SphK1 membrane targeting acts on PKM2 succinylation to fuel synovial hyperplasia in rheumatoid arthritis

Reprogramming of glucose metabolism serves as the driving force behind the abnormal proliferation of fibroblast-like synoviocytes (FLSs), contributing to synovial hyperplasia in Rheumatoid arthritis (RA). However, the underlying mechanisms governing the glycolytic phenotype of RA-FLSs remain unclear. There was a metabolic shift toward glycolysis in synovial tissue of RA patients and CIA rats, as well as in TNF-α-induced MH7A cells. RNA sequencing revealed that pyruvate kinase M2 (PKM2) and sphingosine kinase 1 (SphK1), as differentially expressed glycolysis-related genes, were upregulated in RA synovium. Notably, PKM2 succinylation was significantly elevated in vivo and in vitro, which was attributed to reduced expression of SIRT5 with desuccinylation activity. Succinylation facilitated the conversion of PKM2 from tetramers to dimers. Functionally, this shift promoted glycolysis and cell proliferation, while enhancing mitochondrial fission. Overexpression of SIRT5 weakened the PKM2 succinylation in TNF-α-induced MH7A cells. It was confirmed that Lys311 was a crucial site for PKM2 succinylation modification in MH7A cells. Aberrant SphK1 activation, characterized by increased membrane targeting, was observed both in vivo and in vitro. SphK1 knockdown inhibited this membrane targeting and reduced PKM2 succinylation, concomitant with reactivation of the PKA/PGC1α/SIRT5 signaling axis. In addition, SphK1 inhibition attenuated PKM2 succinylation in synovial tissue and FLSs from CIA rats via upregulating the PKA/PGC1α/SIRT5 signaling axis, resulting in downregulated glycolysis. Overall, PKM2 succinylation promotes glycolysis in RA-FLSs and induces synovial hyperplasia, and this process is regulated by SphK1 membrane targeting, highlighting a novel strategy for safe and effective treatment of RA.