Mechanisms that alter protein phosphatase 2A (PP2A)-dependent lung tumour suppression via the I2PP2A/SET oncoprotein are unknown. We show here that the tumour suppressor ceramide binds I2PP2A/SET selectively in the nucleus and including its K209 and Y122 residues as determined by molecular modelling/simulations and site-directed mutagenesis. Because I2PP2A/SET was found overexpressed, whereas ceramide was downregulated in lung tumours, a sphingolipid analogue drug, FTY720, was identified to mimick ceramide for binding and targeting I2PP2A/SET, leading to PP2A reactivation, lung cancer cell death, and tumour suppression in vivo. Accordingly, while molecular targeting of I2PP2A/SET by stable knockdown prevented further tumour suppression by FTY720, reconstitution of WT-I2PP2A/SET expression restored this process. Mechanistically, targeting I2PP2A/SET by FTY720 mediated PP2A/RIPK1-dependent programmed necrosis (necroptosis), but not by apoptosis. The RIPK1 inhibitor necrostatin and knockdown or genetic loss of RIPK1 prevented growth inhibition by FTY720. Expression of WT- or death-domain-deleted (DDD)-RIPK1, but not the kinase-domain-deleted (KDD)-RIPK1, restored FTY720-mediated necroptosis in RIPK1−/− MEFs. Thus, these data suggest that targeting I2PP2A/SET by FTY720 suppresses lung tumour growth, at least in part, via PP2A activation and necroptosis mediated by the kinase domain of RIPK1.

In this study, the inhibitor 2 of protein phosphatase 2A (I2PP2A) was identified in vitro and in situ as a ceramide-binding protein, which exhibits stereoisomer specificity and fatty acid chain length preference. Site- directed mutagenesis coupled with structural details of I2PP2A suggested that VIK 207-209 residues localized on helix 7 are important for ceramide binding and single mutation of K209D altered this interaction. Notably, I2PP2A-ceramide binding decreased the association between PP2A and the inhibitor, preventing the inhibition of PP2A activity in vitro. In addition, studies in A549 human lung cancer cells revealed that ceramide mediates c-Myc degradation via its PP2A-dependent dephosphorylation at S62, and treatment with okadaic acid and expression of c-Myc mutants with S62A or S62D conversions resulted in resistance to ceramide-mediated degradation. Importantly, whereas down-regulation of I2PP2A enhanced PP2A-mediated c-Myc degradation in response to ceramide, ectopic expression of wild-type I2PP2A but not of its K209D mutant protected this degradation in A549 cells. Moreover, expression of wild-type I2PP2A prevented the growth-inhibitory effects of ceramide both against A549 cells and xenograft-driven tumors in situ and in vivo compared with that in controls. Thus, these results suggest that direct interaction of I2PP2A with ceramide plays important biological roles via the regulation of PP2A activity and signaling, which in turn control ceramide-mediated degradation of c-Myc and antiproliferation.

There is much recent interest in the application of copper-free click chemistry to study a wide range of biological events in vivo and in vitro. Specifically, azide-conjugated fluorescent probes can be used to identify targets which have been modified with bioorthogonal reactive groups. For intracellular applications of this chemistry, the structural and physicochemical properties of the fluorescent azide become increasingly important. Ideal fluorophores should extensively accumulate within cells, have even intracellular distribution, and be free (unbound), allowing them to efficiently participate in bimolecular reactions. We report here on the synthesis and evaluation a set of structurally diverse fluorescent probes to examine their potential usefulness in intracellular click reactions. Total cellular uptake and intracellular distribution profiles were comparatively assessed using both quantitative and qualitative approaches. The intracellular diffusion coefficients were measured using a fluorescence recovery after photobleaching (FRAP)-based method. Many reactive fluorophores exhibited suboptimal properties for intracellular reactions. BODIPY-and TAMRA-based azides had superior cellular accumulation, whereas TAMRA-based probes had the most uniform intracellular distribution and best cytosolic diffusivity. Collectively, these results provide an unbiased comparative evaluation regarding the suitability of azide-linked fluorophores for intracellular click reactions.