Redox takes control.

A study of two enzymes in the brain reveals new insights into how redox reactions regulate the activity of protein kinases.

PM534, an Optimized Target-Protein Interaction Strategy through the Colchicine Site of Tubulin.

Targeting microtubules is the most effective wide-spectrum pharmacological strategy in antitumoral chemotherapy, and current research focuses on reducing main drawbacks: neurotoxicity and resistance. PM534 is a novel synthetic compound derived from the Structure-Activity-Relationship study on the natural molecule PM742, isolated from the sponge of the order Lithistida, family Theonellidae, genus Discodermia (du Bocage 1869). PM534 targets the entire colchicine binding domain of tubulin, covering four of the five centers of the pharmacophore model. Its nanomolar affinity and high retention time modulate a strikingly high antitumor activity that efficiently overrides two resistance mechanisms in cells (detoxification pumps and tubulin ßIII isotype overexpression). Furthermore, PM534 induces significant inhibition of tumor growth in mouse xenograft models of human non-small cell lung cancer. Our results present PM534, a highly effective new compound in the preclinical evaluation that is currently in its first human Phase I clinical trial.

An allosteric switch between the activation loop and a c-terminal palindromic phospho-motif controls c-Src function.

Autophosphorylation controls the transition between discrete functional and conformational states in protein kinases, yet the structural and molecular determinants underlying this fundamental process remain unclear. Here we show that c-terminal Tyr 530 is a de facto c-Src autophosphorylation site with slow time-resolution kinetics and a strong intermolecular component. On the contrary, activation-loop Tyr 419 undergoes faster kinetics and a cis-to-trans phosphorylation switch that controls c-terminal Tyr 530 autophosphorylation, enzyme specificity, and strikingly, c-Src non-catalytic function as a substrate. In line with this, we visualize by X-ray crystallography a snapshot of Tyr 530 intermolecular autophosphorylation. In an asymmetric arrangement of both catalytic domains, a c-terminal palindromic phospho-motif flanking Tyr 530 on the substrate molecule engages the G-loop of the active kinase adopting a position ready for entry into the catalytic cleft. Perturbation of the phospho-motif accounts for c-Src dysfunction as indicated by viral and colorectal cancer (CRC)-associated c-terminal deleted variants. We show that c-terminal residues 531 to 536 are required for c-Src Tyr 530 autophosphorylation, and such a detrimental effect is caused by the substrate molecule inhibiting allosterically the active kinase. Our work reveals a crosstalk between the activation and c-terminal segments that control the allosteric interplay between substrate- and enzyme-acting kinases during autophosphorylation.

Structural and dynamic determinants for highly selective RET kinase inhibition reveal cryptic druggability.

INTRODUCTION: The structural and dynamic determinants that confer highly selective RET kinase inhibition are poorly understood. OBJECTIVES: To explore the druggability landscape of the RET active site in order to uncover structural and dynamic vulnerabilities that can be therapeutically exploited. METHODS: We apply an integrated structural, computational and biochemical approach in order to explore the druggability landscape of the RET active site. RESULTS: We demonstrate that the that the druggability landscape of the RET active site is determined by the conformational setting of the ATP-binding (P-) loop and its coordination with the αC helix. Open and intermediate P-loop structures display additional druggable vulnerabilities within the active site that were not exploited by first generation RET inhibitors. We identify a cryptic pocket adjacent to the catalytic lysine formed by K758, L760, E768 and L772, that we name the post-lysine pocket, with higher druggability potential than the adenine-binding site and with important implications in the regulation of the phospho-tyrosine kinase activity. Crystal structure and simulation data show that the binding mode of highly-selective RET kinase inhibitors LOXO-292 and BLU-667 is controlled by a synchronous open P-loop and αC-in configuration that allows accessibility to the post-lysine pocket. Molecular dynamics simulations show that these inhibitors efficiently occupy the post-lysine pocket with high stability through the simulation time-scale (300 ns), with both inhibitors forming hydrophobic contacts further stabilized by pi-cation interactions with the catalytic K758. Engineered mutants targeting the post-lysine pocket impact on inhibitor binding and sensitivity, as well as RET tyrosine kinase activity. CONCLUSIONS: The identification of the post-lysine pocket as a new druggable vulnerability in the RET kinase and its exploitation by second generation RET inhibitors have important implications for future drug design and the development of personalized therapies for patients with RET-driven cancers.

Hybrid histidine kinase activation by cyclic di-GMP-mediated domain liberation.

Cytosolic hybrid histidine kinases (HHKs) constitute major signaling nodes that control various biological processes, but their input signals and how these are processed are largely unknown. In Caulobacter crescentus, the HHK ShkA is essential for accurate timing of the G1-S cell cycle transition and is regulated by the corresponding increase in the level of the second messenger c-di-GMP. Here, we use a combination of X-ray crystallography, NMR spectroscopy, functional analyses, and kinetic modeling to reveal the regulatory mechanism of ShkA. In the absence of c-di-GMP, ShkA predominantly adopts a compact domain arrangement that is catalytically inactive. C-di-GMP binds to the dedicated pseudoreceiver domain Rec1, thereby liberating the canonical Rec2 domain from its central position where it obstructs the large-scale motions required for catalysis. Thus, c-di-GMP cannot only stabilize domain interactions, but also engage in domain dissociation to allosterically invoke a downstream effect. Enzyme kinetics data are consistent with conformational selection of the ensemble of active domain constellations by the ligand and show that autophosphorylation is a reversible process.

Novel targeted therapeutics for MEN2.

The rearranged during transfection (RET) proto-oncogene was recognized as the multiple endocrine neoplasia type 2 (MEN2) causing gene in 1993. Since then, much effort has been put into a clear understanding of its oncogenic signaling, its biochemical function and ways to block its aberrant activation in MEN2 and related cancers. Several small molecules have been designed, developed or redirected as RET inhibitors for the treatment of MEN2 and sporadic MTC. However, current drugs are mostly active against several other kinases, as they were not originally developed for RET. This limits efficacy and poses safety issues. Therefore, there is still much to do to improve targeted MEN2 treatments. New, more potent and selective molecules, or combinatorial strategies may lead to more effective therapies in the near future. Here, we review the rationale for RET targeting in MEN2, the use of currently available drugs and novel preclinical and clinical RET inhibitor candidates.

Structure and function of RET in multiple endocrine neoplasia type 2.

It has been twenty-five years since the discovery of oncogenic germline RET mutations as the cause of multiple endocrine neoplasia type 2 (MEN2). Intensive work over the last two and a half decades on RET genetics, signaling and cell biology has provided the current bases for the genotype-phenotype and functional correlations within this cancer syndrome. On the contrary, the structural and molecular basis for RET tyrosine kinase domain activation and oncogenic deregulation has remained largely elusive. Recent studies with a strong crystallographic and biochemical focus have started to elucidate key insights into such molecular and atomic details revealing unexpected and private mechanisms of actions and molecular determinants not previously envisioned. This review focuses on the structure and function of the RET receptor, and in particular, on what a more detailed view of the protein itself and what the current structural and molecular information tell us about the genotype and phenotype relationships in the cancer syndrome MEN2.

RET Functions as a Dual-Specificity Kinase that Requires Allosteric Inputs from Juxtamembrane Elements.

Receptor tyrosine kinases exhibit a variety of activation mechanisms despite highly homologous catalytic domains. Such diversity arises through coupling of extracellular ligand-binding portions with highly variable intracellular sequences flanking the tyrosine kinase domain and specific patterns of autophosphorylation sites. Here, we show that the juxtamembrane (JM) segment enhances RET catalytic domain activity through Y687. This phospho-site is also required by the JM region to rescue an otherwise catalytically deficient RET activation-loop mutant lacking tyrosines. Structure-function analyses identified interactions between the JM hinge, αC helix, and an unconventional activation-loop serine phosphorylation site that engages the HRD motif and promotes phospho-tyrosine conformational accessibility and regulatory spine assembly. We demonstrate that this phospho-S909 arises from an intrinsic RET dual-specificity kinase activity and show that an equivalent serine is required for RET signaling in Drosophila. Our findings reveal dual-specificity and allosteric components for the mechanism of RET activation and signaling with direct implications for drug discovery.

Cyclic di-GMP mediates a histidine kinase/phosphatase switch by noncovalent domain cross-linking.

Histidine kinases are key components of regulatory networks in bacteria. Although many of these enzymes are bifunctional, mediating both phosphorylation and dephosphorylation of downstream targets, the molecular details of this central regulatory switch are unclear. We showed recently that the universal second messenger cyclic di-guanosine monophosphate (c-di-GMP) drives Caulobacter crescentus cell cycle progression by forcing the cell cycle kinase CckA from its default kinase into phosphatase mode. We use a combination of structure determination, modeling, and functional analysis to demonstrate that c-di-GMP reciprocally regulates the two antagonistic CckA activities through noncovalent cross-linking of the catalytic domain with the dimerization histidine phosphotransfer (DHp) domain. We demonstrate that both c-di-GMP and ADP (adenosine diphosphate) promote phosphatase activity and propose that c-di-GMP stabilizes the ADP-bound quaternary structure, which allows the receiver domain to access the dimeric DHp stem for dephosphorylation. In silico analyses predict that c-di-GMP control is widespread among bacterial histidine kinases, arguing that it can replace or modulate canonical transmembrane signaling.

Oncogenic RET kinase domain mutations perturb the autophosphorylation trajectory by enhancing substrate presentation in trans.

To decipher the molecular basis for RET kinase activation and oncogenic deregulation, we defined the temporal sequence of RET autophosphorylation by label-free quantitative mass spectrometry. Early autophosphorylation sites map to regions flanking the kinase domain core, while sites within the activation loop only form at later time points. Comparison with oncogenic RET kinase revealed that late autophosphorylation sites become phosphorylated much earlier than wild-type RET, which is due to a combination of an enhanced enzymatic activity, increased ATP affinity, and surprisingly, by providing a better intermolecular substrate. Structural analysis of oncogenic M918T and wild-type RET kinase domains reveal a cis-inhibitory mechanism involving tethering contacts between the glycine-rich loop, activation loop, and αC-helix. Tether mutations only affected substrate presentation but perturbed the autophosphorylation trajectory similar to oncogenic mutations. This study reveals an unappreciated role for oncogenic RET kinase mutations in promoting intermolecular autophosphorylation by enhancing substrate presentation.

Ponatinib is a potent inhibitor of wild-type and drug-resistant gatekeeper mutant RET kinase.

RET kinase is aberrantly activated in thyroid cancers and in rare cases of lung and colon cancer, and has been validated as a molecular target in these tumors. Vandetanib was recently approved for the treatment of medullary thyroid cancer. However, vandetanib is ineffective in vitro against RET mutants carrying bulky aminoacids at position 804, the gatekeeper residue, similarly to drug-resistant BCR-ABL mutants in chronic myeloid leukemia. Ponatinib is a multi-target kinase inhibitor that was recently approved for treatment-refractory Philadelphia-positive leukemia. We show here potent inhibition of oncogenic RET by ponatinib, including the drug-insensitive V804M/L mutants. Ponatinib inhibited the growth of RET+ and BCR-ABL+ cells with similar potency, while not affecting RET-negative cells. Both in biochemical and in cellular assays ponatinib compared favorably with known RET inhibitors, such as vandetanib, cabozantinib, sorafenib, sunitinib and motesanib, used as reference compounds. We suggest that ponatinib should be considered for the treatment of RET+ tumors, in particular those expressing vandetanib-resistant V804M/L mutations.

GDNF-RET signaling in ER-positive breast cancers is a key determinant of response and resistance to aromatase inhibitors.

Most breast cancers at diagnosis are estrogen receptor-positive (ER(+)) and depend on estrogen for growth and survival. Blocking estrogen biosynthesis by aromatase inhibitors has therefore become a first-line endocrine therapy for postmenopausal women with ER(+) breast cancers. Despite providing substantial improvements in patient outcome, aromatase inhibitor resistance remains a major clinical challenge. The receptor tyrosine kinase, RET, and its coreceptor, GFRα1, are upregulated in a subset of ER(+) breast cancers, and the RET ligand, glial-derived neurotrophic factor (GDNF) is upregulated by inflammatory cytokines. Here, we report the findings of a multidisciplinary strategy to address the impact of GDNF-RET signaling in the response to aromatase inhibitor treatment. In breast cancer cells in two-dimensional and three-dimensional culture, GDNF-mediated RET signaling is enhanced in a model of aromatase inhibitor resistance. Furthermore, GDNF-RET signaling promoted the survival of aromatase inhibitor-resistant cells and elicited resistance in aromatase inhibitor-sensitive cells. Both these effects were selectively reverted by the RET kinase inhibitor, NVP-BBT594. Gene expression profiling in ER(+) cancers defined a proliferation-independent GDNF response signature that prognosed poor patient outcome and, more importantly, predicted poor response to aromatase inhibitor treatment with the development of resistance. We validated these findings by showing increased RET protein expression levels in an independent cohort of aromatase inhibitor-resistant patient specimens. Together, our results establish GDNF-RET signaling as a rational therapeutic target to combat or delay the onset of aromatase inhibitor resistance in breast cancer.

Focal adhesion kinase (FAK) binds RET kinase via its FERM domain, priming a direct and reciprocal RET-FAK transactivation mechanism.

Whether RET is able to directly phosphorylate and activate downstream targets independently of the binding of proteins that contain Src homology 2 or phosphotyrosine binding domains and whether mechanisms in trans by cytoplasmic kinases can modulate RET function and signaling remain largely unexplored. In this study, oligopeptide arrays were used to screen substrates directly phosphorylated by purified recombinant wild-type and oncogenic RET kinase domain in the presence or absence of small molecule inhibitors. The results of the peptide array were validated by enzyme kinetics, in vitro kinase, and cell-based experiments. The identification of focal adhesion kinase (FAK) as a direct substrate for RET kinase revealed (i) a RET-FAK transactivation mechanism consisting of direct phosphorylation of FAK Tyr-576/577 by RET and a reciprocal phosphorylation of RET by FAK, which crucially is able to rescue the kinase-impaired RET K758M mutant and (ii) that FAK binds RET via its FERM domain. Interestingly, this interaction is abolished upon RET phosphorylation, indicating that RET binding to the FERM domain of FAK is a priming step for RET-FAK transactivation. Finally, our data indicate that FAK inhibitors could be used as potential therapeutic agents for patients with multiple endocrine neoplasia type 2 tumors because both, treatment with the FAK kinase inhibitor NVP-TAE226 and FAK down-regulation by siRNA reduced RET phosphorylation and signaling as well as the proliferation and survival of tumor and transfected cell lines expressing oncogenic RET.

RET in breast cancer: functional and therapeutic implications.

Recent studies demonstrate that the receptor tyrosine kinase RET is overexpressed in a subset of ER-positive breast cancers and that crosstalk between RET and ER is important in responses to endocrine therapy. The development of small molecular inhibitors that target RET allows the opportunity to consider combination therapies as a strategy to improve response to treatment and to prevent and combat endocrine resistance. This review discusses: (i) the current knowledge about RET, its co-receptors and ligands in breast cancer; (ii) the breast cancer clinical trials involving agents that target RET; and (iii) the challenges that remain in terms of specificity of available inhibitors and in understanding the complex molecular mechanisms that underlie the resistance to endocrine therapy.

A role for glial cell derived neurotrophic factor induced expression by inflammatory cytokines and RET/GFR alpha 1 receptor up-regulation in breast cancer.

By screening a tissue microarray of invasive breast tumors, we have shown that the receptor tyrosine kinase RET (REarranged during Transfection) and its coreceptor GFR alpha 1 (GDNF receptor family alpha-1) are overexpressed in a subset of estrogen receptor-positive tumors. Germ line-activating oncogenic mutations in RET allow this receptor to signal independently of GFR alpha 1 and its ligand glial cell-derived neurotrophic factor (GDNF) to promote a spectrum of endocrine neoplasias. However, it is not known whether tumor progression can also be driven by receptor overexpression and whether expression of GDNF, as has been suggested for other neurotrophic factors, is regulated in response to the inflammatory microenvironment surrounding many epithelial cancers. Here, we show that GDNF stimulation of RET(+)/GFR alpha 1(+) MCF7 breast cancer cells in vitro enhanced cell proliferation and survival, and promoted cell scattering. Moreover, in tumor xenografts, GDNF expression was found to be up-regulated on the infiltrating endogenous fibroblasts and to a lesser extent by the tumor cells themselves. Finally, the inflammatory cytokines tumor necrosis factor-alpha and interleukin-1 beta, which are involved in tumor promotion and development, were found to act synergistically to up-regulate GDNF expression in both fibroblasts and tumor cells. These data indicate that GDNF can act as an important component of the inflammatory response in breast cancers and that its effects are mediated by both paracrine and autocrine stimulation of tumor cells via signaling through the RET and GFR alpha 1 receptors.

Sorafenib functions to potently suppress RET tyrosine kinase activity by direct enzymatic inhibition and promoting RET lysosomal degradation independent of proteasomal targeting.

Germ line missense mutations in the RET (rearranged during transfection) oncogene are the cause of multiple endocrine neoplasia, type 2 (MEN2), but at present surgery is the only treatment available for MEN2 patients. In this study, the ability of Sorafenib (BAY 43-9006) to act as a RET inhibitor was investigated. Sorafenib inhibited the activity of purified recombinant kinase domain of wild type RET and RET(V804M) with IC(50) values of 5.9 and 7.9 nm, respectively. Interestingly, these values were 6-7-fold lower than the IC(50) for the inhibition of B-RAF(V600E). In cell-based assays, Sorafenib inhibited the kinase activity and signaling of wild type and oncogenic RET in MEN2 tumor and established cell lines at a concentration between 15 and 150 nm. In contrast, inhibition of oncogenic B-RAF- or epidermal growth factor-induced ERK1/2 phosphorylation required micromolar concentrations of Sorafenib demonstrating the high specificity of this drug in targeting RET. Moreover, prolonged exposure to Sorafenib resulted in inhibition of cell proliferation and RET protein degradation. Using lysosomal and proteasomal inhibitors, we demonstrate that Sorafenib induces RET lysosomal degradation independent of proteasomal targeting. Furthermore, we provide a structural model of the Sorafenib.RET complex in which Sorafenib binds to and induces the DFG(out) conformation of the RET kinase domain. These results strengthen the argument that Sorafenib may be effective in the treatment of MEN2 patients. In addition, because inhibition of RET is not impaired by mutation of the Val(804) gatekeeper residue, MEN2 tumors may be less susceptible to acquired Sorafenib resistance.

Ras/ERK1/2-mediated STAT3 Ser727 phosphorylation by familial medullary thyroid carcinoma-associated RET mutants induces full activation of STAT3 and is required for c-fos promoter activation, cell mitogenicity, and transformation.

The precise role of STAT3 Ser(727) phosphorylation in RET-mediated cell transformation and oncogenesis is not well understood. In this study, we have shown that familial medullary thyroid carcinoma (FMTC) mutants RET(Y791F) and RET(S891A) induced, in addition to Tyr(705) phosphorylation, constitutive STAT3 Ser(727) phosphorylation. Using inhibitors and dominant negative constructs, we have demonstrated that RET(Y791F) and RET(S891A) induce STAT3 Ser(727) phosphorylation via a canonical Ras/ERK1/2 pathway and that integration of the Ras/ERK1/2/ELK-1 and STAT3 pathways was required for up-regulation of the c-fos promoter by FMTC-RET. Moreover, inhibition of ERK1/2 had a more severe effect on cell proliferation and cell phenotype in HEK293 cells expressing RET(S891A) compared with control and RET(WT)-transfected cells. The transforming activity of RET(Y791F) and RET(S891A) in NIH-3T3 cells was also inhibited by U0126, indicating a role of the ERK1/2 pathway in RET-mediated transformation. To investigate the biological significance of Ras/ERK1/2-induced STAT3 Ser(727) phosphorylation for cell proliferation and transformation, N-Ras-transformed NIH-3T3 cells were employed. These cells displayed elevated levels of activated ERK1/2 and Ser(727)-phosphorylated STAT3, which were inhibited by treatment with U0126. Importantly, overexpression of STAT3, in which the Ser(727) was mutated into Ala (STAT3(S727A)), rescued the transformed phenotype of N-Ras-transformed cells. Immunohistochemistry in tumor samples from FMTC patients showed strong nuclear staining of phosphorylated ERK1/2 and Ser(727) STAT3. These data show that FMTC-RET mutants activate a Ras/ERK1/2/STAT3 Ser(727) pathway, which plays an important role in cell mitogenicity and transformation.

Current concepts in RET-related genetics, signaling and therapeutics.

The receptor tyrosine kinase RET is expressed in cell lineages derived from the neural crest and has a key role in regulating cell proliferation, migration, differentiation and survival during embryogenesis. Germline and somatic mutations in RET that produce constitutively activated receptors cause the cancer syndrome multiple endocrine neoplasia type 2 and several endocrine and neural-crest-derived tumors, whereas mutations resulting in nonfunctional RET or lower expression of RET are found in individuals affected with Hirschsprung disease. This review focuses on the genetics and molecular mechanisms underlying the different inherited human neural-crest-related disorders in which RET dysfunction has a crucial role and discusses RET as a potential therapeutic target.

Identifying candidate Hirschsprung disease-associated RET variants.

Patients with sporadic Hirschsprung disease (HSCR) show increased allele sharing at markers in the 5' region of the RET locus, indicating the presence of a common ancestral RET mutation. In a previous study, we found a haplotype of six SNPs that was transmitted to 55.6% of our patients, whereas it was present in only 16.2% of the controls we used. Among the patients with that haplotype, 90.8% had it on both chromosomes, which led to a much higher risk of developing HSCR than when the haplotype occurred heterozygously. To more precisely define the HSCR-associated region and to identify candidate disease-associated variant(s), we sequenced the shared common haplotype region from 10 kb upstream of the RET gene through intron 1 and exon 2 (in total, 33 kb) in a patient homozygous for the common risk haplotype and in a control individual homozygous for the most common nonrisk haplotype. A comparison of these sequences revealed 86 sequence differences. Of these 86 variations, 8 proved to be in regions highly conserved among different vertebrates and within putative transcription factor binding sites. We therefore considered these as candidate disease-associated variants. Subsequent genotyping of these eight variants revealed a strong disease association for six of the eight markers. These six markers also showed the largest distortions in allele transmission. Interspecies comparison showed that only one of the six variations was located in a region also conserved in a nonmammalian species, making it the most likely candidate HSCR-associated variant.

RET-familial medullary thyroid carcinoma mutants Y791F and S891A activate a Src/JAK/STAT3 pathway, independent of glial cell line-derived neurotrophic factor.

The RET proto-oncogene encodes a receptor tyrosine kinase whose dysfunction plays a crucial role in the development of several neural crest disorders. Distinct activating RET mutations cause multiple endocrine neoplasia type 2A (MEN2A), type 2B (MEN2B), and familial medullary thyroid carcinoma (FMTC). Despite clear correlations between the mutations found in these cancer syndromes and their phenotypes, the molecular mechanisms connecting the mutated receptor to the different disease phenotypes are far from completely understood. Luciferase reporter assays in combination with immunoprecipitations, and Western and immunohistochemistry analyses were done in order to characterize the signaling properties of two FMTC-associated RET mutations, Y791F and S891A, respectively, both affecting the tyrosine kinase domain of the receptor. We show that these RET-FMTC mutants are monomeric receptors which are autophosphorylated and activated independently of glial cell line-derived neurotrophic factor. Moreover, we show that the dysfunctional signaling properties of these mutants, when compared with wild-type RET, involve constitutive activation of signal transducers and activators of transcription 3 (STAT3). Furthermore, we show that STAT3 activation is mediated by a signaling pathway involving Src, JAK1, and JAK2, differing from STAT3 activation promoted by RET(C634R) which was previously found to be independent of Src and JAKs. Three-dimensional modeling of the RET catalytic domain suggested that the structural changes promoted by the respective amino acids substitutions lead to a more accessible substrate and ATP-binding monomeric conformation. Finally, immunohistochemical analysis of FMTC tumor samples support the in vitro data, because nuclear localized, Y705-phosphorylated STAT3, as well as a high degree of RET expression at the plasma membrane was observed.