Hale, A. T. | bioRxiv Science

bioRxiv2024

IPMK regulates HDAC3 activity and histone H4 acetylation in human cells.

Histone deacetylases (HDACs) repress transcription by catalyzing the removal of acetyl groups from histones. Class 1 HDACs are activated by inositol phosphate signaling molecules in vitro, but it is unclear if this regulation occurs in human cells. Inositol Polyphosphate Multikinase (IPMK) is required for production of inositol hexakisphosphate (IP6), pentakisphosphate (IP5) and certain tetrakisphosphate (IP4) species, all known activators of Class 1 HDACs in vitro. Here, we generated IPMK knockout (IKO) human U251 glioblastoma cells, which decreased cellular inositol phosphate levels and increased histone H4-acetylation by mass spectrometry. ChIP-seq showed IKO increased H4-acetylation at IKO-upregulated genes, but H4-acetylation was unchanged at IKO-downregulated genes, suggesting gene-specific responses to IPMK knockout. HDAC deacetylase enzyme activity was decreased in HDAC3 immunoprecipitates from IKO vs. wild-type cells, while deacetylase activity of other Class 1 HDACs had no detectable changes in activity. Wild-type IPMK expression in IKO cells fully rescued HDAC3 deacetylase activity, while kinase-dead IPMK expression had no effect. Further, the deficiency in HDAC3 activity in immunoprecipitates from IKO cells could be fully rescued by addition of synthesized IP4 (Ins(1,4,5,6)P4) to the enzyme assay, while control inositol had no effect. These data suggest that cellular IPMK-dependent inositol phosphates are required for full HDAC3 enzyme activity and proper histone H4-acetylation. Implications for targeting IPMK in HDAC3-dependent diseases are discussed.

biochemistry↗

bioRxiv2021

Sulfation of glycosaminoglycans depends on catalytic activity of a lithium-inhibited phosphatase

Golgi-resident bisphosphate nucleotidase 2 (BPNT2) is a member of a family of magnesium-dependent/lithium-inhibited phosphatases that share a three-dimensional structural motif that directly coordinates metal binding to effect phosphate hydrolysis. BPNT2 is responsible for the breakdown of 3-phosphoadenosine-5-phosphate (PAP), a by-product of glycosaminoglycan (GAG) sulfation. Disruption of BPNT2 in mice leads to skeletal abnormalities due to impaired GAG sulfation, especially chondroitin-4-sulfation. Mutations in BPNT2 have also been found to underlie a chondrodysplastic disorder in humans. The precise mechanism by which loss of BPNT2 impairs sulfation remains unclear. Here, we utilize an in vitro approach using mouse embryonic fibroblasts (MEFs) to test the hypothesis that catalytic activity of BPNT2 is required for GAG sulfation. We show that a catalytic-dead Bpnt2 construct (D108A) does not rescue impairments in intracellular or secreted sulfated GAG, including decreased chondroitin-4-sulfate, present in Bpnt2-knockout MEFs. We also demonstrate that missense mutations in Bpnt2 which are adjacent to the catalytic site (and known to cause chondrodysplasia in humans) recapitulate defects in overall GAG sulfation and chondroitin-4-sulfation in MEF cultures. We further show that treatment of MEFs with lithium inhibits GAG sulfation, and that this effect depends on the presence of BPNT2. This work demonstrates that the catalytic activity of an enzyme potently inhibited by lithium can modulate GAG sulfation and therefore extracellular matrix composition, revealing new insights into lithium pharmacology and the pathophysiology of psychiatric disorders responsive to lithium.

biochemistry↗