Chiang J. Li

ARQ 197, a Novel and Selective Inhibitor of the Human c-Met Receptor Tyrosine Kinase with Antitumor Activity

The met proto-oncogene is functionally linked with tumorigenesis and metastatic progression. Validation of the receptor tyrosine kinase c-Met as a selective anticancer target has awaited the emergence of selective c-Met inhibitors. Herein, we report ARQ 197 as the first non-ATP–competitive small molecule that selectively targets the c-Met receptor tyrosine kinase. Exposure to ARQ 197 resulted in the inhibition of proliferation of c-Met–expressing cancer cell lines as well as the induction of caspase-dependent apoptosis in cell lines with constitutive c-Met activity. These cellular responses to ARQ 197 were phenocopied by RNAi-mediated c-Met depletion and further demonstrated by the growth inhibition of human tumors following oral administration of ARQ 197 in multiple mouse xenograft efficacy studies. Cumulatively, these data suggest that ARQ 197, currently in phase II clinical trials, is a promising agent for targeting cancers in which c-Met-driven signaling is important for their survival and proliferation. Mol Cancer Ther; 9(6); 1544–53. ©2010 AACR.

Short hairpin RNA–expressing bacteria elicit RNA interference in mammals

RNA-interference (RNAi) is a potent mechanism, conserved from plants to humans for specific silencing of genes, which holds promise for functional genomics and gene-targeted therapies. Here we show that bacteria engineered to produce a short hairpin RNA (shRNA) targeting a mammalian gene induce trans-kingdom RNAi in vitro and in vivo. Nonpathogenic Escherichia coli were engineered to transcribe shRNAs from a plasmid containing the invasin gene Inv and the listeriolysin O gene HlyA, which encode two bacterial factors needed for successful transfer of the shRNAs into mammalian cells. Upon oral or intravenous administration, E. coli encoding shRNA against CTNNB1 (catenin β-1) induce significant gene silencing in the intestinal epithelium and in human colon cancer xenografts in mice. These results provide an example of trans-kingdom RNAi in higher organisms and suggest the potential of bacteria-mediated RNAi for functional genomics, therapeutic target validation and development of clinically compatible RNAi-based therapies.

Selective killing of cancer cells by β-lapachone: Direct checkpoint activation as a strategy against cancer

Most chemotherapeutic drugs kill cancer cells by indirectly activating checkpoint-mediated apoptosis after creating nonselective damage to DNA or microtubules, which accounts for their toxicity toward normal cells. We seek to target cancer cells by directly activating checkpoint regulators without creating such damage. Here, we show that β-lapachone selectively induces apoptosis in cancer cells without causing the death of nontransformed cells in culture. This unusual selectivity against cancer cells is preceded by activation of S-phase checkpoint and selective induction of E2F1, a regulator of checkpoint-mediated apoptosis. This study suggests direct checkpoint activation as a strategy against cancer.

Suppression of cancer relapse and metastasis by inhibiting cancer stemness

Significance Current cancer treatments ultimately fail owing to metastasis and relapse. The discovery of therapeutic approaches that counteract relapse and metastasis is, therefore, extremely important for advancing cancer medicine. Hypermalignant cancer cells, termed cancer stem cells or stemness-high cancer cells, have been isolated from patients with a variety of tumor types and found to be highly malignant, tumorigenic, and resistant to chemotherapies. Our data that BBI608, a cancer stemness inhibitor in clinical development, effectively blocks cancer relapse and metastasis in xenografted human cancers, suggest targeting cancer stemness as a novel approach to develop the next generation of cancer therapeutics to suppress cancer relapse and metastasis. Partial or even complete cancer regression can be achieved in some patients with current cancer treatments. However, such initial responses are almost always followed by relapse, with the recurrent cancer being resistant to further treatments. The discovery of therapeutic approaches that counteract relapse is, therefore, essential for advancing cancer medicine. Cancer cells are extremely heterogeneous, even in each individual patient, in terms of their malignant potential, drug sensitivity, and their potential to metastasize and cause relapse. Indeed, hypermalignant cancer cells, termed cancer stem cells or stemness-high cancer cells, that are highly tumorigenic and metastatic have been isolated from cancer patients with a variety of tumor types. Moreover, such stemness-high cancer cells are resistant to conventional chemotherapy and radiation. Here we show that BBI608, a small molecule identified by its ability to inhibit gene transcription driven by Stat3 and cancer stemness properties, can inhibit stemness gene expression and block spherogenesis of or kill stemness-high cancer cells isolated from a variety of cancer types. Moreover, cancer relapse and metastasis were effectively blocked by BBI608 in mice. These data demonstrate targeting cancer stemness as a novel approach to develop the next generation of cancer therapeutics to suppress cancer relapse and metastasis.

Asymmetric RNA duplexes mediate RNA interference in mammalian cells

RNA interference (RNAi) has become an indispensable technology for biomedical research and has demonstrated the potential to become a new class of therapeutic. Current RNAi technology in mammalian cells relies on short interfering RNA (siRNA) consisting of symmetrical duplexes of 19–21 base pairs (bp) with 3′ overhangs. Here we report that asymmetric RNA duplexes with 3′ and 5′ antisense overhangs silence mammalian genes effectively. An asymmetric interfering RNA (aiRNA) of 15 bp was incorporated into the RNA-induced silencing complex (RISC) and mediated sequence-specific cleavage of the target mRNA between base 10 and 11 relative to the 5′ end of the antisense strand. The gene silencing mediated by aiRNA was efficacious, durable and correlated with reduced off-target silencing by the sense strand. These results establish aiRNA as a scaffold structure for designing RNA duplexes to induce RNAi in mammalian cells.

Potent induction of apoptosis by beta-lapachone in human multiple myeloma cell lines and patient cells.

BackgroundHuman multiple myeloma (MM) remains an incurable hematological malignancy. We have reported that β-lapachone, a pure compound derived from a plant, can induce cell death in a variety of human carcinoma cells, including ovary, colon, lung, prostate, pancreas, and breast, suggesting a wide spectrum of anticancer activity.Materials and MethodsWe first studied anti-survival effects of β-lapachone in human MM cells by colony formation assay. To determine whether the differential inhibition of colony formation occurs through antiproliferative activity, we performed MTT assays. The cytotoxicity of β-lapachone on human peripheral blood mononuclear cells was also measured by MTT assay. To determine whether the cell death induced by β-lapachone occurs through necrosis or apoptosis, we used the propidium iodide staining procedure to determine the sub-G1 fraction, Annexin-V staining for externalization of phosphatidylserine, and fragmentation of cellular genomic DNA subjected to gel electrophoresis. To investigate the mechanism of anti-MM activity, we examined Bcl-2 expression, cytochrome C release, and poly (ADP ribose) polymerase cleavage by Western blot assay.ResultsWe found that β-lapachone (less than 4 µM) inhibits cell survival and proliferation by triggering cell death with characteristics of apoptosis in ARH-77, HS Sultan, and MM.1S cell lines, in freshly derived patient MM cells (MM.As), MM cell lines resistant to dexamethasone (MM.1R), doxorubicin (DOX.40), mitoxantrone (MR.20), and mephalan (LR5). Importantly, after treatment with β-lapachone, we observed no apoptosis in peripheral blood mononuclear cells in either quiescent or proliferative states, freshly isolated from healthy donors. In β-lapachone treated ARH-77, cytochrome C was released from mitochondria to cytosol, and poly (ADP ribose) polymerase was cleaved, signature events of apoptosis. Finally, the apoptosis induced by β-lapachone in MM cells was not blocked by either interleukin-6 or Bcl-2, which confer multidrug resistance in MM.ConclusionsOur results suggest potential therapeutic application of β-lapachone against MM, particularly to overcome drug resistance in relapsed patients.

Dual induction of apoptosis and senescence in cancer cells by Chk2 activation : Checkpoint activation as a strategy against cancer

The human checkpoint kinase 2 (Chk2) plays a central role in regulation of the cellular response to DNA damage, resulting in cell cycle arrest, DNA repair, or apoptosis depending on severity of DNA damage and the cellular context. Chk2 inhibitors are being developed as sensitizers for chemotherapeutic agents. In contrast, here we report that direct activation of Chk2 alone (without chemotherapeutic agents) led to potent inhibition of cancer cell proliferation. In the absence of de novo DNA damage, checkpoint activation was achieved by increased Chk2 expression, as evidenced by its phosphorylation at Thr68, resulting in senescence and apoptosis of cancer cells (DLD1 and HeLa). The Chk2-induced apoptosis was p53 independent and was mediated by caspase activation triggered by loss of mitochondrial potential. The Chk2-induced senescence was also p53 independent and was associated with induction of p21. These results suggest that direct activation of checkpoint kinases may be a novel approach for cancer therapy.

Induction of cancer cell stemness by chemotherapy

Recent studies indicate that cancer stem cells (CSCs) exist in most hematological and solid tumors. CSCs are characterized by their ability to self-renew and their capacity to differentiate into the multitude of cells that comprise the tumor mass. Moreover, these cells have been shown to be intrinsically resistant to conventional anticancer therapies. Despite their fundamental role in cancer pathogenesis, the cellular origin of CSCs remains highly controversial. The aim of this study was to examine whether heterogeneous cancer cells can acquire stem cell-like properties in response to chemotherapy. We demonstrate that carboplatin can induce the self-renewal (spherogenesis) and pluripotency (Sox2 and Oct3/4 expression) of hepatocellular carcinoma (HCC) cells grown under stem cell culture conditions. Moreover, we show that non-CSC cells, obtained by side population flow cytometric sorting using Hoechst 33342, can acquire stem-like properties after exposure to carboplatin. Finally, we show that knockdown of Sox2 and Oct3/4 gene expression in HCC cells can reduce carboplatin-mediated increases in sphere formation and increase cellular sensitivity to chemotherapy. Taken together, our data indicate that bulk cancer cells may be an important source of CSCs during tumor development, and that targeting Sox2 and/or Oct3/4 may be a promising approach for targeting CSCs in clinical cancer treatment.

Selective Induction of Necrotic Cell Death in Cancer Cells by β-Lapachone through Activation of DNA Damage Response Pathway

Most efforts thus far have been devoted to develop apoptosis inducers for cancer treatment. However, apoptotic pathway deficiencies are a hallmark of cancer cells. We propose that one way to bypass defective apoptotic pathways in cancer cells is to induce necrotic cell death. Here we show that selective induction of necrotic cell death can be achieved by activation of the DNA damage response pathways. While β-lapachone induces apoptosis through E2F1 checkpoint pathways, necrotic cell death can be selectively induced by β-lapachone in a variety of cancer cells. We found that β-lapachone, unlike DNA damaging chemotherapeutic agents, transiently activates PARP1, a main regulator of the DNA damage response pathway, both in vitro and in vivo. This occurs within minutes of exposure to β-lapachone, resulting in selective necrotic cell death. Inhibition of PAR blocked β-lapachone-induced necrosis. Furthermore, necrotic cell death induced by β-lapachone was significantly reduced in PARP1 knockout cell lines. Our data suggest that selective necrotic cell death can be induced through activation of DNA damage response pathways, supporting the idea of selective necrotic cell death as a therapeutic strategy

Regulation in S Phase by E2F

The DNA synthetic S phase of the unperturbed cell cycle is a closed system, as compared to regulation of G1 by external growth factors. The E2F family provides internal control in S phase by transcribing genes required for deoxynucleotide triphosphate (dNTP) and DNA synthesis. Furthermore, over expression of E2F-1 activates programmed cell death (apoptosis), a safeguarding signal of aberrant growth of cells that have become carcinogenic. Mechanisms for control of E2F-1 are thus essential. The hypothesis is proposed that deoxythymidine triphosphate (dTTP) allosterically feedback controls E2F-1 to regulate both DNA synthesis and apoptosis. This may act either upon production of E2F-1 or its degradation.