Time course of pulmonary response of rats to inhalation of crystalline silica:: NF-κB activation, inflammation, cytokine production, and damage

In vitro studies suggest that silica-induced lung disease may be linked to processes regulated by nuclear factor-kappaB (NF-kappaB) activation, but this has not been examined in vivo. Rats were exposed to a silica aerosol of 15 mg/m(3) (6h/day, 5 days/wk) for 116 days, and bronchoalveolar lavage (BAL) was conducted at various times during the exposure. Silica-induced pulmonary inflammation and damage were determined by measuring BAL cell differentials and first BAL fluid lactate dehydrogenase (LDH) activity and serum albumin concentrations, respectively. NF-kappaB activation and production of tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 (IL-1) by BAL cells were also measured. The results demonstrate that NF-kappaB activation occurred after 5 days exposure, and continued to increase thereafter. BAL cell production of IL-1 and TNF-alpha had increased incrementally by 10 and 30 days of exposure, respectively. This elevation continued through 79 days of exposure before further increasing at 116 days of exposure. Pulmonary inflammation and damage in silica-exposed rats were also significantly elevated at 5 days of exposure, further increased at a slow rate through 41 days of exposure, and dramatically increased thereafter. Taken together, the results indicate that the initial molecular response of NF-kappaB activation in BAL cells occurs in response to low levels of silica deposition in the lung and increases more rapidly versus exposure duration than silica-induced pulmonary inflammation, cellular damage, and cytokine production by BAL cells. This suggests that NF-kappaB activation in BAL cells may play an important role in the initiation and progression of silica-induced pulmonary inflammation, cellular damage, and fibrosis.