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 a cquired refractoriness of the heme oxygenase-l 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 betwe en the first inducing event and the second (challenge) dose, Complete refractoriness was observed at 48 h, We also studied development of re fractoriness after UVA, sodium arsenite and H2O2 treatment in all poss ible combinations and demonstrated that only UVA led to refractoriness . Ultraviolet radiation induced partial refractoriness to H2O2 inducti on but did not change the response to sodium arsenite. In an investiga tion of the mechanism of development of refractoriness we used the hem e 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 me mbranes is also probably not involved in the refractoriness mechanism, Because either hemin alone or UVA radiation are able to lead to a ref ractoriness of the heme oxygenase-l gene to reinduction by a second ex posure to one or the other agent in human fibroblasts, we conclude tha t heme, or an as yet unidentified heme derivative, is involved in the refractoriness response.