PDE4D restricts cAMP signaling in a cancer cachexia in vitro model

Cachexia is a complex cancer-associated comorbidity characterized by severe weight loss and skeletal muscle wasting, which negatively impacts patient survival and quality of life. However, the limited understanding of the intricate molecular mechanisms underlying cancer cachexia has hindered the development of effective therapeutic strategies, thus making cachexia an urgent medical need. Previous studies have shown that cyclic adenosine monophosphate (cAMP) signaling pathway plays a crucial role in muscle homeostasis, and unpublished data from Graziani Lab indicate that its dysregulation is involved in cancer cachexia. In this thesis, we investigated the role of cAMP-specific phosphodiesterases 4 (PDE4), key negative regulators of the cAMP signaling, in the establishment of cancer-induced β-adrenergic resistance in skeletal muscles. Using CRISPR/Cas9 gene editing technology, Pde4b or Pde4d knockout C2C12 myoblast cell lines were generated. Results indicated a strong dependency of myoblasts on PDE4D for cAMP hydrolysis and thus for the termination of the cAMP signaling, especially in pro-cachectic conditions. Indeed, genetic ablation of Pde4d resulted in increased cAMP levels and a subsequent boost of the downstream cAMP/PKA/CREB1 signaling pathway, which is impaired in the pro-cachectic environment. Furthermore, through gene set enrichment and gene ontology analysis of RNA-seq data from cachectic muscle treated/untreated with rolipram, a PDE4-specific inhibitor, we showed that defective cAMP signaling contributes to the cancer-induced transcriptional reprogramming of cachectic muscle by impairing the expression of genes encoding mitochondrial proteins. Collectively, these data support the potential of targeting PDE4D as a prospective therapeutic intervention to counteract or delay mitochondrial dysfunction, one of the key features of muscle wasting in cancer cachexia.