Distinct roles for the α, β and γ1 isoforms of protein phosphatase 1 in the outside-in αIIbβ3 integrin signalling-dependent functions

Although protein kinases and phosphatases participate in integrin alpha(IIb)beta(3) signalling, whether integrin functions are regulated by the catalytic subunit of protein phosphatase 1 (PP1c) isoforms are unclear. We show that siRNA mediated knockdown of all PP1c isoforms (alpha, beta and gamma 1) in 293 alpha(IIb)beta(3) cells decreased adhesion to immobilised fibrinogen and fibrin clot retraction. Selective knockdown of only PP1c gamma 1 did not alter adhesion or clot retraction, while depletion of PP1c beta decreased both functions. Unexpectedly, knockdown of PP1c alpha enhanced alpha(IIb)beta(3) adhesion to fibrinogen and clot retraction. Protein interaction studies revealed that all PP1c isoforms can interact with the integrin alpha(IIb) subunit. Phospho-profiling studies revealed an enhanced activation of mitogen-activated protein kinase (MAPK) p38 in the PP1c alpha depleted cells. Enhanced adhesive phenotype displayed by the PP1c alpha-depleted 293 alpha(IIb)beta(3) cells was blocked by pharmacological inhibition of p38. Conversely, the decreased adhesion of PP1c alpha overexpressing cells was rescued by the expression of constitutively active p38 alpha or p38 gamma. Thus, PP1c isoforms have distinct contribution to the outside-in alpha(IIb)beta(3) signalling-dependent functions in 293 alpha(IIb)beta(3) cells. Moreover, PP1c alpha negatively regulates integrin function by suppressing the p38 pathway.