Effects of Particulate and Soluble Cadmium Species on Biochemical and Functional Parameters in Cultured Murine Macrophages

Abstract

Cultured murine macrophages (RAW 264.7) were used to evaluate the temporal relationships between cytotoxicity, phagocytosis, tumor necrosis factor-α (TNF-α), and nitric oxide (NO) production, and alterations in expression of stress proteins after exposure to cadmium oxide (CdO) or cadmium chloride (CdCl₂), particulate and soluble forms of cadmium, respectively. Macrophages were exposed in vitro to CdO (25 or 50 μg) or CdCl₂ (30 or 40 μM) for 2 to 72 h. Cytotoxicity was not evident until 18 h when exposed to 30 μM CdCl₂ or 25 μg CdO, but occurred as early as 12 h after exposure to 40 μM CdCl₂ or 50 μg CdO. Relative to untreated controls, phagocytic activity decreased progressively from 2 to 24 h after exposure to both forms of cadmium. TNF-α levels increased to 2- to 3-fold after 4 h and remained elevated until 24 h after exposure to 25 and 50 μg CdO and 30 μM CdCl2, but decreased by 18-24 h at 40 μM CdCl₂. CdCl₂ or CdO alone did not induce NO; how ever, both cadmium species reduced lipopolysaccharide (LPS)-stimulated NO production in a dose-dependent manner. Enhanced de novo synthesis of 70- and 90- kD heat shock, or stress, proteins was observed 2 to 8 h after exposure to both CdCl₂ and CdO; however, synthesis of these proteins returned to control levels by 24 h. Stress protein synthesis was enhanced by CdCl₂ or CdO prior to cytotoxicity, but coincided with a decrease in phagocytic capacity and an increase in TNF-α levels. The data suggest that cultured macrophages respond similarly in vitro to a particulate form and a soluble form of cadmium in a cell type that plays a pivotal role in inflammatory and immune responses.