Autophagy receptor p62/SQSTM1 signals a complex network that links autophagy-lysosomal system to proteasome. Phosphorylation of p62 on Serine 349 (P-Ser349 p62) is involved in a cell protective, antioxidant pathway. We have shown previously that P-Ser349 p62 occurs and is rapidly degraded during human synovial fibroblasts autophagy. In this work we observed that fingolimod (FTY720), used as a medication for multiple sclerosis, induced coordinated expression of p62, P-Ser349 p62 and inhibitory TFEB form, phosphorylated on Serine 211 (P-Ser211 TFEB), in human synovial fibroblasts. These effects were mimicked and potentiated by proteasome inhibitor MG132. In addition, FTY720 induced autophagic flux, LC3B-II up-regulation, Akt phosphorylation inhibition on Serine 473 but down-regulated TFEB, suggesting stalled autophagy. FTY720 decreased cytoplasmic fraction contained TFEB but induced TFEB in nuclear fraction. FTY720-induced P-Ser211 TFEB was mainly found in membrane fraction. Autophagy and VPS34 kinase inhibitor, autophinib, further increased FTY720-induced P-Ser349 p62 but inhibited concomitant expression of P-Ser211 TFEB. These results suggested that P-Ser211 TFEB expression depends on autophagy. Overexpression of GFP tagged TFEB in HEK293 cells showed concomitant expression of its phosphorylated form on Serine 211, that was down-regulated by autophinib. These results suggested that autophagy might be autoregulated through P-Ser211 TFEB as a negative feedback loop. Of interest, overexpression of p62, p62 phosphorylation mimetic (S349E) mutant and phosphorylation deficient mutant (S349A) in HEK293 cells markedly induced P-Ser211 TFEB. These results showed that p62 is involved in regulation of TFEB phosphorylation on Serine 211 but that this involvement does not depend on p62 phosphorylation on Serine 349.
Keywords: TFEB; autophagy; p62/SQSTM1; phosphorylation/dephosphorylation.
© 2021 The Author(s).