Tumorigenic transformation by CPI-17 through inhibition of a merlin phosphatase

The tumour suppressor protein merlin ( encoded by the neurofibromatosis type 2 gene NF2) is an important regulator of proliferation in many cell and tissue types(1-4). Merlin is activated by dephosphorylation at serine 518 (S518), which occurs on serum withdrawal or on cell - cell or cell - matrix contact(5,6). However, the relevant phosphatase that activates merlin's tumour suppressor function is unknown. Here we identify this enzyme as the myosin phosphatase (MYPT-1-PP1 delta). The cellular MYPT-1-PP1 delta - specific inhibitor CPI-17 causes a loss of merlin function characterized by merlin phosphorylation, Ras activation and transformation. Constitutively active merlin (S518A) reverses CPI-17-induced transformation, showing that merlin is the decisive substrate of MYPT-1-PP1 delta in tumour suppression. In addition we show that CPI-17 levels are raised in several human tumour cell lines and that the downregulation of CPI-17 induces merlin dephosphorylation, inhibits Ras activation and abolishes the transformed phenotype. MYPT-1-PP1 delta and its substrate merlin are part of a previously undescribed tumour suppressor cascade that can be hindered in two ways, by mutation of the NF2 gene and by upregulation of the oncoprotein CPI-17.