PCT No. PCT/GB96/00256 Sec. 371 Date Mar. 3, 1998 Sec. 102(e) Date Mar. 3, 1998 PCT Filed Feb. 7, 1996 PCT Pub. No. WO96/25487 PCT Pub. Date Aug. 22, 1996This application relates to cell lines transfected with (a) nucleic acid encoding one or more hormone receptors, the receptors being capable of transmitting a signal, and (b) nucleic acid encoding a desired polypeptide, the nucleic acid encoding the polypeptide being under the control of a promoter containing regulatory elements responsive to the signal transmitted by the receptor(s) so that transcription of the nucleic acid encoding the desired polypeptide is modulated by the level of the hormone(s) in the patient. Preferably, the receptors are the GIP or GLP-1 receptors controlling the production of insulin in the cells in response to changes in the metabolic status of a patient. The application also relates to compositions comprising these cells and their use in methods of medical treatment.