Hypoxia-inducible factor-1 (HIF-1) up-regulates adrenomedullin expression in human tumor cell lines during oxygen deprivation: A possible promotion mechanism of carcinogenesis

Little is known about the molecular mechanisms that control adrenomedullin (AM) production in human cancers. We demonstrate here that the expression o f AM mRNA in a variety of human tumor cell lines is highly induced in a tim e-dependent manner by reduced oxygen tension (1% O-2) or exposure to hypoxi a mimetics such as desferrioxamine mesylate (DFX) or CoCl2. This AM express ion seems to be under hypoxia-inducible factor-1 (HIF-1) transcriptional re gulation, since HIF-1 alpha and HIF-1 beta knockout mouse cell lines had an ablated or greatly reduced hypoxia AM mRNA induction. Similarly, inhibitio n or enhancement of HIF-1 activity in human tumor cells showed an analogous modulation of AM mRNA. Under hypoxic conditions, immunohistochemical analy sis of tumor eel lines revealed elevated levels of AM and HIF-1 alpha as co mpared with normoxia, and we also found an increase of immunoreactive AM in the conditioned medium of tumor cells analyzed by RIA. AM mRNA stabilizati on was shown to be partially responsible for the hypoxic up-regulated expre ssion of AM. In addition, we have identified several putative hypoxia respo nse elements (HREs) in the human AM gene, and reporter studies with selecte d HREs were capable of enhancing luciferase expression after exposure to DF X. Furthermore, transient coexpression of HIF-1 alpha resulted in an augmen ted transactivation of the reporter gene after DFX treatment. Given that mo st solid human tumors have focal hypoxic areas and that AM functions as a m itogen, angiogenic factor, and apoptosis-survival factor, our findings impl icate the HIF-1/AM link as a possible promotion mechanism of carcinogenesis .