{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nPHPT1, a 14‐kDa phosphohistidine phosphatase first characterized for its selective dephosphorylation of phosphorylated histidine residues, has been shown to play a central role in regulating protein histidine phosphorylation in eukaryotic cells. Early studies established its existence and substrate specificity for phosphohistidine‐containing peptides, while subsequent mutagenesis experiments identified key active site residues critical for catalysis and substrate anchoring, thereby illuminating its catalytic mechanism and structure–function relationship. These investigations, complemented by high‐resolution structural analyses that pinpointed essential interactions such as a salt bridge needed for optimal ligand binding, have provided the molecular framework for understanding PHPT1’s enzymatic activity and regulation (see."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "1", "end_ref": "7"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nBeyond its biochemical properties, PHPT1 has emerged as a versatile regulatory enzyme in several physiological processes. In the immune system, PHPT1 directly binds to and dephosphorylates a specific histidine residue on the calcium‐activated K⁺ channel KCa3.1, thereby negatively modulating T lymphocyte Ca²⁺ influx and downstream signaling events. Similarly, in renal physiology, PHPT1 counteracts the activation of the TRPV5 Ca²⁺ channel—which is stimulated by nucleoside diphosphate kinase B—to influence active Ca²⁺ reabsorption. Additionally, PHPT1 targets key metabolic enzymes such as ATP-citrate lyase, with its dephosphorylation activity impacting cellular energy metabolism and associated cell viability in neural and pancreatic beta cells (see."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "8"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nPHPT1’s influence extends into disease contexts where its activity and expression are critical. In cancer biology, overexpression of PHPT1 has been linked to enhanced lung cancer cell migration and invasion, with evidence also suggesting its oncogenic potential in lung and renal carcinomas. In parallel, overactive PHPT1 in endothelial and neuronal systems can promote apoptosis and impair cell viability via reduced ATP-citrate lyase activity, highlighting its role in cellular homeostasis. Furthermore, in liver fibrosis, PHPT1 expression is upregulated in activated hepatic stellate cells and contributes to their migration through the PI3Kγ/AKT/Rac1 pathway. Notably, alterations in PHPT1 expression or post-translational modifications—such as oxidation at a key methionine residue, as well as the production of unstable splice variants—further illustrate the complex regulation of its function. Finally, sensitive assays developed to quantify PHPT1 activity and studies on its transcriptional response to genotoxic stress underscore its potential as a biomarker of cellular damage (see."}, {"type": "fg", "children": [{"type": "fg_fs", "start_ref": "10", "end_ref": "19"}]}, {"type": "t", "text": "\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Pia Ek, Gunilla Pettersson, Bo Ek, et al. 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