Synthetic metalloporphyrins, which competitively inhibit heme oxygenase (HO) activity, are currently being studied as potential therapeutic compounds for the suppression of neonatal jaundice. In addition to this inhibitory activity, these heme analogs may also increase transcription of the HO-1 gene. Activation of the gene encoding the target enzyme may adversely affect the clinical use of these compounds. We have developed rapid cell culture and transgenic mouse assays to evaluate the effects of metalloporphyrins on HO-1 transcription. Two stable NIH 3T3 lines containing the luciferase reporter gene expressed from either the full length promoter (HO15luc) or a truncated promoter (HO1.4luc) were generated. In addition, we have introduced the luc fusion utilizing the full-length promoter into mice to create a HO-luc transgenic (Tg) line. The cell lines were exposed to 12 different metalloporphyrins over a 4-fold concentration range. Each metalloporphyrin contained one of three metals (Cr, Sn, Zn) and each had one of four ring substituents [deutero- (DP); meso- (MP); bis glycol- (BG); proto-, (PP)]. Most of the metalloporphyrins increased HO15luc to varying degrees, but none increased the truncated HO1.4luc in cell culture. Metal ions themselves (Cr3+, Sn4+, Zn2+) did not increase transcription. These results suggest that one or both of the two distal enhancer elements located at -4 and -10 Kb upstream from transcriptional start are required for HO-1 induction by heme and its analogs. In culture, HO-1 expression increased nearly two-fold over 10h of exposure in response to SnMP and ZnPP. There was no apparent effect on HO-1 expression by ZnBG. These three metalloporphyrins and heme were then evaluated in the transgenic indicator mice following tail-vein injection. In the Tg animal assay, SnMP and ZnPP appeared to increase transcription in the abdominal area. A ten-fold increase was noted for SnMP and two-fold for ZnPP. Alterations in porphyrin ring substituents and the central metal may affect the extent of gene activation, suggesting the involvement of a common heme-binding protein (HBP). HBPs are involved in initiation of signaling pathways leading to increased HO-1 expression. The subtle differences among metalloporphyrins may impact the HBP binding affinity; likely affecting the extent of gene activation. Furthermore, these data indicate that evaluation of metalloporphyrins in animal models is required for understanding the full range of their effects in vivo.
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Zhang, W., Contag, P., Contag, C. et al. Transcriptional Regulation of Heme Oxygenase-1 by Metalloporphyrins. Pediatr Res 45, 72 (1999) doi:10.1203/00006450-199904020-00434