Phosphatase 2A is involved in endothelial cell microtubule remodeling and barrier regulation

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Abstract
We have recently shown that microtubule (MT) inhibitor, nocodazole (2–5 μM) significantly increases endothelial cells (EC) actomyosin contraction and permeability indicating the importance of MT in maintaining the EC barrier (Verin et al. [2001]: Cell Mol Physiol 281:L565–L574). Okadaic acid (OA, 2–5 nM), a powerful inhibitor of protein phosphatase 2A (PP2A), significantly potentiates the effect of submaximal concentrations of nocodazole (50–200 nM) on transendothelial electrical resistance (TER) suggesting the involvement of PP2A activity in the MT‐mediated EC barrier regulation. Immunofluorescent staining of EC revealed that in control cells PP2A distributes in a pattern similar to MT. Consistent with these results, we demonstrated that significant amounts of PP2A were present in MT‐enriched EC fractions indicating tight association of PP2A with MT in endothelium. Treatment of EC with OA leads to disappearance of MT‐like PP2A staining suggesting dissociation of PP2A from the MT network. Next, we examined the effect of PP2A inhibition on phosphorylation status of MT‐associated protein tau, which in its unphosphorylated form promotes MT assembly. OA caused significant increases in tau phosphorylation confirming that tau is a substrate for PP2A in endothelium. Immunofluorescent experiments demonstrated that the OA‐induced increases in tau phosphorylation strongly correlated with translocation of phospho‐tau to cell periphery and disassembly of peripheral MT. These results suggest the involvement of PP2A‐mediated tau dephosphorylation in alteration of EC MT structure and highlight the potential importance of PP2A in the regulation of EC the MT cytoskeleton and barrier function.
Funding Information
  • National Heart, Lung, and Blood Institutes (HL67307, HL68062, HL58064)
  • American Heart Association
  • Hungarian Science Research Fund (OTKA T043133)

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