Ki11502, a novel multitargeted receptor tyrosine kinase inhibitor, induces growth arrest and apoptosis of human leukemia cells in vitro and in vivo

PDGF-BB signaling via PDGFR-β regulates the maturation of blood vessels generated upon vasculogenic differentiation of dental pulp stem cells

A functional vascular network requires that blood vessels are invested by mural cells. We have shown that dental pulp stem cells (DPSC) can undergo vasculogenic differentiation, and that the resulting vessels anastomize with the host vasculature and become functional (blood carrying) vessels. However, the mechanisms underlying the maturation of DPSC-derived blood vessels remains unclear. Here, we performed a series of studies to understand the process of mural cell investment of blood vessels generated upon vasculogenic differentiation of dental pulp stem cells. Primary human DPSC were co-cultured with primary human umbilical artery smooth muscle cells (HUASMC) in 3D gels in presence of vasculogenic differentiation medium. We observed DPSC capillary sprout formation and SMC recruitment, alignment and remodeling that resulted in complex vascular networks. While HUASMC enhanced the number of capillary sprouts and stabilized the capillary network when co-cultured with DPSC, HUASMC by themselves were unable to form capillary sprouts. In vivo, GFP transduced human DPSC seeded in biodegradable scaffolds and transplanted into immunodeficient mice generated functional human blood vessels invested with murine smooth muscle actin (SMA)-positive, GFP-negative cells. Inhibition of PDGFR-β signaling prevented the SMC investment of DPSC-derived capillary sprouts in vitro and of DPSC-derived blood vessels in vivo. In contrast, inhibition of Tie-2 signaling did not have a significant effect on the SMC recruitment in DPSC-derived vascular structures. Collectively, these results demonstrate that PDGF-BB signaling via PDGFR-β regulates the process of maturation (mural investment) of blood vessels generated upon vasculogenic differentiation of human dental pulp stem cells.

Pdgf signalling guides neural crest contribution to the haematopoietic stem cell specification niche

Haematopoietic stem cells (HSCs) support maintenance of the haematopoietic and immune systems throughout the life of vertebrates, and are the therapeutic component of bone marrow transplants. Understanding native specification of HSCs, to uncover key signals that might help improve in vitro directed differentiation protocols, has been a longstanding biomedical goal. The current impossibility of specifying true HSCs in vitro suggests that key signals remain unknown. We speculated that such signals might be presented by surrounding “niche” cells, but no such cells have been defined. Here we demonstrate in zebrafish, that trunk neural crest (NC) physically associate with HSC precursors in the dorsal aorta (DA) just prior to initiation of the definitive haematopoietic programme. Preventing association of the NC with the DA leads to loss of HSCs. Our results define NC as key cellular components of the HSC specification niche that can be profiled to identify unknown HSC specification signals.

Patterning mechanisms of the sub-intestinal venous plexus in zebrafish

Despite considerable interest in angiogenesis, organ-specific angiogenesis remains less well characterized. The vessels that absorb nutrients from the yolk and later provide blood supply to the developing digestive system are primarily venous in origin. In zebrafish, these are the vessels of the Sub-intestinal venous plexus (SIVP) and they represent a new candidate model to gain an insight into the mechanisms of venous angiogenesis. Unlike other vessel beds in zebrafish, the SIVP is not stereotypically patterned and lacks obvious sources of patterning information. However, by examining the area of vessel coverage, number of compartments, proliferation and migration speed we have identified common developmental steps in SIVP formation. We applied our analysis of SIVP development to obd mutants that have a mutation in the guidance receptor PlexinD1. obd mutants show dysregulation of nearly all parameters of SIVP formation. We show that the SIVP responds to a unique combination of pathways that control both arterial and venous growth in other systems. Blocking Shh, Notch and Pdgf signaling has no effect on SIVP growth. However Vegf promotes sprouting of the predominantly venous plexus and Bmp promotes outgrowth of the structure. We propose that the SIVP is a unique model to understand novel mechanisms utilized in organ-specific angiogenesis.

Blockade of the PDGFR family together with SRC leads to diminished proliferation of colorectal cancer cells

Among the family of receptor tyrosine kinases (RTKs), platelet-derived growth factor receptor (PDGFR) has attracted increasing attention as a potential target of anti-tumor therapy in colorectal cancer (CRC). To study the function of PDGFRβ in CRC cell lines, SW480, DLD-1 and Caco-2 cells showing high PDGFRβ expression were used for receptor down-regulation by small interfering RNA (siRNA) and using the pharmacological inhibitor of PDGFRβ Ki11502. Blockade of PDGFRβ using both approaches led to moderate inhibition of proliferation and diminished activation of the downstream PI3K-signaling pathway in all three cell lines. Surprisingly, incubation with Ki11502 resulted in an arrest of SW480 cells in the G2 phase of the cell cycle, whereas the siRNA approach did not result in this effect. To address this difference, we analyzed the involvement of the PDGFRβ family member c-KIT in Ki11502 effectiveness, but siRNA and proliferation studies in SW480 and DLD-1 cells could not prove the involvement of c-KIT inactivation during Ki11502 treatment.

Hence, an RTK activation antibody array on SW480 cells led us to the identification of the non-receptor tyrosine kinase SRC, which is inactivated after Ki11502 treatment but not after the siRNA approach. Further studies using the SRC-specific inhibitor PP2 showed that SRC inhibition upon treatment with the inhibitor Ki11502 is responsible for the observed effects of Ki11502 in SW480 and DLD-1 CRC cells. In summary, our results demonstrate that the inhibition of PDGFRβ alone using siRNA has only moderate cellular effects in CRC cell lines; however, the multi-target inhibition of PDGFRβ, c-KIT and SRC, e.g., using Ki11502, represents a promising therapeutic intervention for the treatment of CRC.

Inhibitory effect of turmeric curcuminoids on FLT3 expression and cell cycle arrest in the FLT3-overexpressing EoL-1 leukemic cell line

Leukemia is a hematologic malignancy with a frequent incidence and high mortality rate. Previous studies have shown that the FLT3 gene is overexpressed in leukemic blast cells, especially in acute myeloid leukemia. In this study, a commercially available curcuminoid mixture (1), pure curcumin (2), pure demethoxycurcumin (3), and pure bisdemethoxycurcumin (4) were investigated for their inhibitory effects on cell growth, FLT3 expression, and cell cycle progression in an FLT3-overexpressing EoL-1 leukemic cell line using an MTT assay, Western blotting, and flow cytometry, respectively. The mixture (1) and compounds 2-4 demonstrated cytotoxic effects with IC50 values ranging from 6.5 to 22.5 μM. A significant decrease in FLT3 protein levels was found after curcuminoid treatment with IC20 doses, especially with mixture 1 and compound 2. In addition, mixture 1 and curcumin (2) showed activity on cell cycle arrest at the G0/G1 phase and decreased the FLT3 and STAT5A protein levels in a dose-dependent manner. Compound 2 demonstrated the greatest potential for inhibiting cell growth, cell cycle progression, and FLT3 expression in EoL-1 cells. This investigation has provided new findings regarding the effect of turmeric curcuminoids on FLT3 expression in leukemic cells.

Inhibition of platelet-derived growth factor receptor signaling regulates Oct4 and Nanog expression, cell shape, and mesenchymal stem cell potency

Defining the signaling mechanisms that regulate the fate of adult stem cells is an essential step toward their use in regenerative medicine. Platelet-derived growth factor receptor (PDGFR) signaling plays a crucial role in specifying mesenchymal stem cell (MSC) commitment to mesenchymal lineages. Based on the hypothesis that selective inhibition of signaling pathways involved in differentiation may increase stem cell potency, we examined the role of PDGFR signaling in controlling the fate of human MSCs. Using a small molecular PDGFR inhibitor that induced MSCs toward a more rounded shape, expression of Oct4 and Nanog were markedly upregulated. In these PDGFR inhibitor-treated MSCs, Oct4 and Nanog expression and cell shape were regulated by janus kinase (JAK), MAPK kinase (MEK), and epidermal growth factor receptor (EGFR) signaling. Under defined differentiation conditions, these PDGFR-inhibited MSCs expressed definitive endodermal, ectodermal, and mesodermal markers. We also confirmed that depletion of individual PDGF receptors upregulated expression of Oct4A and Nanog. This study identifies PDGFR signaling as a key regulator of Oct4 and Nanog expression and of MSC potency. Thus, inhibiting these specific receptor tyrosine kinases, which play essential roles in tissue formation, offers a novel approach to unlock the therapeutic capacity of MSCs. STEM CELLS 2012;30:548–560

Sorafenib treatment of FLT3-ITD(+) acute myeloid leukemia: favorable initial outcome and mechanisms of subsequent nonresponsiveness associated with the emergence of a D835 mutation

Internal tandem duplication (ITD) of the fms-related tyrosine kinase-3 (FLT3) gene occurs in 30% of acute myeloid leukemias (AMLs) and confers a poor prognosis. Thirteen relapsed or chemo-refractory FLT3-ITD(+) AML patients were treated with sorafenib (200-400 mg twice daily). Twelve patients showed clearance or near clearance of bone marrow myeloblasts after 27 (range 21-84) days with evidence of differentiation of leukemia cells. The sorafenib response was lost in most patients after 72 (range 54-287) days but the FLT3 and downstream effectors remained suppressed. Gene expression profiling showed that leukemia cells that have become sorafenib resistant expressed several genes including ALDH1A1, JAK3, and MMP15, whose functions were unknown in AML. Nonobese diabetic/severe combined immunodeficiency mice transplanted with leukemia cells from patients before and during sorafenib resistance recapitulated the clinical results. Both ITD and tyrosine kinase domain mutations at D835 were identified in leukemia initiating cells (LICs) from samples before sorafenib treatment. LICs bearing the D835 mutant have expanded during sorafenib treatment and dominated during the subsequent clinical resistance. These results suggest that sorafenib have selected more aggressive sorafenib-resistant subclones carrying both FLT3-ITD and D835 mutations, and might provide important leads to further improvement of treatment outcome with FLT3 inhibitors.

Nilotinib and imatinib are comparably effective in reducing growth of human eosinophil leukemia cells in a newly established xenograft model

We developed a xenograft model of human Chronic Eosinophilic Leukemia (CEL) to study disease progression and remission-induction under therapy with tyrosine kinase inhibitors using imatinib and nilotinib as examples. The FIP1L1/PDGFRA+ human CEL cell lineEOL-1 was injected intravenously into scid mice, and MR imaging and FACS analysis of mouse blood samples were performed to monitor disease development and the effects of imatinib and nilotinib. Organ infiltration was analyzed in detail by immunohistochemistry after sacrifice. All animals developed CEL and within one week of therapy, complete remissions were seen with both imatinib and nilotinib, resulting in reduced total tumor volumes by MR-imaging and almost complete disappearance of EOL-1 cells in the peripheral blood and in tissues. The new model system is feasible for the evaluation of new tyrosine kinase inhibitors and our data suggest that nilotinib may be a valuable additional targeted drug active in patients with FIP1L1/PDGFRA+ CEL.

Activated leukemic oncogenes AML1-ETO and c-kit: role in development of acute myeloid leukemia and current approaches for their inhibition

Acute myeloid leukemia (AML) is a malignant blood disease caused by different mutations that enhance the proliferative activity and survival of blood cells and affect their differentiation and apoptosis. The most frequent disorders in AML are translocations between chromosomes 21 and 8 leading to production of a chimeric oncogene, AML1-ETO, and hyperexpression of the receptor tyrosine kinase KIT. Mutations in these genes often occur jointly. The presence in cells of two activated oncogenes is likely to trigger their malignization. The current approaches for treatment of oncologic diseases (bone marrow transplantation, radiotherapy, and chemotherapy) have significant shortcomings, and thus many laboratories are intensively developing new approaches against leukemias. Inhibiting expression of activated leukemic oncogenes based on the principle of RNA interference seems to be a promising approach in this field.

The challenge of developing robust drugs to overcome resistance

Drug resistance is problematic in microbial disease, viral disease and cancer. Understanding at the outset that resistance will impact the effectiveness of any new drug that is developed for these disease categories is imperative. In this Feature, we detail approaches that have been taken with selected drug targets to reduce the susceptibility of new drugs to resistance mechanisms. We will also define the concepts of robust drugs and resilient targets, and discuss how the design of robust drugs and the selection of resilient targets can lead to successful strategies for combating resistance.

Long-term exposure of leukemia cells to multi-targeted tyrosine kinase inhibitor induces activations of AKT, ERK and STAT5 signaling via epigenetic silencing of the PTEN gene

Imatinib induces complete molecular response in patients with chronic myeloid leukemia (CML) and chronic eosinophilic leukemia (CEL). However, development of resistance to imatinib has emerged as an important clinical problem for molecular-targeted therapy in CML and CEL. In this study, we have established the imatinib-resistant CEL EOL-1 sub-lines (designated as EOL-1R) by culturing cells with increasing concentrations of imatinib for 6 months. Interestingly, EOL-1R cells showed epigenetic silencing of the phosphatase and tensin homolog deleted on chromosome ten (PTEN) gene. Exposure of EOL-1R cells to imatinib failed to dephosphorylate AKT, ERK and STAT5, although PDGFRalpha was effectively inactivated. The forced expression of PTEN negatively regulated these signal pathways and sensitized EOL-1R cells to imatinib. Notably, hypermethylation of the promoter region of the PTEN gene in association with the downregulation of this gene's transcripts was identified in imatinib-resistant leukemia cells isolated from individuals with CEL, CML and Philadelphia-positive acute lymphoblastic leukemia. In addition, anti-epigenetic agents restored PTEN expression, resulting in the sensitization of EOL-1R cells to imatinib. Taken together, epigenetic silence of PTEN is one of the mechanisms that cause drug resistance in individuals with leukemia after exposure to imatinib. Anti-epigenetic agents may be useful for overcoming drug resistance in such a case.

Platelet-derived growth factor receptor beta is critical for zebrafish intersegmental vessel formation

Background

Platelet-derived growth factor receptor β (PDGFRβ) is a tyrosine kinase receptor known to affect vascular development. The zebrafish is an excellent model to study specific regulators of vascular development, yet the role of PDGF signaling has not been determined in early zebrafish embryos. Furthermore, vascular mural cells, in which PDGFRβ functions cell autonomously in other systems, have not been identified in zebrafish embryos younger than 72 hours post fertilization.

Methodology/Principal Findings

In order to investigate the role of PDGFRβ in zebrafish vascular development, we cloned the highly conserved zebrafish homolog of PDGFRβ. We found that pdgfrβ is expressed in the hypochord, a developmental structure that is immediately dorsal to the dorsal aorta and potentially regulates blood vessel development in the zebrafish. Using a PDGFR tyrosine kinase inhibitor, a morpholino oligonucleotide specific to PDGFRβ, and a dominant negative PDGFRβ transgenic line, we found that PDGFRβ is necessary for angiogenesis of the intersegmental vessels.

Significance/Conclusion

Our data provide the first evidence that PDGFRβ signaling is required for zebrafish angiogenesis. We propose a novel mechanism for zebrafish PDGFRβ signaling that regulates vascular angiogenesis in the absence of mural cells.

Discovery of a small-molecule type II inhibitor of wild-type and gatekeeper mutants of BCR-ABL, PDGFRalpha, Kit, and Src kinases: novel type II inhibitor of gatekeeper mutants

Many clinically validated kinases, such as BCR-ABL, c-Kit, PDGFR, and EGFR, become resistant to adenosine triphosphate-competitive inhibitors through mutation of the so-called gatekeeper amino acid from a threonine to a large hydrophobic amino acid, such as an isoleucine or methionine. We have developed a new class of adenosine triphosphate competitive inhibitors, exemplified by HG-7-85-01, which is capable of inhibiting T315I- BCR-ABL (clinically observed in chronic myeloid leukemia), T670I-c-Kit (clinically observed in gastrointestinal stromal tumors), and T674I/M-PDGFRalpha (clinically observed in hypereosinophilic syndrome). HG-7-85-01 is unique among all currently reported kinase inhibitors in having the ability to accommodate either a gatekeeper threonine, present in the wild-type forms of these kinases, or a large hydrophobic amino acid without becoming a promiscuous kinase inhibitor. The distinctive ability of HG-7-85-01 to simultaneously inhibit both wild-type and mutant forms of several kinases of clinical relevance is an important step in the development of the next generation of tyrosine kinase inhibitors.

FLT3 inhibition and mechanisms of drug resistance in mutant FLT3-positive AML

An appealing therapeutic target in AML is constitutively activated, mutant FLT3, which is expressed in a subpopulation of AML patients and is generally a poor prognostic indicator in patients under the age of 65. There are currently several FLT3 inhibitors that are undergoing clinical investigation. However, the discovery of drug-resistant leukemic blast cells in FLT3 inhibitor-treated AML patients has prompted the search for novel, structurally diverse FLT3 inhibitors that could be alternatively used to circumvent drug resistance. Here, we provide an overview of FLT3 inhibitors under preclinical and clinical investigation, and we discuss mechanisms whereby AML cells develop resistance to FLT3 inhibitors, and the ways in which combination therapy could potentially be utilized to override drug resistance. We discuss how the cross-talk between major downstream signaling pathways, such as PI3K/PTEN/Akt/mTOR, RAS/Raf/MEK/ERK, and Jak/STAT, can be exploited for therapeutic purposes by targeting key signaling molecules with selective inhibitors, such as mTOR inhibitors, HSP90 inhibitors, or farnesyltransferase inhibitors, and identifying those agents with the ability to positively combine with inhibitors of FLT3, such as PKC412 and sunitinib. With the widespread onset of drug resistance associated with tyrosine kinase inhibitors, due to mechanisms involving development of point mutations or gene amplification of target proteins, the use of a multi-targeted therapeutic approach is of potential clinical benefit.