Increases in intracellular calcium dephosphorylate histone H3 at serine 10 in human hepatoma cells: Potential role of protein phosphatase 2A–protein kinase CβII complex

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Abstract
We present evidence that increases in intracellular calcium, induced by treatment with calcium ionophore A23187 or the endoplasmic reticulum calcium‐ATPase inhibitor thapsigargin, dephosphorylated histone H3 at serine10 (histone H3‐Ser10) in a dose‐dependent manner in human hepatoma HepG2 cells. Inhibition of p42/44MAPK, pp90RSK, or p38MAPK did not affect the ability of A23187 to dephosphorylate histone H3‐Ser10. This response is significantly blocked by okadaic acid, indicating a requirement for protein phosphatase 2A (PP2A). A23187 increased the activity of PP2A towards phosphorylated histone H3‐Ser10. Furthermore, pretreatment with calphostin C, a selective protein kinase C (PKC) inhibitor, blocked A23187‐dependent dephosphorylation of histone H3‐Ser10, and coimmunoprecipitation analysis showed PP2A association with the PKCβII isoform. Unlike untreated cells, coimmunoprecipitated complex from A23187‐treated cells showed greater dephosphorylation of histone H3‐Ser10 in a PP2A‐dependent manner. Inhibition of PP2A increased phosphorylation at Ser660 that determines calcium sensitivity and activity of PKCβII isoform, thus supporting a role for intracomplex regulation. Finally, chromatin immunoprecipitation assays following exposure to A23187 and okadaic acid revealed regulatory role of histone H3‐Ser10 phosphorylation in selective gene induction. Altogether, our findings suggest a novel role for calcium in modulating histone H3‐Ser10 phosphorylation level and led us to propose a model emphasizing PP2A activation, occurring downstream following perturbations in calcium homeostasis, as key event in dephosphorylating histone H3‐Ser10 in mammalian cells.