Development of refractoriness of induced human heme oxygenase-1 gene expression to reinduction by UVA irradiation and hemin.

Using primary human fibroblasts we have observed the existence of an acquired refractoriness of the heme oxygenase-1 gene to induction by a second dose of UVA (320-380 nm) radiation. We studied the kinetics of development of refractoriness over a time interval of up to 72 h between the first inducing event and the second (challenge) dose. Complete refractoriness was observed at 48 h. We also studied development of refractoriness after UVA, sodium arsenite and H2O2 treatment in all possible combinations and demonstrated that only UVA led to refractoriness. Ultraviolet radiation induced partial refractoriness to H2O2 induction but did not change the response to sodium arsenite. In an investigation of the mechanism of development of refractoriness we used the heme oxygenase inhibitor, tin-protoporphyrin IX and showed that induction of heme oxygenase enzymatic activity is a crucial step. However, the induction of ferritin, which is known to play a key role in protection against oxidative stress, did not appear to be involved. Damage to membranes is also probably not involved in the refractoriness mechanism. Because either hemin alone or UVA radiation are able to lead to a refractoriness of the heme oxygenase-1 gene to reinduction by a second exposure to one or the other agent in human fibroblasts, we conclude that heme, or an as yet unidentified heme derivative, is involved in the refractoriness response.