Regulation of the human heme oxygenase 1 gene by solar UVA radiation - a potential role for Bach proteins and heme turnover

Repeated acute (e.g. sunburn) or chronic inflammatory events can lead to permanent tissue damage and disease so that agents which suppress inflammation are used to treat disease. Among the body¿s own anti-inflammatory proteins is heme oxygenase 1 which is induced in many types of tissue following damage, including damage by the ultraviolet A component of sunlight. The protein is able to break down heme, a molecule which can have damaging side-effects when in excess and unbound to protein. Understanding how this protein is regulated and how it is able to protect cells and tissues should provide information of value in treating inflammatory disease. We intend to use cells derived and cultured from human skin to study responses to the UVA radiation component of sunlight and will determine the factors and enzymatic pathways involved in the UVA regulation of this inducible anti-inflammatory response. By defining and understanding such pathways, we hope not only to advance our basic knowledge of cell biology but also to facilitate the development of more sophisticated strategies of disease prevention based on controlling the inflammatory response.

The inducible heme catabolising enzyme, heme oxygenase 1 (HO-1), has a major anti-inflammatory role and increased expression is associated with increased resistance to tissue injury in various pathologies. Regulation of the mouse promoter by heavy metals and heme appears to occur via the Maf regulatory network involving two upstream enhancers with Nrf-2, acting as an activator protein and Bach1 acting as a suppressor protein. Heme modifies the activity of both proteins. Solar UVA radiation, a powerful oxidant, releases heme and strongly activates HO-1 in human skin fibroblasts resulting in protection against oxidative membrane damage. UVA treatment also leads to an exceptionally powerful down-regulation of the gene manifested as a total refractoriness to re-induction, a phenomenon not observed after heavy metal treatment. The UVA model will be used to examine the role of heme, Bach proteins and HO itself in up-regulation of HO in human skin fibroblasts and, in particular, to examine the hypothesis that the same factors and possibly heme-independent suppressor proteins are involved in both the strong down-regulation and refractoriness to re-induction observed in fibroblasts and the absence of basal expression or inducible up-regulation in human skin keratinocytes. Action spectra will be undertaken to further clarify the relationship between heme release, HO-1 induction and protection against oxidative membrane damage.