{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nPhosphodiesterase 5A (PDE5A) is a cGMP‐specific enzyme that plays a central role in modulating intracellular cyclic guanosine monophosphate (cGMP) levels by hydrolyzing cGMP, thus terminating nitric oxide (NO)/cGMP signaling. Structural and biochemical studies have revealed that PDE5A contains distinct catalytic and regulatory GAF domains that bind cGMP allosterically, and its activity is further controlled by phosphorylation (which increases its affinity for cGMP), cyclic nucleotide levels, and even by direct binding of inhibitors. These molecular features not only explain its responsiveness to drugs—such as sildenafil, tadalafil, and vardenafil—but also illustrate how PDE5A serves as a tunable negative feedback regulator in diverse cell types."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "9"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nIn smooth muscle tissues such as the corpus cavernosum, vas deferens, bladder, and vascular endothelium, PDE5A’s cGMP‐degrading activity is critical for regulating muscle tone and contractility. In the corpora cavernosa, for instance, androgen‐dependent upregulation of PDE5A explains its high expression in male genital tissue and underpins the efficacy of PDE5 inhibitors in treating erectile dysfunction, while in the bladder and prostate, PDE5A modulates NO/cGMP signaling to affect smooth muscle relaxation and lower urinary tract function. Moreover, in cardiac myocytes, increased PDE5A expression (or its enhanced activation via oxidative stress and phosphorylation) has been linked to pathological remodeling, hypertrophy, and heart failure, emphasizing its significance in cardiovascular physiology."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "10", "end_ref": "17"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nBeyond its classical roles in smooth muscle and cardiovascular tissues, dysregulation of PDE5A is increasingly recognized in non‐canonical settings including cancer and neural tissues. In melanoma cells, oncogenic BRAF downregulates PDE5A, leading to elevated cGMP and intracellular Ca²⁺ that drive invasive behavior. In several cancers—such as breast, thyroid, colon, glioblastoma, and even Hodgkin lymphoma—altered PDE5A expression correlates with changes in cell proliferation, survival, and apoptotic pathways, making it a promising target for chemoprevention and anticancer strategies. In platelets and neurons, compartmentalized regulation of PDE5A (and the development of biosensors based on its GAF domains) underscores its broader role in fine-tuning cellular responses to NO and cGMP. This multifunctional profile of PDE5A, modulated by its expression levels, post-translational modifications, and inhibitor binding, establishes it as a key therapeutic target for conditions ranging from pulmonary hypertension, erectile dysfunction, and lower urinary tract symptoms to cardiac hypertrophy, neurodegeneration, and cancer."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "18", "end_ref": "32"}]}, {"type": "t", "text": ""}]}]}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "C-S Lin, S Chow, A Lau, et al. 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