Heme oxygenase-1 gene expression as a stress index to ocular irritation

Predicting the toxic potential of compounds to the ocular surface has depended on the Draize test for the past half century. Alternatives to Draize testing have recently been sought for a number of reasons. Stress gene expression has emerged as a means of quantifying cellular reaction and, thus, the toxic potential of the compound in question. This study examines the expression of the major stress response gene heme oxygenase-1 (HO-1) in a human corneal epithelial cell line (HCE-T) following challenges with a number of known ocular irritants.

HCE-T was used to investigate the effect of ocular irritants on cell viability and HO-1 expression. Irritants tested included hydrogen peroxide, isopropyl alcohol, sodium hydroxide and trichloroacetic acid. HCE-T cells were grown to 80% confluency and treated with the listed irritants at a concentration range of 10-100 microM. Cell viability and northern blot analysis were performed following a 24 and 48 hr incubation period.

HCE-T cells expressed HO-1 mRNA and HO activity similar to other human cell lines. Northern blot analysis demonstrated that levels of HO-1 mRNA transcripts increased regularly after exposure to the irritants in a concentration-dependent manner. Studies on the effect of various inhibitors and inducers of HO-1 on cell viability showed that inhibition of HO-1 potentiates the cytotoxic effect of ocular irritants. In contrast, pre-induction of HO-1 in HCE-T decreases the effect of various irritants on cell viability.

These results are consistent with the idea that HO-1 mRNA levels may be used as an indicator of toxicity resulting from ocular irritants and that HCE-T cells respond to stress in a fashion similar to other human cell lines. This strategy for testing may be important in the development of an alternative to Draize testing. The results of this study also suggest that HO-1 may constitute a part of the protective defense mechanism against chemical injury.