Current Cancer Drug Targets - Volume 16, Issue 7, 2016

The process of protein prenylation involves the covalent linkage of either farnesyl (15-carbon) or geranylgeranyl (20-carbon) isoprenoid lipds to conserved cysteine residues in the carboxyl-terminus of proteins. Protein geranylgeranyltransferase I (GGTase-I) is the enzyme that catalyzes the addition of the geranylgeranyl moiety from geranylgeranyl pyrophosphate to the target protein, which contains a Cterminal consensus sequence termed a CaaX motif. Geranylgeranylation is important to the function of a number of proteins, including the majority of Rho GTPases, G protein gamma subunits, and several other regulatory proteins. Studies over the past two decades have revealed that many of these proteins contribute to tumor development and metastasis. Blocking Rho GTPase activity through inhibition of GGTase-I in particular has been advanced as a potential strategy for disease therapy. This review will provide an overview of the CaaX prenyltransferases, the rationale for targeting GGTase-I in cancer in particular, and the current status of GGTase-I inhibitor (GGTI) development.

Membranous Met is classically identified with its role in cancer metastases, while nuclear Met is associated with a more invasive, aggressive and proliferative form of cancer. Full-length Met or N-terminal transmembrane domain cleaved Met can translocate into nucleus in a cell growth and pH dependent but both ligand-dependent (full length Met) and -independent (cleaved Met) manner. nMET may play greater essential roles in cancer recurrence than membranous Met. For example, in prostate cancer, it has been found that androgen receptor (AR) may inhibit the expression of membranous Met so anti-androgen based prostate cancer therapy may promote the expression of nuclear Met (nMET). We recently found a novel nMET/SOX9/ β-Catenin/AR pathway in relapsed prostate cancer which may contribute to the formation of the feedback loop of AR reactivation via MET/nMET. Emerging evidence suggests the possibility of nMET as a prognostic marker in relapsed cancer. This review summarizes recent findings about nMET and its unique role in recurrent cancer.

RBM15, an RNA-binding protein, plays important roles in the growth and apoptosis of cells, especially blood cells through regulating multiple signal pathways such as Notch and Wnt. An increasing body of evidence has suggested that RBM15 may play a key function on the development of various blood diseases, such as acute/chronic myeloid leukemia and kaposi′s sarcoma. In this review, we will focus on the progress of the association between RBM15 and its related blood diseases.

Brachyury is an important transcription factor of the T-box gene family with an evolutionarily-conserved function in mesoderm development in the embryo. Recent research has demonstrated that, in various human carcinomas, overexpression of Brachyury is associated with epithelial-mesenchymal transition (EMT), tumor metastasis, expression of markers for cancer stem cells, and resistance to chemotherapy and radiotherapy. Brachyury is a diagnostic and prognostic biomarker, and its expression in tumor tissues is associated with increasing tumor grade, stage, invasiveness, metastasis and poor prognosis. Targeting of Brachyury-positive tumor cells may modulate the extent of EMT and stop invasiveness. Fibroblast growth factor, transforming growth factor-β and other EMT signalling factors are involved in the molecular pathways of Brachyury in tumorigenesis and development. Experimentally, Brachyury knockdown resulted in downregulation of EMT and stem cell markers, formation of tumor spheroids, and invasiveness. Treatment with recombinant yeast-Brachyury vector-based vaccine can activate and expand Brachyury-specific CD4+ and CD8+ T-cells in vitro, with an outcome of lysis of human tumor cells expressing the Brachyury protein. Further understanding of the characteristics of Brachyury and its associated signaling pathways might help in developing novel therapeutic strategies against EMT.

The Interferon Regulatory Factor (IRF) family consists of multiple transcription factors involved in the regulation of a variety of biological processes. Originally identified as transcriptional regulators of the type I interferon system, IRFs play a pivotal role in adaptive immunity, cell growth, differentiation and tumorigenesis. Hence, understanding IRF biology has important implications in the host response to cancer development and progression. Many lines of evidence suggest that different IRFs are involved in the pathogenesis of Chronic Myeloid Leukemia (CML), a myeloproliferative disorder caused by the BCR-ABL oncoprotein. BCR-ABL displays constitutive tyrosine kinase activity that favors cell proliferation, inhibits apoptosis and allows cell survival even in the absence of proper adhesion to the extracellular matrix. Different BCR-ABL tyrosine kinase inhibitors are currently available for CML treatment. These drugs are able to generate eight year CML-specific overall survival rates >90%, only a minority of patients will achieve molecular responses compatible with drug discontinuation. Thus, there is an unmet need for additional therapeutic targets that may lead to the cure of most patients diagnosed with CML. A growing body of evidence has suggested a role for both IRF4 and IRF8 in the pathogenesis of CML. Furthermore, IRF1 is consistently deleted at one or both alleles in patients with leukemia and myelodysplasia. Finally, we have recently demonstrated that IRF5 is a target of BCR-ABL kinase activity and reduces CML cell proliferation. In this article, we provide an update on the current knowledge of the role of the IRFs in CML.

Triple-negative breast cancer (TNBC) is defined by the absence of expression of estrogen receptor (ER), progesterone receptor (PR), and a lack of overexpression or amplification of human epidermal growth factor receptor 2 (HER2). The clinicopathological characteristics of TNBC include a high grading, a high rate of cell proliferation and a greater degree of chromosomal rearrangement. Patients with triple-negative breast cancer are more likely to be drug resistant and more difficult to treat, and are also frequently BRCA1 mutants. Methylation of the BRCA1 promoter region is associated with a reduction of the BRCA1 mRNA level. TNBC patients with a methylated BRCA1 had a better disease-free survival compared with those with non-methylated BRCA1. From a therapeutic perspective, the expression level of BRCA1 has been a major determinant of the responses to different classes of chemotherapy. BRCA1-dysfunctional tumors are hypersensitive to DNA damaging chemotherapeutic agents like platinum drugs. Although platinum based drugs are currently widely used as conventional chemotherapeutic drugs in breast cancer chemotherapy, their use has several disadvantages. It is therefore of interest to seek out alternative therapeutic metal-based compounds that could overcome the limitations of these platinum based drugs. Ruthenium-based compounds could be the most promising alternative to the platinum drugs. This review highlights the use of BRCA1 as a predictive marker as well as for a potential drug target for anticancer ruthenium compounds.

Membrane type 1-matrix metalloproteinase (MT1-MMP, MMP-14) is associated with cancer invasion and metastasis leading to poor patient prognosis. MT1-MMP mediates cancer cell invasion via degradation of basement membrane and extracellular matrix, and induction of cell migration. However, MT1-MMP expression in the cancer stroma can drive invasion of carcinoma cells in vivo, suggesting MT1-MMP may also promote cancer invasiveness via paracrinemediated mechanisms. A major step in cancer cell metastasis is thought to be an epithelial-mesenchymal transition (EMT), in which carcinoma cells evolve from a stationary epithelial phenotype to a more motile mesenchymal phenotype. We demonstrate here that EMT is triggered by MT1-MMP-mediated activation of TGF-β signaling, involving induction of CUTL1 and subsequently, of Wnt5a. Mesenchymal-like cancer cells expressing endogenous MT1-MMP reverted to an epithelial phenotype when MT1-MMP, SMAD4, CUTL1, or Wnt5a expression or TGF-β activity was inhibited. Wnt5a knockdown in MT1- MMP expressing LNCaP cells caused decreased cell migration and cell growth in soft agar. While MT1-MMP expression did not affect total TGF-β level, MT1-MMP catalytic activity increased the availability of active TGF-β, enabling MT1-MMP-expressing cells to activate the EMT in nearby cells. MT1-MMP-expressing cells induced co-cultured non-MT1-MMP-expressing cells to undergo EMT by a TGF-β-dependent process. These results highlight a pathway by which tumor invasiveness may be expanded via MT1-MMP-mediated activation of TGF-β signaling, enabling autocrine and paracrine-mediated induction of EMT.

To test the role of STAT3 in human rhabdomyosarcoma cells, genetic approaches were used to either knockdown the expression of STAT3 and GP130, an upstream activator of STAT3 using short hairpin RNA (shRNA) or express persistently active STAT3 protein. Knockdown expression of GP130 or STAT3 sensitized cells to anti-cancer drugs doxorubicin, cisplatin, and MEK inhibitor AZD6244. On the other hand, expression of the constitutively active STAT3 protein reduced the sensitivity of rhabdomyosarcoma cells to those drugs. In addition, we tested a small molecule STAT3 inhibitor LY5 and a GP130 inhibitor bazedoxifene in rhabdomyosarcoma cells. Our data demonstrated that the combination of LY5 or bazedoxifene with doxorubicin, cisplatin, and AZD6244 showed stronger inhibitory effects than single agent alone. In summary, our results demonstrated that GP130/STAT3 signaling contributes to the resistance of these drugs in rhabdomyosarcoma cells. They also suggested a potentially novel cancer therapeutic strategy using the combination of inhibitors of GP130/STAT3 signaling with doxorubicin, cisplatin, or AZD6244 for rhabdomyosarcoma treatments.

One approach to further improve the therapeutic efficacy of nanoparticles is employment of active targeting strategies. Bispecific antibodies (bsAbs) that bind to both tumor specific antigens on the cell surface and to haptens such as digoxigenin (Dig) can direct digoxigeninylated payloads to tumor cells. In this study, we investigate the potential of dendritic polyglycerol (dPG) conjugates, which consist of a doxorubicin (DOX) prodrug, Dig moiety, and a poly (ethylene glycol) (PEG) shell, in combination with bsAb, as a novel approach for targeted prodrug delivery. We could show successful binding of the bsAbs to dPGDigMal-DOX-PEG conjugates, as well as binding of these complexes to the cell surface of Lewis Y (LeY) expressing MCF-7 cells. Using flow cytometry, we could show the preferential binding of the targeting complex over the complex of control conjugate lacking Dig moieties. At concentrations that are usually applied for drug delivery, antibodycomplexed nanoparticles (independent of antibody specificity) released cytotoxic compounds into cells to the same degree as unmodified nanoparticles. This indicates that antibody-attachment does not interfere with the inherent cell binding and drug delivery properties of nanoparticles. At low doxorubicin concentrations and short incubation times, however, we were able to see a slightly increased target specific cytotoxicity in vitro which is mediated by complexation of the digoxigeninylated NP with the Dig-binding moiety of a bsAb that in turns direct the complexed bsAb to target cells. This study demonstrates the potential of digoxigeninylated dPG prodrug conjugates in combination with bsAbs as a new platform for targeted prodrug delivery into cancerous tissues. However the nanoparticle design needs to be further optimized for significant targeted delivery.