Kit/stem cell factor receptor-induced phosphatidylinositol 3'-kinase signalling is not required for normal development and function of interstitial cells of Cajal in mouse gastrointestinal tract

For Better or Worse: The Potential for Dose Limiting the On-Target Toxicity of PI 3-Kinase Inhibitors

The hyper-activation of the phosphoinositide (PI) 3-kinase signaling pathway is a hallmark of many cancers and overgrowth syndromes, and as a result, there has been intense interest in the development of drugs that target the various isoforms of PI 3-kinase. Given the key role PI 3-kinases play in many normal cell functions, there is significant potential for the disruption of essential cellular functions by PI 3-kinase inhibitors in normal tissues; so-called on-target drug toxicity. It is, therefore, no surprise that progress within the clinical development of PI 3-kinase inhibitors as single-agent anti-cancer therapies has been slowed by the difficulty of identifying a therapeutic window. The aim of this review is to place the cellular, tissue and whole-body effects of PI 3-kinase inhibition in the context of understanding the potential for dose limiting on-target toxicities and to introduce possible strategies to overcome these.

KIT as a therapeutic target for non-oncological diseases

KIT is a receptor tyrosine kinase that after binding to its ligand stem cell factor activates signaling cascades linked to biological processes such as proliferation, differentiation, migration and cell survival. Based on studies performed on SCF and/or KIT mutant animals that presented anemia, sterility, and/or pigmentation disorders, KIT signaling was mainly considered to be involved in the regulation of hematopoiesis, gametogenesis, and melanogenesis. More recently, novel animal models and ameliorated cellular and molecular techniques have led to the discovery of a widen repertoire of tissue compartments and functions that are being modulated by KIT. This is the case for the lung, heart, nervous system, gastrointestinal tract, pancreas, kidney, liver, and bone. For this reason, the tyrosine kinase inhibitors that were originally developed for the treatment of hemato-oncological diseases are being currently investigated for the treatment of non-oncological disorders such as asthma, rheumatoid arthritis, and alzheimer's disease, among others. The beneficial effects of some of these tyrosine kinase inhibitors have been proven to depend on KIT inhibition. This review will focus on KIT expression and regulation in healthy and pathologic conditions other than cancer. Moreover, advances in the development of anti-KIT therapies, including tyrosine kinase inhibitors, and their application will be discussed.

Bioengineered intestinal muscularis complexes with long-term spontaneous and periodic contractions

Although critical for studies of gut motility and intestinal regeneration, the in vitro culture of intestinal muscularis with peristaltic function remains a significant challenge. Periodic contractions of intestinal muscularis result from the coordinated activity of smooth muscle cells (SMC), the enteric nervous system (ENS), and interstitial cells of Cajal (ICC). Reproducing this activity requires the preservation of all these cells in one system. Here we report the first serum-free culture methodology that consistently maintains spontaneous and periodic contractions of murine and human intestinal muscularis cells for months. In this system, SMC expressed the mature marker myosin heavy chain, and multipolar/dipolar ICC, uniaxonal/multipolar neurons and glial cells were present. Furthermore, drugs affecting neural signals, ICC or SMC altered the contractions. Combining this method with scaffolds, contracting cell sheets were formed with organized architecture. With the addition of intestinal epithelial cells, this platform enabled up to 11 types of cells from mucosa, muscularis and serosa to coexist and epithelial cells were stretched by the contracting muscularis cells. The method constitutes a powerful tool for mechanistic studies of gut motility disorders and the functional regeneration of the engineered intestine.

Direct engagement of the PI3K pathway by mutant KIT dominates oncogenic signaling in gastrointestinal stromal tumor

Gastrointestinal stromal tumors (GISTs) predominantly harbor activating mutations in the receptor tyrosine kinase KIT. To genetically dissect in vivo the requirement of different signal transduction pathways emanating from KIT for tumorigenesis, the oncogenic Kit^V558Δ mutation was combined with point mutations abrogating specific phosphorylation sites on KIT. Compared with single-mutant Kit^V558Δ/+ mice, double-mutant Kit^{V558Δ;Y567F/Y567F} knock-in mice lacking the SRC family kinase-binding site on KIT (pY567) exhibited attenuated MAPK signaling and tumor growth. Surprisingly, abrogation of the PI3K-binding site (pY719) in Kit^{V558Δ;Y719F/Y719F} mice prevented GIST development, although the interstitial cells of Cajal (ICC), the cells of origin of GIST, were normal. Pharmacologic inhibition of the PI3K pathway in tumor-bearing Kit^V558Δ/+ mice with the dual PI3K/mTOR inhibitor voxtalisib, the pan-PI3K inhibitor pilaralisib, and the PI3K-alpha-restricted inhibitor alpelisib each diminished tumor proliferation. The addition of the MEK inhibitor PD-325901 or binimetinib further decreased downstream KIT signaling. Moreover, combining PI3K and MEK inhibition was effective against imatinib-resistant Kit^{V558Δ;T669I/+} tumors.

Interstitial cells: regulators of smooth muscle function

Smooth muscles are complex tissues containing a variety of cells in addition to muscle cells. Interstitial cells of mesenchymal origin interact with and form electrical connectivity with smooth muscle cells in many organs, and these cells provide important regulatory functions. For example, in the gastrointestinal tract, interstitial cells of Cajal (ICC) and PDGFRα(+) cells have been described, in detail, and represent distinct classes of cells with unique ultrastructure, molecular phenotypes, and functions. Smooth muscle cells are electrically coupled to ICC and PDGFRα(+) cells, forming an integrated unit called the SIP syncytium. SIP cells express a variety of receptors and ion channels, and conductance changes in any type of SIP cell affect the excitability and responses of the syncytium. SIP cells are known to provide pacemaker activity, propagation pathways for slow waves, transduction of inputs from motor neurons, and mechanosensitivity. Loss of interstitial cells has been associated with motor disorders of the gut. Interstitial cells are also found in a variety of other smooth muscles; however, in most cases, the physiological and pathophysiological roles for these cells have not been clearly defined. This review describes structural, functional, and molecular features of interstitial cells and discusses their contributions in determining the behaviors of smooth muscle tissues.

microRNA-218 increase the sensitivity of gastrointestinal stromal tumor to imatinib through PI3K/AKT pathway

To detect the expressions of microRNA-218 (miR-218) in an imatinib mesylate-sensitive human gastrointestinal stromal tumor (GIST) cells (GIST882) and an imatinib mesylate-resistant cell line (GIST430) and explore the roles of miR-218 and GIST cells in the sensitivity of gastrointestinal stromal tumor to imatinib mesylate and its potential signaling pathways, with an attempt to provide new insights for the treatment of GIST. The GIST cell lines (GIST882 and GIST430) were cultured in vitro. Quantitative real-time PCR (qRT-PCR) was utilized to determine the expression profiles of miR-218 in both GIST cell lines. Forty-eight hours after the transfection of the miR-218 mimic or miR-218 inhibitor in the GIST cells, the changes in the expression of miR-218 in the GIST cells were detected with qRT-PCR. The effects of the ectopic expression of miR-218 in GIST882 or GIST430 cells on the imatinib mesylate-induced GIST cell viability were determined by MTT. The effects of miR-218 ectopic expression on the apoptosis of imatinib mesylate-induce GIST cells were determined by Annexin V/PI double staining method and flow cytometry. The effects of miR-218 ectopic expression on the AKT and phospho-AKT (p-AKT) expressions of imatinib mesylate-induce GIST cells were determined by Western blot and flow cytometry with the PI3K pathway inhibitor Wortmannin. As shown by qRT-PCR, compared with that in the imatinib mesylate-sensitive GIST882, the expression of miR-218 in imatinib mesylate-resistant GIST430 was significantly decreased (P < 0.01). Compared with the control group, the expression of miR-218 significantly increased in the GIST882 48 h after the transfection of miR-218 mimic (P < 0.01) and significantly declined after the transfection of miR-218 inhibitor (P < 0.01). As shown by MTT and flow cytometry, after the expression of miR-218 was inhibited in GIST882 under the effect of imatinib mesylate, the cell viability significantly increased (P < 0.01) and the number of apoptotic cells significantly decreased (P < 0.05); on the contrary, the over-expression of miR-218 in GIST430 under the effect of imatinib mesylate resulted in the significantly decreased cell viability (P < 0.01) and the significantly increased number of apoptotic cells (P < 0.05). Western blot and flow cytometry showed that, in comparison to the control group, Wortmannin could significantly inhibit the expression of p-AKT in GIST430 cells (P < 0.01) and stimulated apoptosis (P < 0.01). The expression of miR-218 is down-regulated in an imatinib mesylate-resistant GIST cell line (GIST430), whereas miR-218 over-expression can improve the sensitivity of GIST cells to imatinib mesylate, with PI3K/AKT signaling pathway possibly involved in the mechanism.

Adenovirus-mediated stem cell leukemia gene transfer induces rescue of interstitial cells of Cajal in ICC-loss mice

OBJECTIVE

Interaction of c-Kit and its ligand stem cell factor (SCF) is necessary for appropriate development and survival of interstitial cells of Cajal (ICC) in the intestine. Blockade of c-Kit will cause ICC loss in vivo. Stem cell leukemia (SCL) gene acts as a positive regulator of upstream transcription of c-Kit expression. This study aimed to explore whether the restoration of c-Kit expression promoted by SCL gene transfer could rescue ICC in vivo.

MATERIALS AND METHODS

A modified ICC-loss mouse model was created by continual administration of anti-c-Kit antibody (ACK2) to obtain a steady status of ICC loss, and a recombinant adenovirus vector containing SCL gene (Ad-SCL) was designed to rescue ICC in these mice. Western blot analysis and immunofluorescence labeling assays were performed to analyze the SCL and c-Kit expression in vitro and in vivo. The distribution and configuration of ICC were observed with immunohistochemistry and electromicroscope.

RESULTS

Western blot analysis and immunofluorescence labeling assays showed that SCL gene was successfully delivered to cultured HeLa and ICC cells in vitro. Moreover, significantly increased c-Kit expression could be detected in the colon of Ad-SCL-infected ICC-loss mice. Furthermore, rescue of the ICC network and ICC with typical ultrastructural features could be detected in Ad-SCL-infected ICC-loss mice at day 37.

CONCLUSIONS

Ad-SCL was able to enhance c-Kit expression, reactivate the c-Kit/SCF pathway, and rescue ICC in ICC-loss mice. Since loss and defects of ICC are associated with many human gut motility disorders, Ad-SCL may be of potential use in gene therapy of these patients.

Signaling of c-kit in dendritic cells influences adaptive immunity

The binding of the receptor tyrosine kinase, c-kit, to its ligand, stem cell factor (SCF), mediates numerous biological functions. Important roles for c-kit in hematopoiesis, melanogenesis, erythropoiesis, spermatogenesis, and carcinogenesis are well documented. Similarly, activation of granulocytes, mast cells, and of eosinophils in particular, by c-kit ligation has long been known to result in degranulation with concomitant release of pro-inflammatory mediators, including cytokines. However, recent work from a number of laboratories, including our own, highlights previously unappreciated functions for c-kit in immunologic processes. These novel findings strongly suggest that signaling through the c-kit-SCF axis could have a significant impact on the pathogenesis of diseases associated with an immunologic component. In our own studies, c-kit upregulation on dendritic cells via T helper (Th)2- and Th17-inducing stimuli led to c-kit activation and immune skewing toward these T helper subsets and away from Th1 responses. Others have shown that dendritic cell treatment with inhibitors of c-kit activation, such as imatinib mesylate (Gleevec), favored breaking of T-cell tolerance, skewing of responses toward production of Th1 cytokines, and activation of natural killer cells. These data all indicate that deeper understanding of, and ability to control, the c-kit-SCF axis could lead to improved treatment modalities aimed at redirecting unwanted and/or deleterious immune responses in a wide variety of conditions.

Emerging functions of c-kit and its ligand stem cell factor in dendritic cells: regulators of T cell differentiation

The receptor tyrosine kinase, c-kit, and its ligand, stem cell factor (SCF), function in a diverse range of biological functions. The role of c-kit in the maintenance and survival of hematopoietic stem cells and of mast cells is well recognized. c-kit also plays an important role in melanogenesis, erythropoiesis and spermatogenesis. Recent work from our laboratory highlights an important role of c-kit in the regulation of expression of two molecules in dendritic cells (DCs), interleukin-6 (IL-6) and Jagged-2 (a ligand of Notch), which are known to regulate T helper cell differentiation. Our study shows that induction of c-kit expression and its signaling in DCs promotes Th2 and Th17 responses but not Th1 response. c-kit inhibition by imatinib mesylate (Gleevec) in DCs was previously shown to promote natural killer cell activation which may be due to dampening of IL-6 production by the DCs. Since dysregulation of c-kit function has been associated with various disease states including cancer, in this perspective we have focused on known and novel functions of c-kit to include molecules such as IL-6 and Notch that were not previously recognized to be within the purview of c-kit biology. We have also reviewed the differential expression pattern of SCF and c-kit on various cell types and its variation during development or pathology. The recognition of previously unappreciated roles for c-kit will provide better insights into its function within and beyond the immune system and pave the way for developing better therapeutic strategies.

Regulation of interstitial cells of Cajal in the mouse gastric body by neuronal nitric oxide

The factors underlying the survival and maintenance of interstitial cells of Cajal (ICC) are not well understood. Loss of ICC is often associated with loss of neuronal nitric oxide synthase (nNOS) in humans, suggesting a possible link. The aim of this study was to determine the effect of neuronal NO on ICC in the mouse gastric body. The volumes of ICC were determined in nNOS(-/-) and control mice in the gastric body and in organotypic cultures using immunohistochemistry, laser scanning confocal microscopy and three-dimensional reconstruction. ICC numbers were determined in primary cell cultures after treatment with an NO donor or an NOS inhibitor. The volumes of myenteric c-Kit-immunoreactive networks of ICC from nNOS(-/-) mice were significantly reduced compared with control mice. No significant differences in the volumes of c-Kit-positive ICC were observed in the longitudinal muscle layers. ICC volumes were either decreased or unaltered in the circular muscle layer after normalization for the volume of circular smooth muscle. The number of ICC was increased after incubation with S-nitroso-N-acetylpenicillamine and decreased by N(G)-nitro-l-arginine. Neuronally derived NO modulates ICC numbers and network volume in the mouse gastric body. NO appears to be a survival factor for ICC.

Computer aided classification of cell nuclei in the gastrointestinal tract by volume and principal axis

Normal function of the gastrointestinal tract involves the coordinated activity of several cell types Human disorders of motor function of the gastrointestinal tract are often associated with changes in the number of these cells. For example, in diabetic patients, abnormalities in gastrointestinal transit are associated with changes in nerves and interstitial cells of Cajal (ICC), two key cells that generate and regulate motility. ICC are cells of mesenchymal origin that function as pacemakers and amplify neuronal signals in the gastrointestinal tract. Quantifying the changes in number of specific cell types in tissues from patients with motility disorders is challenging and requires immunolabeling for specific antigens. The shape of nuclei differs between the cell types in the wall of the gastrointestinal tract. Therefore the objective of this study was to determine whether cell nuclei can be classified by analyzing the 3D morphology of the nuclei. Furthermore, the orientation of the long axis of nuclei changes within and between the muscle layers. These features can be used to classify and differentially label the nuclei in confocal volume images of the tissue by computing the principal axis of the coordinates of the set of voxels forming each nucleus and thereby to identify cells by their nuclear morphology. Using this approach, we were able to separate and quantify nuclei in the smooth muscle layers of the tissue. Therefore we conclude that computer-aided classification of cell nuclei can be used to identify changes in the cell types expressed in gastrointestinal smooth muscle.

Subtractive hybridization unravels a role for the ion cotransporter NKCC1 in the murine intestinal pacemaker

In the small intestine, interstitial cells of Cajal (ICC) surrounding the myenteric plexus generate the pacemaking slow waves that are essential for an efficient intestinal transit. The underlying molecular mechanisms of the slow wave are poorly known. Our aim was to identify ICC-specific genes and their function in the mouse jejunum. Suppression subtractive hybridization using two independent ICC-deficient mouse models identified 56 genes putatively downregulated in the muscularis propria compared with wild-type littermates. Differential expression was confirmed by real-time quantitative PCR for the tyrosine kinase receptor KIT, the established marker for ICC, and for the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1). Immunoreactivity for NKCC1 was detected in myenteric ICC but not in the ICC population located at the deep muscular plexus. NKCC1 was also expressed in enteric neurons and mucosal crypts. Bumetanide, an NKCC1 inhibitor, reversibly affected the shape, amplitude, and frequency of the slow waves. Similar alterations were observed in NKCC1 knockout mice. These data support the hypothesis that NKCC1 expressed in myenteric ICC is involved in the mechanism of slow waves in the murine jejunum.

A clinical and biological overview of gastrointestinal stromal tumors

In the last few years a body of knowledge has been generated on the molecular basis of gastrointestinal stromal tumors (GIST). These mesenchymal tumors are characterized by the expression of KIT protein and because they have an activating mutation in a class III receptor tyrosine kinase gene (KIT or PDGFRA). Several KIT-activating mutations, which are largely responsible for the development of this tumor, promote cell survival, proliferation, and migration through different pathways such as MAPK p42/44, AKT, S6K, STAT1, and STAT3. Likewise, gene-activating mutations in the gene PDGFRalpha which codes for the receptor tyrosine kinase, Platelet-derived growth factor receptor alpha have been identified in GIST lacking KIT mutations. This means that KIT and PDGFRalpha mutations appear to be alternative and mutually exclusive oncogenic pathways for GIST development. These tumors may occur anywhere along the gastrointestinal tract (GI). The most frequently involved sites are stomach and small intestine. They are typically chemo- and radioresistant. The discovery of a specific inhibitor of this tyrosine kinase, imatinib mesylate, has radically changed the prognosis of patients with unresectable disease. Only 4 yr after the first patient was successfully treated with imatinib, multiple phase II and III trials have been published and, currently, imatinib mesylate is the only effective systemic treatment available of these tumors. Response rates are approximately 70-90% with acceptable toxicity. GIST are the first model of a solid tumor efficiently treated with a molecular-targeted agent. This review summarizes the clinical and biological aspects of this unique neoplasm.

Dissociation, culture and morphologic changes of interstitial cells of Cajal in vitro

AIM: To study the method of dissociation, culture and investigate its morphologic changes in vitro of interstitial cells of Cajal (ICC).

METHODS: Enzymatic digestion and Ficoll density centrifu-gation were used to dissociate ICC from the ileal segment of mice. Factors including contamination, Ca²⁺, Mg²⁺ and collagenase, and stem cell factor, etc., were investigated. ACK2, the antibody of c-kit, was used to identify the cultured ICC. Both light microscope and fluorescence microscope were used to observe the changes of ICC in vitro.

RESULTS: The method for dissociation and culture of ICC in vitro was successfully established. After 24 h, cultured ICC exhibited a few axis-cylinders, and longer axis-cylinders were observed to form synapse of each other after 3 d. More widespread connections formed within 7 d in vitro. The changes of its morphologic character were obvious within 7 d; however, there were no obvious morphologic changes after 30 d.

CONCLUSION: Many factors can influence the dissociation and culture of ICC.

Reference Systems for Kinase Drug Discovery: Chemical Genetic Approaches to Cell-Based Assays

Protein kinases play key roles in a number of diseases, including cancer, inflammation, and diabetes. Disregulation of kinase-based signal transduction networks results in aberrant cell differentiation, activation, proliferation, and invasion. The growing importance of kinases as a major class of drug targets across multiple large clinical indications, together with the large number of kinases in the genome (~518), has generated a critical need for technologies that enable the identification of potent and selective kinase inhibitors with good drug-like properties. In this review, we describe methods used for developing cell-based assays for kinase inhibitors, discuss advantages and disadvantages of each approach, and describe new chemical genetic methods as reference systems for establishing cell-based assays and their use for functional selectivity profiling of kinase inhibitors.