COPPER-INDUCED ACTIVATION OF TRANSCRIPTION FACTOR EB AND ITS DUAL EFFECT ON LYSOSOMAL EXOCYTOSIS

Transition metals toxicity is an important factor in pathogenesis of numerous human disorders, including neurodegenerative diseases. Lysosomes have emerged as important factors for transition metal toxicity because they can handle transition metals via endocytosis, autophagy, absorption from the cytoplasm and exocytosis. Transcription factor EB (TFEB) regulates lysosomal biogenesis and the expression of lysosomal proteins in response to lysosomal and/or metabolic stresses. Since transition metals cause lysosomal dysfunction, I proposed that TFEB may be activated in order to drive gene expression in response to transition metal exposure and may influence transition metal toxicity. I found that transition metals copper (Cu) and iron (Fe) activate overexpressed TFEB and stimulate the expression of TFEB- dependent genes in TFEB-overexpressing cells. Specifically, in cells that show robust lysosomal exocytosis, TFEB was cytoprotective at moderate levels of Cu exposure, decreasing oxidative stress as reported by the expression of HMOX1 gene. However, at high levels of Cu exposure, particularly in cells with low levels of lysosomal exocytosis, activation of overexpressed TFEB was toxic, increasing oxidative stress and mitochondrial damage. In addition, I found that Cu has a dual effect on lysosomal exocytosis: when cells were exposed to Cu for a prolonged period of time, lysosomal exocytosis was reduced, whereas brief exposure to Cu activated lysosomal exocytosis. Cu-induced lysosomal exocytosis depends on calcium (Ca) and the lysosomal SNARE VAMP7. Furthermore, depletion of ATP7B, a Cu pump recently shown to be involved in lysosomal exocytosis, suppressed the basal lysosomal exocytosis, but did not affect the ability of Cu to activate lysosomal exocytosis. ATP7B knockdown was associated with sustained oxidative stress. The removal of Ca from the extracellular medium, but not the extracellular addition of Ca channel blocker lanthanum (La) suppressed the Cu-dependent component of the lysosomal exocytosis. Based on these data, I conclude that the TFEB-driven gene network participates in the cellular response to transition metals and that exposure to Cu promotes lysosomal exocytosis in order to avoid the toxic effects associated with prolonged exposure to Cu.