Background: In skeletal muscle, transcript levels of proteins regulating the ubiquitin proteasome system (UPS) increase with atrophy and decrease with hypertrophy. Whether the same is true for heart Muscle is not known. Aim of the study: We set out to characterize the transcriptional profile of regulators of the UPS during atrophy-, hypertrophy-, and hypoxia-induced remodeling of the heart. Methods and results: Cardiac atrophy was induced by heterotopic transplantation of the rat heart. Left ventricular hypertrophy was induced by banding of the ascending aorta in rats. To Study the effects of hypoxemia on the left ventricle, rats were exposed to hypobaric hypoxia. Transcript levels of six known regulators of the UPS, ubiquitin B (UbB), the ubiquitin conjugating enzymes UbcH2 and E2-14 kDa, the ubiquitin ligases Mafbx/Atrogin-1 and MuRF-1, and the proteasomal subunit PSMB4 were measured using quantitative RT-PCR. Unloading-induced atrophy increased mRNA levels of UbB and decreased levels of both ubiquitin ligases. Transcript levels of all UPS genes investigated increased in the hypertrophied and hypoxic heart (with the exception of E2-14 kDa). Conclusions: Cardiac atrophy, hypertrophy, and hypoxemia all increase myocardial UbB expression, suggesting that UbB is a transcriptional marker for load-induced and hypoxia-mediated cardiac remodeling. (c) 2006 Elsevier Inc. All rights reserved.