Introduction: Expression of the brown adipocyte specific gene, uncoupling protein 1 (UCP1), is regulated by a distal enhancer interacting with the proximal promoter. Expression from the human promoter is stimulated by isoproterenol and thiazolidinediones but only in the present of retinoic acid whereas expression of the mouse UCP 1 promoter can be stimulated without retinoic acid. To establish the molecular mechanism for this species difference we have stably transduced mouse UCP1 promoter reporter constructs and examined their activity in the differentiated mouse cell line. Methods: Mouse and human enhancer-proximal promoter luciferase pLenti reporter constructs were transiently transfected into undifferentiated HIB-1B cells, or used to produce lentiviral particles which were used to produce stably transduced cells that were differentiated for 7 days using a standard protocol of IBMX, dexamethasone, triiodthyronine and insulin. HIB1B cells were then cultured in the presence and absence of retinoic acid, forskolin (to stimulate cAMP) and the thiazolidinedione, rosiglitazone. Luciferase assays were performed up to 48h later. Results: Transcription from both mouse and human UCP1 promoter luciferase reporter constructs transiently transfected into undifferentiated HIB1B cells was strongly upregulated by the separate addition of forskolin, retinoic acid or rosiglitazone to the media, although combined addition of these drugs enhanced luciferase activity. In differentiated HIB1B cells stably transduced with the mouse construct, addition of forskolin only weakly stimulated transcription but strongly upregulated luciferase activity, when combined with retinoic acid or rosiglitazone, Conclusions: Transcription of a UCP1 enhancer-promoter reporter construct stably transduced into differentiated HIB1B cells is only fully responsive to cAMP simulation in the presence of retinoic acid or rosiglitazone and this may have importance in the strategy to treat human obesity by increasing the activity of UCP1 in adipose tissue. Conflict of Interest: None Funding: Saudi Arabian government and Nottingham University