity (18). Importantly, NiCl₂ proved to increase EPO mRNA expression and EPO production in the human hepatoma cell line, Hep3B (5). The intraperitoneal infusion of NiCl₂ failed to stimulate EPO production in rats (17). Likewise, the subcutaneous injection of nickel failed to stimulate renal EPO mRNA expression in mice (10).
Note, here, that dietary nickel was found to increase the blood hemoglobin concentration and to cause pulmonary hypertension in broiler chickens (12). No such effect has been reported with respect to increased nickel supplementation in mammals. It can be stated that the effect of Ni^₂₊ differs from that of Co²⁺, as the latter clearly stimulates EPO synthesis and erythropoiesis following oral administration (7).

HIFs do not only stimulate erythropoiesis, but are involved in a number of other processes which protect the cells from oxygen and glucose deprivation. More than 1000 HIF target genes have been identified so far (15). For example, hypoxia, nickel, and cobalt stimulate VEGF expression in osteoblast cell cultures by a similar mechanism (16). GeneChip analysis of mouse embryonic fibroblasts exposed to Ni²⁺ has demonstrated increased expression of genes involved in glucose metabolism, including glycolytic enzymes and genes responsible for glucose transport (14).

Nickel can stimulate HIF-dependent processes. However, there are no data in support of the assumption that nickel can stimulate EPO production when administered systemically (oral, subcutaneous, or intravenous routes). Actually, nickel is contained in numerous food products including corn, nuts, fruits, tea, etc. and thus an integral part of the human diet. Increased nickel supplementation, however, can cause severe side effects. Extensive nickel consumption can result in allergic reactions and (irreversible) nickel sensitivity, digestion problems, increased red blood cell counts, kidney failure and cancer, primarily lung cancer (3). These facts alone should discourage athletes from a potentially deleterious doping practice with nickel. In addition, anti-doping laboratories have established methods for detection of transition metals including nickel in biological samples (19, 20).