Synergistic anti-tumor effect of 17AAG with the PI3K/mTOR inhibitor NVP-BEZ235 on human melanoma

Drug resistance by MAPK signaling recovery or activation of alternative signaling pathways, such as PI3K/AKT/mTOR, is an important factor that limits the long-term efficacy of targeted therapies in melanoma patients. In the present study, we investigated the phospho-proteomic profile of RTKs and its correlation with downstream signaling pathways in human melanoma. We found that tyrosine kinase receptors expression correlated with the expression of pivotal downstream components of the RAS/RAF/MAPK and PI3K/AKT/mTOR pathways in melanoma cell lines and tumors. We also found high expression of HSP90 and the PI3K/AKT/mTOR pathway proteins, 4EBP1 and AKT compared with healthy tissue and this correlated with poor overall survival of melanoma patients. The combination of the HSP90 inhibitor 17AAG with the PI3K/mTOR inhibitor NVP-BEZ235 showed a synergistic activity decreasing melanoma cell growth, inducing apoptosis and targeting simultaneously the MAPK and PI3K/AKT/mTOR pathways. These results demonstrate that the combination of HSP90 and PI3K/mTOR inhibitors could be an effective therapeutic strategy that target the main survival pathways in melanoma and must be considered to overcome resistance to BRAF inhibitors in melanoma patients.

Genetic Dissection of the Signaling Cascade that Controls Activation of the Shigella Type III Secretion System from the Needle Tip

Many Gram-negative bacterial pathogens use type III secretion systems (T3SSs) for virulence. The Shigella T3SS consists of a hollow needle, made of MxiH and protruding from the bacterial surface, anchored in both bacterial membranes by multimeric protein rings. Atop the needle lies the tip complex (TC), formed by IpaD and IpaB. Upon physical contact with eukaryotic host cells, T3S is initiated leading to formation of a pore in the eukaryotic cell membrane, which is made of IpaB and IpaC. Through the needle and pore channels, further bacterial proteins are translocated inside the host cell to meditate its invasion. IpaD and the needle are implicated in transduction of the host cell-sensing signal to the T3S apparatus. Furthermore, the sensing-competent TC seems formed of 4 IpaDs topped by 1 IpaB. However, nothing further is known about the activation process. To investigate IpaB’s role during T3SS activation, we isolated secretion-deregulated IpaB mutants using random mutagenesis and a genetic screen. We found ipaB point mutations in leading to defects in secretion activation, which sometimes diminished pore insertion and host cell invasion. We also demonstrated IpaB communicates intramolecularly and intermolecularly with IpaD and MxiH within the TC because mutations affecting these interactions impair signal transduction.

Role of the central region of NifL in conformational switches that regulate nitrogen fixation

The NifL regulatory protein is an anti-activator that tightly regulates transcription of genes required for nitrogen fixation in Azotobacter vinelandii by controlling the activity of its partner protein NifA through the formation of a protein-protein complex. NifL modulates the activity of NifA in response to the redox, carbon and nitrogen status to ensure that nitrogen fixation occurs only under physiological conditions that are appropriate for nitrogenase activity. The domain architecture of NifL is similar to that of some histidine protein kinases, with two N-terminal PAS (PER, ARNT, SIM) domains, one of which contains an FAD cofactor that senses the redox status, and a C-terminal domain containing conserved residues that constitutes the nucleotide-binding domain of the GHKL (gyrase, Hsp90, histidine kinase, MutL) superfamily of ATPases. We have evidence that the central region of NifL, which is located between the PAS domains and the C-terminal GHKL nucleotide-binding domain, plays a crucial role in the propagation of signals perceived in response to the redox and fixed nitrogen status and that this region participates in conformational changes that switch NifL between active and inactive states. We have identified a critical arginine residue in the central region of NifL that participates in the conformational switch that activates NifL. Mutations in the central region of NifL that disable the redox-sensing function of NifL but leave the protein competent to respond to the nitrogen signal conveyed by the signal transduction protein GlnK have also been isolated. Our results suggest that the topological relationship between the central region and the GHKL domain may alter as a consequence of conformational changes induced in response to signal perception.

Nitrogen fixation: key genetic regulatory mechanisms

The necessity to respond to the level of fixed nitrogen and external oxygen concentrations and to provide sufficient energy for nitrogen fixation imposes common regulatory principles amongst diazotrophs. The NifL-NifA system in Azotobacter vinelandii integrates the signals of redox, fixed-nitrogen and carbon status to regulate nif transcription. Multidomain signalling interactions between NifL and NifA are modulated by redox changes, ligand binding and interaction with the signal-transduction protein GlnK. Under adverse redox conditions (excess oxygen) or when fixed nitrogen is in excess, NifL forms a complex with NifA in which transcriptional activation is prevented. Oxidized NifL forms a binary complex with NifA to inhibit NifA activity. When fixed nitrogen is in excess, the non-covalently modified form of GlnK interacts with NifL to promote the formation of a GlnK-NifL-NifA ternary complex. When the cell re-encounters favourable conditions for nitrogen fixation, it is necessary to deactivate the signals to ensure that the NifL-NifA complex is dissociated so that NifA is free to activate transcription. This is achieved through interactions with 2-oxoglutarate, a key metabolic signal of the carbon status, which binds to the N-terminal GAF (cGMP-specific and stimulated phosphodiesterases, Anabaena adenylate cyclases and Escherichia coli FhlA) domain of NifA.