Increased expression and phosphorylation of the two S100A9 isoforms in mononuclear cells from patients with systemic lupus erythematosus: A proteomic signature for circulating low-density granulocytes

Proteins differentially expressed in peripheral blood mononuclear cells (PBMCs) from systemic lupus erythematosus (SLE) patients versus Normal controls were identified by 2-DE and MALDI-MS. Thus, S100A9 expression was significantly increased in SLE PBMCs relative to Normal PBMCs at both mRNA and protein levels. Increased S100A9 levels in SLE PBMCs correlated positively with the abnormal presence of low-density granulocytes (LDGs) detected by flow-cytometry in the mononuclear cell fractions. Another set of proteins that were differentially expressed in SLE PBMCs formed S100A9-independent clusters, suggesting that these differences in protein expression are in fact reflecting changes in the abundance of specific cell types. In SLE PBMCs spots of the two S100A9 isoforms, S100A9-l and S100A9-s, and their phosphorylated counterparts were identified and confirmed to be phosphorylated at Thr 113 by MS/MS analyses. In addition, the phorbol ester PMA alone or in combination with ionomycin induced a stronger increase in threonine phosphorylation of S100A9 in SLE than in Normal PBMCs, while the same stimuli caused the opposite effect on phosphorylation and activation of Erk1/2, suggesting the existence of an abnormal S100A9 signaling in SLE PBMCs. Therefore, the expansion and activation of LDGs in SLE seems to underlie this prominent S100A9 signature. © 2011 Elsevier B.V.This work was supported in part by the European Commission in collaboration with the following Funding Agencies: Ministerio de Ciencia e Innovación (MICINN) del Gobierno de España (SAF2005-06056-C02-01, and SAF2008-03685) (to J.S.), Consejería de Economía, Innovación y Ciencia de la Junta de Andalucía (P05-CVI-00908, and P08-CTS-04046) (to J.S.), and Ministerio de Sanidad, Política Social e Igualdad (MSPSI) ISCIII-FIS, Grants: FIS03/0389; UIPY-1465/07; MICINN-ISCIII-FIS-Grant: FIS06/1502, (to M.Z.), and MICINN-CSIC; Grant number: PI-200820I216 (to M.Z.). E.J.P. and R.P. were supported by grant-contracts from MICINN. S.G.R. was supported by a JAE-Doc contract. E.Z. and A.R.-V. were suported by felllowship-contracts from the Consejería de Economía, Innovación y Ciencia de la Junta de Andalucía.Peer Reviewe