FoxP1 is sufficient and required for skeletal muscle wasting

Skeletal muscle wasting is a devastating consequence of cancer that affects up to 80% of cancer patients and associates with reduced survival. Herein we identified the transcriptional repressor protein, Forkhead box P1 (FoxP1), as a downstream target gene of FoxO1 whose skeletal muscle expression is elevated in multiple models of cancer cachexia and in patients with cancer who exhibit cachexia. Through generation of inducible skeletal muscle-specific FoxP1 over-expressing (FoxP1iSkmTg/Tg) mice, we demonstrate that FoxP1 upregulation is sufficient to induce features of cachexia, including body and skeletal muscle wasting characterized by reduced muscle fiber cross-sectional area of type IIX/B muscle fibers. Muscles from FoxP1iSkmTg/Tg mice also showed significant muscle damage and myopathy characterized by the accumulation of p62 and cellular material-filled vesicles, the presence of centrally nucleated myofibers, and were significantly weaker than controls. In the context of cancer cachexia, blocking FoxP1 upregulation prevented the cancer-induced repression of target genes critical to muscle structural integrity and repair, including Myocyte enhancer factor 2c (Mef2c), improved muscle ultrastructure and significantly attenuated muscle fiber atrophy. We further show that the muscle wasting phenotype induced by FoxP1 required the activity of histone deacetylase (HDAC) proteins, which are well-established to cooperate with FoxP1 to mediate gene repression, and which were necessary for FoxP1-dependent repression of Mef2c. In summary, we identify FoxP1 as a negative transcriptional regulator of skeletal muscle mass and function, whose up-regulation mediates cancer-induced muscle wasting. We used microarrays to investigate the genome-wide transcriptional networks regulated by the FoxO1 and FoxP1 transcription factors in skeletal muscle of tumor-bearing mice. Tibialis anterior muscles of 10-week old male mice were directly injected with AAV9 vectors encoding scrambled-shRNA, FoxO1-shRNA or FoxP1-shRNA. Two weeks later mice were subcutaneously inoculated with either C26 cells into both flanks (C26 groups), or PBS (Sham group). Muscle tissues from all mice were subsequently harvested at tumor endpoint (26 days post-C26 inoculation) for RNA extraction and RT-qPCR for confirmation of successful knockdown of FoxO1 or FoxP1 in groups receiving AAV9-FoxO1-shRNA or AAV9-FoxP1-shRNA vectors. Equal amounts of RNA were subsequently pooled from muscle tissue from three animals per group, followed by hybridization on Affymetrix microarrays.