Abstract
Intracellular free zinc ([Zn(2+)](i)) is mobilized in neuronal and non-neuronal cells under physiological and/or pathophysiological conditions; therefore, [Zn(2+)](i) is a component of cellular signal transduction in biological systems. Although several transporters and ion channels that carry Zn(2+) have been identified, proteins that are involved in Zn(2+) supply into cells and their expression are poorly understood, particularly under inflammatory conditions. Here, we show that the expression of Zn(2+) transporters ZIP8 and ZIP14 is increased via the activation of hypoxia-induced factor 1α (HIF-1α) in inflammation, leading to [Zn(2+)](i) accumulation, which intrinsically activates transient receptor potential ankyrin 1 (TRPA1) channel and elevates basal [Zn(2+)](i). In human fibroblast-like synoviocytes (FLSs), treatment with inflammatory mediators, such as tumor necrosis factor-α (TNF-α) and interleukin-1α (IL-1α), evoked TRPA1-dependent intrinsic Ca(2+) oscillations. Assays with fluorescent Zn(2+) indicators revealed that the basal [Zn(2+)](i) concentration was significantly higher in TRPA1-expressing HEK cells and inflammatory FLSs. Moreover, TRPA1 activation induced an elevation of [Zn(2+)](i) level in the presence of 1 μM Zn(2+) in inflammatory FLSs. Among the 17 out of 24 known Zn(2+) transporters, FLSs that were treated with TNF-α and IL-1α exhibited a higher expression of ZIP8 and ZIP14. Their expression levels were augmented by transfection with an active component of nuclear factor-κB P65 and HIF-1α expression vectors, and they could be abolished by pretreatment with the HIF-1α inhibitor echinomycin (Echi). The functional expression of ZIP8 and ZIP14 in HEK cells significantly increased the basal [Zn(2+)](i) level. Taken together, Zn(2+) carrier proteins, TRPA1, ZIP8, and ZIP14, induced under HIF-1α mediated inflammation can synergistically change [Zn(2+)](i) in inflammatory FLSs.