Age-related changes in adrenomedullin expression and hypoxia-inducible factor 1 activity in the rat lung and their responses to hypoxia

Abstract

Objective: To investigate the effects of hypoxia on adrenomedullin (AM) gene expression during aging. Methods: Male rats aged 3 months, 12 months and 20 months were subjected to breathing 8% of oxygen for 6 hours. AM levels in the lung were measured by RIA while mRNA levels of preproAM, its receptor component proteins and hypoxia-inducible factor-1a (HIF-1a) were determined by RNase protection assay and/or RT-PCR. HIF binding to DNA was measured by electrophoretic mobility shift. Results: In the lung, there was an age-related increase in basal levels of preproAM mRNA and AM and of the binding of hypoxia-inducible factor-1a (HIF-1a) to DNA. Upon hypoxic stimulation, HIF binding to DNA increased in the young and middle-aged rats, but not in the old rats. AM gene expression increased in response to hypoxia in rats of all ages but the increase was much less in the old rats. AM peptide levels in the lung decreased with age in hypoxia. In male rats aged 3 months and 20 months subjected to hypoxia, preproAM, CRLR (calcitonin receptorlike receptor), RAMP (receptor activity modifying proteins) mRNA and HIF-1 mRNA levels showed an increase in basal levels but a diminished response to hypoxia in the old rats. Polysome profiling demonstrated decreases in the percentages of translatable preproAM mRNA in response to hypoxia, with a greater decrease in the old than the young rats. Conclusion: The age-dependent decrease in the hypoxic response of the AM system in the lung was associated with high basal levels of HIF activity and AM expression in the old rats, but less translatable preproAM mRNA in the old rats in response to hypoxia. Thus the HIF-AM pathway may be impaired in the aged lung and other mechanisms may be present to maintain an AM response to hypoxia. (This study was supported by a CRCG grant from the University of Hong Kong).