A Role for Forkhead Transcription Factors (FOXOs) in Disuse Atrophy of the Human Diaphragm (DIA) Associated with Mechanical Ventilation (MV)

Abstract

We (Levine et al, NEJM, in press) have demonstrated that the combination of complete DIA inactivity and MV (i.e. DIA inact + MV) elicits marked atrophy of human DIA myofibers due to increased proteolysis. To assess the role of FOXOs 1 and 3 in effecting this atrophy, we biopsied the costal DIA of 10 brain dead organ donors prior to organ harvest (i.e., case subjects) and compared them to intra‐operative biopsies obtained from the DIA of 10 patients undergoing lung surgery (i.e., controls). Cases exhibited DIA inact + MV for much longer periods of time than controls (i.e., 18–69 hours versus 2–3 hours). We then isolated RNA from each DIA biopsy and we carried out real‐time reverse transcriptase PCR to assess the relative gene expressions of FOXOs 1 and 3, Atrogin‐1, and MuRF‐1 genes. In comparison to controls, case diaphragms exhibited the following fold increases in relative transcript number: FOXO 1, 5.6 (p<0.001); FOXO 3, 2.0 (p<0.04); Atrogin‐1, 2.6 (p<0.05), and MuRF‐1, 4.9 (p<0.01). These results are consistent with the concept that (a) case diaphragms increase transcription of FOXOs 1 and 3 that bind to the promoter region of the ubiquitin ligase genes (i.e., Atrogin‐1 and MuRF‐1); (b) increased mRNA expression of these latter genes increases the rate of proteolysis via the ubiquitin‐proteasome pathway; and (c) thereby accounts for some portion of the myofiber atrophy noted in case DIA. Supported by NIH Grant R01‐HL‐078834.