DUSP26 negatively affects the proliferation of epithelial cells, an effect not mediated by dephosphorylation of MAPKs.

Dual specificity phosphatases are characterised by their ability to dephosphorylate both phosphotyrosine and phosphoserine/threonine residues within the one substrate. The aim of this study was to characterise the phosphatase activity of the atypical dual specificity phosphatase, DUSP26 on MAP kinases, and to determine its expression, regulation and function in cancer cells. Overexpression and knockdown of DUSP26 in epithelial cells and in vitro phosphatase assays were used to demonstrate that, contrary to several published reports, DUSP26 does not act as a dual specificity phosphatase on ERK, JNK or p38 MAPKs. However, overexpression of DUSP26 in MCF10A epithelial cells suppressed colony formation and acinar growth in 3D culture, effects dependent on its phosphatase activity, while knockdown of DUSP26 in HOSE17.1 cells enhanced colony formation and cellular proliferation. DUSP26 mRNA expression was reduced in neuroblastoma, brain and ovarian cancer cell lines. Consistent with epigenetic silencing of DUSP26, expression was enhanced by treatment of cells with 5-aza-2-deoxycitidine and trichostatin A, and a CpG island upstream of the DUSP26 transcriptional start site was variably methylated in cancer cell lines. Together, these results help to clarify confusion in the literature relating to DUSP26 substrate specificity and support recent reports that substrates other than MAPKs are the primary substrates of this phosphatase. In addition, they indicate that DUSP26 may function as a tumour suppressor in particular cancers.