Nickel, cobalt and chromium-induced cytotoxicity and intracellular accumulation in human hacat keratinocytes

Nickel, cobalt and chromium can induce allergic contact dermatitis (ACD) and may provoke irritant reactions in the skin. This study aimed at investigating cytotoxicity and cell viability along with intracellular metal accumulation in HaCaT human keratinocytes exposed to soluble forms of nickel, cobalt or chromium. The EC 50 (24 h) values as detected by MTT test were 30 μM for sodium chromate (Na 2CrO 4), 475 μM for cobalt chloride (CoCl 2) and 600 μM for nickel chloride (NiCl 2). Chromium chloride (CrCl 3) was not toxic up to 1 mM. No clear effects were observed after 4 h, but 24-h treatments with 1 mM CoCl 2 or 10 μM Na 2CrO 4 were found to almost completely abolish the ability of the cells to form colonies, whilst 1 mM NiCl 2 reduced cellular survival to only 70% of control cultures. Intracellular accumulation of metals was evaluated by the use of radioisotopes at the EC 50 value and at 1/10–1/5 of this concentration. Accumulation of Na 2 51CrO 4 was linear with increasing dose. This was not the case for 63NiCl 2 and 58CoCl 2. All the metals were accumulated preferentially in the cytosols; 96% or more for 63NiCl 2, approximately 90% for 58CoCl 2 and 60–70% for Na 2 51CrO 4. Finally, it was observed that HaCaT human keratinocytes can concentrate the metals present in the media up to 3.9 and 12.5 times for NiCl 2 and CoCl 2, respectively, and up to 167 for Na 2CrO 4. These striking metal intracellular accumulation patterns, which have not been earlier described in keratinocytes, highlight the relevance of searching for specific biomarkers of early cellular toxic effects, such as cytosolic proteins that bind the metals.