Regulation of RNA polymerase III transcription by the tumour suppressor p53

The tumour suppressor protein p53 is inactivated in a large proportion of human cancers. p53 controls growth and proliferation, through multiple mechanisms, including the ability to regulate transcription. p53 can function as a general repressor of RNA polymerase (pol) III transcription. Pol III is responsible for transcribing a variety of small stable RNAs including tRNA, 5S rRNA, and the adenoviral VA1 RNA. p53 targets TFIIIB, a TBP-containing factor that is essential for recruitment of pol III to its templates. This study investigates the TFIIIB-p53 interaction, and how it serves to regulate pol III-transcribed genes. p53 does not disrupt the interaction between the TFIIIB subunits, TBP and Brf1. It does, however, prevent association of TFIIIB with the pol III specific factor TFIIIC2, and pol III itself Immobilised template and chromatin immunoprecipitation analysis show that p53 prevents the recruitment of TFIIIB, but not TFIIIC2, to the tRNA promoter. Pol III repression cannot be attributed to one clearly defined region of p53. Sequence within both the N- and C-termini are essential, and the central core domain is also implicated in playing a role. Evidence is provided here that p53-mediated repression of tRNA genes occurs via a trichostatin A-sensitive histone deacetylase independent mechanism. p53 is deregulated or mutated in the vast majority of human cancers. Individuals who inherit mutant forms of p53 can suffer from Li-Fraumeni Syndrome (LFS), a familial cancer syndrome associated with a range of malignancies. Here is shown that pol III transcriptional activity is often highly elevated in primary fibroblasts from Li-Fraumeni patients, especially if the germline p53 mutation is followed by loss of the remaining allele. Deregulation of p53 function through the action of various oncoproteins can also contribute to carcinogenesis. E6 from human papilloma virus can bind to p53 and neutralise its function and E6 releases pol III from p53-mediated repression. Induction of the Mdm2 regulating protein p14ARF results in a p53-mediated repression of pol III activity. p53 does not interfere with normal cellular growth and development; it is, however, rapidly induced in response to cellular stress. Here it is shown that the DNA-damaging agent MMS provokes a p53 response that correlates with a dramatic pol III transcriptional repression. Collectively the data presented here support the idea that p53 can directly repress pol III transcription through interactions with the basal pol III machinery. p53 status can have a profound effect upon pol III activity; the precise circumstances under which such control becomes physiologically important however, remains to be determined

p53 directly regulates the glycosidase FUCA1 to promote chemotherapy-induced cell death

p53 is a central factor in tumor suppression as exemplified by its frequent loss in human cancer. p53 exerts its tumor suppressive effects in multiple ways, but the ability to invoke the eradication of damaged cells by programmed cell death is considered a key factor. The ways in which p53 promotes cell death can involve direct activation or engagement of the cell death machinery, or can be via indirect mechanisms, for example though regulation of ER stress and autophagy. We present here another level of control in p53-mediated tumor suppression by showing that p53 activates the glycosidase, FUCA1, a modulator of N-linked glycosylation. We show that p53 transcriptionally activates FUCA1 and that p53 modulates fucosidase activity via FUCA1 up-regulation. Importantly, we also report that chemotherapeutic drugs induce FUCA1 and fucosidase activity in a p53-dependent manner. In this context, while we found that over-expression of FUCA1 does not induce cell death, RNAi-mediated knockdown of endogenous FUCA1 significantly attenuates p53-dependent, chemotherapy-induced apoptotic death. In summary, these findings add an additional component to p53s tumor suppressive response and highlight another mechanism by which the tumor suppressor controls programmed cell death that could potentially be exploited for cancer therapy

Activation of p73 and induction of Noxa by DNA damage requires NF-kappa B

Although the transcription factor NF-κB is most clearly linked to the inhibition of extrinsic apoptotic signals such as TNFα by upregulating known anti-apoptotic genes, NF-κB has also been proposed to be required for p53-induced apoptosis in transformed cells. However, the involvement of NF-κB in this process is poorly understood. Here we investigate this mechanism and show that in transformed MEFs lacking NF-κB (p65-null cells) genotoxin-induced cytochrome c release is compromised. To further address how NF-κB contributes to apoptosis, gene profiling by microarray analysis of MEFs was performed, revealing that NF-κB is required for expression of Noxa, a pro-apoptotic BH3-only protein that is induced by genotoxins and that triggers cytochrome c release. Moreover, we find that in the absence of NF-κB, genotoxin treatment cannot induce Noxa mRNA expression. Noxa expression had been shown to be regulated directly by genes of the p53 family, like p73 and p63, following genotoxin treatment. Here we show that p73 is activated after genotoxin treatment only in the presence of NF-κB and that p73 induces Noxa gene expression through the p53 element in the promoter. Together our data provides an explanation for how loss of NF-κB abrogates genotoxin-induced apoptosis

LIM kinase inhibitors disrupt mitotic microtubule organization and impair tumor cell proliferation

The actin and microtubule cytoskeletons are critically important for cancer cell proliferation, and drugs that target microtubules are widely-used cancer therapies. However, their utility is compromised by toxicities due to dose and exposure. To overcome these issues, we characterized how inhibition of the actin and microtubule cytoskeleton regulatory LIM kinases could be used in drug combinations to increase efficacy. A previously-described LIMK inhibitor (LIMKi) induced dose-dependent microtubule alterations that resulted in significant mitotic defects, and increased the cytotoxic potency of microtubule polymerization inhibitors. By combining LIMKi with 366 compounds from the GSK Published Kinase Inhibitor Set, effective combinations were identified with kinase inhibitors including EGFR, p38 and Raf. These findings encouraged a drug discovery effort that led to development of CRT0105446 and CRT0105950, which potently block LIMK1 and LIMK2 activity in vitro, and inhibit cofilin phosphorylation and increase αTubulin acetylation in cells. CRT0105446 and CRT0105950 were screened against 656 cancer cell lines, and rhabdomyosarcoma, neuroblastoma and kidney cancer cells were identified as significantly sensitive to both LIMK inhibitors. These large-scale screens have identified effective LIMK inhibitor drug combinations and sensitive cancer types. In addition, the LIMK inhibitory compounds CRT0105446 and CRT0105950 will enable further development of LIMK-targeted cancer therapy

A novel small molecule inhibitor of MRCK prevents radiation-driven invasion in glioblastoma

Glioblastoma (GBM) is an aggressive and incurable primary brain tumor that causes severe neurological, cognitive, and psychological symptoms. Symptoms are caused and exacerbated by the infiltrative properties of GBM cells, which enable them to pervade the healthy brain and disrupt normal function. Recent research has indicated that, while radiotherapy (RT) remains the most effective component of multimodality therapy for GBM patients, it can provoke a more infiltrative phenotype in GBM cells that survive treatment. Here we demonstrate an essential role of the actin-myosin regulatory kinase myotonic dystrophy kinase-related CDC42- binding kinase (MRCK) in mediating the pro-invasive effects of radiation. MRCK-mediated invasion occurred via downstream signaling to effector molecules MYPT1 and MLC2. MRCK was activated by clinically relevant doses per fraction of radiation, and this activation was concomitant with an increase in GBM cell motility and invasion. Furthermore, ablation of MRCK activity either by RNAi or by inhibition with the novel small molecule inhibitor BDP-9066 prevented radiation-driven increases in motility both in vitro and in a clinically relevant orthotopic xenograft model of GBM. Crucially, treatment with BDP-9066 in combination with RT significantly increased survival in this model and markedly reduced infiltration of the contralateral cerebral hemisphere

Discovery of potent and selective MRCK inhibitors with therapeutic effect on skin cancer

The myotonic dystrophy-related Cdc42-binding kinases MRCKα and MRCKβ contribute to the regulation of actin-myosin cytoskeleton organization and dynamics, acting in concert with the Rho-associated coiled-coil kinases ROCK1 and ROCK2. The absence of highly potent and selective MRCK inhibitors has resulted in relatively little knowledge of the potential roles of these kinases in cancer. Here we report the discovery of the azaindole compounds BDP8900 and BDP9066 as potent and selective MRCK inhibitors that reduce substrate phosphorylation, leading to morphological changes in cancer cells along with inhibition of their motility and invasive character. In over 750 human cancer cell lines tested, BDP8900 and BDP9066 displayed consistent anti-proliferative effects with greatest activity in hematological cancer cells. Mass spectrometry identified MRCKα S1003 as an autophosphorylation site, enabling development of a phosphorylation-sensitive antibody tool to report on MRCKα status in tumor specimens. In a two-stage chemical carcinogenesis model of murine squamous cell carcinoma, topical treatments reduced MRCKα S1003 autophosphorylation and skin papilloma outgrowth. In parallel work, we validated a phospho-selective antibody with the capability to monitor drug pharmacodynamics. Taken together, our findings establish an important oncogenic role for MRCK in cancer, and they offer an initial preclinical proof of concept for MRCK inhibition as a valid therapeutic strategy

LIM kinases are required for invasive path generation by tumor and tumor-associated stromal cells

Leading cells require LIMK for matrix degradation and invadopodia formation during collective cell migration

Co-Crystal Structures of Inhibitors with MRCKβ, a Key Regulator of Tumor Cell Invasion

MRCKα and MRCKβ (myotonic dystrophy kinase-related Cdc42-binding kinases) belong to a subfamily of Rho GTPase activated serine/threonine kinases within the AGC-family that regulate the actomyosin cytoskeleton. Reflecting their roles in myosin light chain (MLC) phosphorylation, MRCKα and MRCKβ influence cell shape and motility. We report further evidence for MRCKα and MRCKβ contributions to the invasion of cancer cells in 3-dimensional matrix invasion assays. In particular, our results indicate that the combined inhibition of MRCKα and MRCKβ together with inhibition of ROCK kinases results in significantly greater effects on reducing cancer cell invasion than blocking either MRCK or ROCK kinases alone. To probe the kinase ligand pocket, we screened 159 kinase inhibitors in an in vitro MRCKβ kinase assay and found 11 compounds that inhibited enzyme activity >80% at 3 µM. Further analysis of three hits, Y-27632, Fasudil and TPCA-1, revealed low micromolar IC50 values for MRCKα and MRCKβ. We also describe the crystal structure of MRCKβ in complex with inhibitors Fasudil and TPCA-1 bound to the active site of the kinase. These high-resolution structures reveal a highly conserved AGC kinase fold in a typical dimeric arrangement. The kinase domain is in an active conformation with a fully-ordered and correctly positioned αC helix and catalytic residues in a conformation competent for catalysis. Together, these results provide further validation for MRCK involvement in regulation of cancer cell invasion and present a valuable starting point for future structure-based drug discovery efforts

Trailblazing LIM kinases take the lead in collective tumor cell invasion

The spread of tumor cells from primary sites often occurs as associated cell collectives. In this form of invasion, the contribution of cells leading the way may differ from those that follow. By implication, proteins that regulate the actin cytoskeleton, a major driver of cell motility, may have different roles depending on whether they are in leading or following cells. The LIM kinases 1 and 2 (LIMK) phosphorylate and inactivate the filamentous actin severing function of cofilin proteins. Using siRNA or pharmacological inhibitors, LIMK was found to be required in leading cells of collectively invading tumor cells, or in cancer-associated stromal fibroblasts, for effective extracellular matrix degradation that facilitates three-dimensional invasion. The decreased extracellular matrix degrading activities were associated with an inability to form the stable filamentous actin structures necessary to make matrix-degrading protrusive structures. However, LIMK was not required for cell motility or for path-following in associated collectives. These findings show that leading and following cells in collective invasion have different properties and indicate that targeting the activities in leading cells is sufficient to significantly inhibit tumor cell invasiveness