ATP/IL-33-Co-Sensing by Mast Cells (MCs) Requires Activated c-Kit to Ensure Effective Cytokine Responses.

Mast cells (MCs) are sentinel cells which represent an important part of the first line of defense of the immune system. MCs highly express receptors for danger-associated molecular patterns (DAMPs) such as the IL-33R and P2X7, making MCs to potentially effective sensors for IL-33 and adenosine-triphosphate (ATP), two alarmins which are released upon necrosis-induced cell damage in peripheral tissues. Besides receptors for alarmins, MCs also express the stem cell factor (SCF) receptor c-Kit, which typically mediates MC differentiation, proliferation and survival. By using bone marrow-derived MCs (BMMCs), ELISA and flow cytometry experiments, as well as p65/RelA and NFAT reporter MCs, we aimed to investigate the influence of SCF on alarmin-induced signaling pathways and the resulting cytokine production and degranulation. We found that the presence of SCF boosted the cytokine production but not degranulation in MCs which simultaneously sense ATP and IL-33 (ATP/IL-33 co-sensing). Therefore, we conclude that SCF maintains the functionality of MCs in peripheral tissues to ensure appropriate MC reactions upon cell damage, induced by pathogens or allergens.

Skin wound closure delay in metabolic syndrome correlates with SCF deficiency in keratinocytes.

Poor wound closure due to diabetes, aging, stress, obesity, alcoholism, and chronic disease affects millions of people worldwide. Reasons wounds will not close are still unclear, and current therapies are limited. Although stem cell factor (SCF), a cytokine, is known to be important for wound repair, the cellular and molecular mechanisms of SCF in wound closure remain poorly understood. Here, we found that SCF expression in the epidermis is decreased in mouse models of delayed wound closure intended to mimic old age, obesity, and alcoholism. By using SCF conditionally knocked out mice, we demonstrated that keratinocytes' autocrine production of SCF activates a transient c-kit receptor in keratinocytes. Transient activation of the c-kit receptor induces the expression of growth factors and chemokines to promote wound re-epithelialization by increasing migration of skin cells (keratinocytes and fibroblasts) and immune cells (neutrophils) to the wound bed 24-48 h post-wounding. Our results demonstrate that keratinocyte-produced SCF is essential to wound closure due to the increased recruitment of a unique combination of skin cells and immune cells in the early phase after wounding. This discovery is imperative for developing clinical strategies that might improve the body's natural repair mechanisms for treating patients with wound-closure pathologies.

KIT ligand protects against both light-induced and genetic photoreceptor degeneration.

Photoreceptor degeneration is a major cause of blindness and a considerable health burden during aging but effective therapeutic or preventive strategies have not so far become readily available. Here, we show in mouse models that signaling through the tyrosine kinase receptor KIT protects photoreceptor cells against both light-induced and inherited retinal degeneration. Upon light damage, photoreceptor cells upregulate Kit ligand (KITL) and activate KIT signaling, which in turn induces nuclear accumulation of the transcription factor NRF2 and stimulates the expression of the antioxidant gene Hmox1. Conversely, a viable Kit mutation promotes light-induced photoreceptor damage, which is reversed by experimental expression of Hmox1. Furthermore, overexpression of KITL from a viral AAV8 vector prevents photoreceptor cell death and partially restores retinal function after light damage or in genetic models of human retinitis pigmentosa. Hence, application of KITL may provide a novel therapeutic avenue for prevention or treatment of retinal degenerative diseases.

Preliminary evaluation of neoblast-like stem cell factor and transcript expression profiles in Schistosoma japonicum.

Neoblast-like stem cell factors and transcripts are essential for cell proliferation, self-renewal, and differentiation. Recent studies have demonstrated that nanos, sox, and vasa-like transcription factors are associated with neoblast-like stem cells in Schistosoma mansoni and play crucial roles in the regulation of worm development. However, these neoblast-like stem cell factors and transcripts and their expression profiles remain unknown in Schistosoma japonicum. In this study, we identified orthologs of 11 neoblast-like stem cell factors and transcripts in S. japonicum using bioinformatics and confirmed them by PCR. The expression profiles of neoblast-like stem cell factors and transcripts revealed that some of them were highly expressed in certain stages. Sex-based expression analysis revealed that nanos, polo-like kinase, PCNA, cyclin B, and H2A showed significantly higher expression in female worms, whereas ago and bruli showed higher expression in male worms. In addition, we noted that ago, bruli, and pp32 exhibited higher expression in the testes, while nanos, polo-like kinase, cyclin B, H2A, and H2B showed notable higher expression in both isolated ovaries and testes. Our preliminary results are expected to provide important information about the regulatory roles of these stem cell factors in parasite development and sexual maturation.

Contributions of inflammation and tumor microenvironment to neurofibroma tumorigenesis.

Neurofibromatosis type 1 associates with multiple neoplasms, and the Schwann cell tumor neurofibroma is the most prevalent. A hallmark feature of neurofibroma is mast cell infiltration, which is recruited by chemoattractant stem cell factor (SCF) and has been suggested to sustain neurofibroma tumorigenesis. In the present study, we use new, genetically engineered Scf mice to decipher the contributions of tumor-derived SCF and mast cells to neurofibroma development. We demonstrate that mast cell infiltration is dependent on SCF from tumor Schwann cells. However, removal of mast cells by depleting the main SCF source only slightly affects neurofibroma progression. Other inflammation signatures show that all neurofibromas are associated with high levels of macrophages regardless of Scf status. These findings suggest an active inflammation in neurofibromas and partly explain why mast cell removal alone is not sufficient to relieve tumor burden in this experimental neurofibroma model. Furthermore, we show that plexiform neurofibromas are highly associated with injury-prone spinal nerves that are close to flexible vertebras. In summary, our study details the role of inflammation in neurofibromagenesis. Our data indicate that prevention of inflammation and possibly also nerve injury at the observed tumor locations are therapeutic approaches for neurofibroma prophylaxis and that such treatment should be explored.

[Expressions of stem cell factor and hypoxia inducible factor 1 alpha in pancreatic cancer cells and related mechanism].

Objective To study the correlation between the expressions of stem cell factor (SCF) and hypoxia inducible factor 1 alpha (HIF-1α) in pancreatic cancer, and investigate the mechanism by which SCF regulates the expression of HIF-1α. Methods Immunohistochemistry was used to detect the expressions of SCF and HIF-1α in pancreatic cancer specimens and to analyze the correlation between SCF and HIF-1α expressions. Pancreatic cancer PANC-1 cells were treated with different doses of SCF (0, 1, 10, 100 ng/mL) alone or combined with c-KIT inhibitor Gleevec (5 µmol/L). Real-time fluorescent quantitative PCR (qRT-PCR) was performed to detect the level of HIF-1α mRNA, and Western blotting to detect the HIF-1α protein level, the phosphorylation levels of ERK1/2 and AKT. Results SCF and HIF-1α were up-regulated in pancreatic cancer samples and they had an obvious positive correlation. In PANC-1 cells, SCF didn't affect the expression of HIF-1α mRNA, but up-regulated the expression of HIF-1α protein in a dose-dependent manner. Gleevec inhibited the SCF-induced up-regulation of HIF-1α protein, but did not affect the mRNA. And Gleevec blocked the phosphorylation of AKT and ERK1/2. Conclusion SCF/c-KIT can up-regulate the protein expression of HIF-1α by activating AKT and ERK signaling pathways in pancreatic cancer cells.

Thrombopoietin contributes to the formation and the maintenance of hematopoietic progenitor-containing cell clusters in the aorta-gonad-mesonephros region.

In the midgestation mouse embryo, hematopoietic cell clusters containing hematopoietic stem/progenitor cells arise in the aorta-gonad-mesonephros (AGM) region. We have previously reported that forced expression of the Sox17 transcription factor in CD45lowc-Kithigh AGM cells, which are the hematopoietic cellular component of the cell clusters, and subsequent coculture with OP9 stromal cells in the presence of three cytokines, stem cell factor (SCF), interleukin-3 (IL-3), and thrombopoietin (TPO), led to the formation and the maintenance of cell clusters with cells at an undifferentiated state in vitro. In this study, we investigated the role of each cytokine in the formation of hematopoietic cell clusters. We cultured Sox17-transduced AGM cells with each of the 7 possible combinations of the three cytokines. The size and the number of Sox17-transduced cell clusters in the presence of TPO, either alone or in combination, were comparable to that observed with the complete set of the three cytokines. Expression of TPO receptor, c-Mpl was almost ubiquitously expressed and maintained in Sox17-transduced hematopoietic cell clusters. In addition, the expression level of c-Mpl was highest in the CD45lowc-Kithigh cells among the Sox17-transduced cell clusters. Moreover, c-Mpl protein was highly expressed in the intra-aortic hematopoietic cell clusters in comparison with endothelial cells of dorsal aorta. Finally, stimulation of the endothelial cells prepared from the AGM region by TPO induced the production of hematopoietic cells. These results suggest that TPO contributes to the formation and the maintenance of hematopoietic cell clusters in the AGM region.

Stem cell factor promotes in vitro ovarian follicle development in the domestic cat by upregulating c-kit mRNA expression and stimulating the phosphatidylinositol 3-kinase/AKT pathway.

In the present study we examined the effects of stem cell factor (SCF; 50 vs 100ngmL-1) alone or in combination with epidermal growth factor (EGF; 100ngmL-1) on: (1) the in vitro viability and growth of cat follicles within ovarian cortices; (2) phosphatidylinositol 3-kinase (PI3K)/AKT and mitogen-activated protein kinase (MAPK) phosphorylation; and (3) c-kit and FSH receptor (FSHr) mRNA expression. At 100ngmL-1, SCF increased (P≤0.05) the percentage and size of secondary follicles after 14 days of in vitro culture and sustained AKT phosphorylation after 3 days incubation. EGF suppressed this beneficial effect and reduced (P≤0.05) the percentage of structurally normal follicles and FSHr expression when combined with 100ngmL-1 SCF. Expression of c-kit mRNA was higher (P≤0.05) in the presence of 100ngmL-1 SCF compared with fresh follicles and cohorts cultured under other conditions. A c-kit inhibitor suppressed follicle growth and reduced AKT phosphorylation. Collectively, the results demonstrate that SCF promotes cat follicle development by upregulating c-kit mRNA expression and AKT phosphorylation. EGF suppresses the stimulating effect of SCF, leading to downregulation of FSHr expression.

Aberrant expressions of stem cell factor/c-KIT in rat testis with varicocele.

BACKGROUND/PURPOSE: Varicocele (VC) is considered by the World Health Organization as the main cause of male infertility. Studies have shown that VC can affect spermatogenesis and then result in male infertility. But the exact mechanism by which VC affects spermatogenesis is still unclear. Stem cell factor (SCF) and c-KIT receptor are crucial molecules during spermatogenesis in testis. This study aims to investigate whether SCF/c-KIT signaling is involved in the pathophysiology of VC on spermatogenesis. METHODS: Rat models of VC were built (n = 13), and sham-operated rats were used as controls (n = 8). The seminiferous tubules of the testis were observed with hematoxylin and eosin staining, expression of SCF was analyzed via enzyme-linked immunosorbent assay and Western blot, and expression of c-KIT was assessed with Western blot and immunofluorescence. RESULTS: Compared with controls, the seminiferous epithelium was disorganized and had significantly fewer cells in the testes of rats with VC. Expression of SCF increased in testes of VC rats, while expression of c-KIT was decreased. CONCLUSION: These results suggest that sperm counts in seminiferous epithelium are affected by VC, and the SCF/c-KIT system is aberrantly expressed in VC testis, which could be involved in male infertility caused by VC.

SCF/c-kit transactivates CXCR4-serine 339 phosphorylation through G protein-coupled receptor kinase 6 and regulates cardiac stem cell migration.

C-kit positive cardiac stem cells (CSCs) have been shown to contribute to myocardial regeneration after infarction. Previously, we have shown that the c-kit ligand stem cell factor (SCF) can induce CSC migration into the infarcted area during myocardial infarction (MI). However, the precise mechanism involved is not fully understood. In this study, we found that CSCs also express C-X-C chemokine receptor type 4 (CXCR4), which is a typical member of the seven transmembrane-spanning G protein-coupled receptor (GPCR). In vitro, activation of c-kit signalling by SCF promotes migration of CSCs with increased phosphorylation of CXCR4-serine 339, p38 mitogen-activated protein kinase (p38 MAPK) and extracellular regulated protein kinases 1/2 (ERK1/2). Knockdown of CXCR4 expression by siRNA reduces SCF/c-kit-induced migration and downstream signalling. As previously reported, CXCR4-serine 339 phosphorylation is mainly regulated by GPCR kinase 6 (GRK6); thus, silencing of GRK6 expression by siRNA impairs CXCR4-serine 339 phosphorylation and migration of CSCs caused by SCF. In vivo, knockdown of GRK6 impairs the ability of CSCs to migrate into peri-infarcted areas. These results demonstrate that SCF-induced CSC migration is regulated by the transactivation of CXCR4-serine 339 phosphorylation, which is mediated by GRK6.

Cancer cell motility is affected through 3D cell culturing and SCF/c-Kit pathway but not by X-irradiation.

BACKGROUND AND PURPOSE: Success of radiotherapy is often limited by therapy resistance and metastasis resulting from cancer cell motility. It was tested in vitro whether this cancer cell motility is affected by growth condition, active SCF/c-Kit pathway or X-irradiation. MATERIALS AND METHODS: Cell motility was measured with BioCoat™ Matrigel™ invasion chamber using four different cancer cell lines (NSCLC: H23, H520, H226 and PrCa: DU145). Cells were grown in 2D or 3D, SCF was knocked down by siRNA and cells were irradiated with 2 or 6Gy. RESULTS: All cell lines except H520 showed a 2-3-fold increase in cell motility when grown in 3D. This effect was considered to result from the EMT-like change seen when cells were grown in 3D as indicated by the enhanced expression of vimentin and N-cadherin and reduction of E-cadherin. Just the opposite trends were found for H520 cells. Knockdown of SCF was found to result in reduced cell motility for both 2D and 3D. In contrast, X-irradiation did not modulate cell motility neither under 2D nor 3D. In line with this, X-irradiation did neither induce the expression of EMT-associated genes nor SCF. CONCLUSION: X-irradiation affects neither the expression of important EMT genes such as vimentin, E-cadherin and N-cadherin nor SCF/c-Kit signaling and, as a consequence, does not alter cell motility.

The development of human mast cells. An historical reappraisal.

The understanding of mast cell (MC) differentiation is derived mainly from in vitro studies of different stages of stem and progenitor cells. The hematopoietic lineage development of human MCs is unique compared to other myeloid-derived cells. Human MCs originate from CD34(+)/CD117(+)/CD13(+)multipotent hematopoietic progenitors, which undergo transendothelial recruitment into peripheral tissues, where they complete differentiation. Stem cell factor (SCF) is a major chemotactic factor for MCs and their progenitors. SCF also elicits cell-cell and cell-substratum adhesion, facilitates the proliferation, and sustains the survival, differentiation, and maturation, of MCs. Because MC maturation is influenced by local microenvironmental factors, different MC phenotypes can develop in different tissues and organs.

Regulation of FOXO3 subcellular localization by Kit ligand in the neonatal mouse ovary.

PURPOSE: Foxo3 protein is required in the oocyte nucleus for the maintenance of primordial follicles in a dormant state. PI3K/AKT-dependent phosphorylation of Foxo3 leads to its relocalization to the cytoplasm and subsequent follicular activation. However, the nature of the upstream signals controlling Foxo3 activity and subcellular localization remains unknown. We aimed to study the in vitro effects of Kit ligand (stem cell factor) on the subcellular localization of Foxo3 in primordial follicles within the postnatal mouse ovary. METHODS: This was an in vitro study using explants of intact neonatal mouse ovaries. The study was performed in laboratory animal facility and basic science research laboratory at a University Hospital. The animals used for this study were FVB mice. Neonatal FVB mice ovaries at postnatal day 7 (PD7) were harvested and incubated in culture medium (DMEM) at 37 °C and 5 % CO(2) for 60-90 min with (n = 3) or without (n = 3) Kit ligand at 150 ng/mL (8 nM). Similar experimental conditions were used to establish a dose-response curve for the effects of Kit ligand and assess the effects of imatinib (small molecule inhibitor of the Kit receptor). Immunofluorescence was used to identify the subcellular location of Foxo3 in oocytes. Proportions of cytoplasmic versus nuclear Foxo3 in primordial follicles were determined. RESULTS: Kit ligand treatment increased the cytoplasmic localization of Foxo3 from 40 % in the untreated ovaries to 74 % in the treated group (p = 0.007 in paired samples and p = 0.03 in unpaired samples). Furthermore, this effect was reversible with imatinib (p = 0.005). A dose-response curve for Kit ligand treatment showed that maximum effect was seen at 150 ng/mL. CONCLUSION: Kit ligand treatment in vitro increases the proportion of cytoplasmic Foxo3 in primordial follicles at PD7, lending support to the idea that Kit receptor/ligand controls Foxo3 activity in the context of primordial follicle activation.

Cellular and molecular regulation of the activation of mammalian primordial follicles: somatic cells initiate follicle activation in adulthood.

BACKGROUND: The first small follicles to appear in the mammalian ovaries are primordial follicles. The initial pool of primordial follicles serves as the source of developing follicles and oocytes for the entire reproductive lifespan of the animal. Although the selective activation of primordial follicles is critical for female fertility, its underlying mechanisms have remained poorly understood. METHODS: A search of PubMed was conducted to identify peer-reviewed literature pertinent to the study of mammalian primordial follicle activation, especially recent reports of the role of primordial follicle granulosa cells (pfGCs) in regulating this process. RESULTS: In recent years, molecular mechanisms that regulate the activation of primordial follicles have been elucidated, mostly through the use of genetically modified mouse models. Several molecules and pathways operating in both the somatic pfGCs and oocytes, such as the phosphatidylinositol 3 kinase (PI3K) and the mechanistic target of rapamycin complex 1 (mTORC1) pathways, have been shown to be important for primordial follicle activation. More importantly, recent studies have provided an updated view of how exactly signaling pathways in pfGCs and in oocytes, such as the KIT ligand (KL) and KIT, coordinate in adult ovaries so that the activation of primordial follicles is achieved. CONCLUSIONS: In this review, we have provided an updated picture of how mammalian primordial follicles are activated. The functional roles of pfGCs in governing the activation of primordial follicles in adulthood are highlighted. The in-depth understanding of the cellular and molecular mechanisms of primordial follicle activation will hopefully lead to more treatments of female infertility, and the current progress indicates that the use of existing primordial follicles as a source for obtaining fertilizable oocytes as a new treatment for female infertility is just around the corner.

Kit ligand decreases the incidence of apoptosis in cultured vitrified whole mouse ovaries.

To investigate the development of follicles and incidence of apoptosis in vitrified cultured mouse ovaries in the presence and absence of Kit ligand, 1-week-old mouse ovaries were cultured in the presence or absence of Kit ligand for 7 days. Development and function of ovarian follicles was evaluated by histology and hormonal analysis. Apoptosis assessment was conducted by analysis of DNA laddering, TdT (terminal deoxynucleotidyl transferase)-mediated dUDP nick-end-labelling and caspase-3/7 activity. The proportion of preantral follicles and the level of 17-ß oestradiol, progesterone and dehydroepiandrosterone were increased in all cultured groups, and it was significantly higher in Kit ligand treated groups than in the control (P < 0.001). The number of apoptotic signals in both vitrified samples is significantly higher than in the non-vitrified control (P < 0.01), and these signals are significantly lower in both Kit ligand treated groups than in non-Kit ligand treated groups (P < 0.001). The level of caspase-3/7 activity was higher in vitrified cultured ovaries than non-vitrified group (P < 0.01). Kit ligand was shown to improve in-vitro development of follicles, and also acted as an anti-apoptotic factor in vitrified ovaries. The developmental potential of follicles in vitrified groups was lower than that in fresh ovaries.

KIT is a frequent target for epigenetic silencing in cutaneous melanoma.

The receptor tyrosine kinase KIT and its ligand, stem cell factor (SCF), are essential for the proliferation and survival of normal melanocytes. In melanomas arising on mucosal, acral, and chronically sun-damaged skin, activating KIT mutations have been identified as oncogenic drivers and potent therapeutic targets. Through an initial whole-genome screen for aberrant promoter methylation in melanoma, we identified the KIT promoter as a target for hypermethylation in 43/110 melanoma cell lines, and in 3/12 primary and 11/29 metastatic cutaneous melanomas. Methylation density at the KIT promoter correlated inversely with promoter activity in vitro and in vivo, and the expression of KIT was restored after treatment with the demethylating agent 5-aza-2'-deoxycytidine. Hypermethylation of KIT showed no direct or inverse correlations with well-documented melanoma drivers. Growth of melanoma cells in the presence of SCF led to reduced KIT expression and increased methylation density at the KIT promoter, suggesting that SCF may exert a selection pressure for the loss of KIT. The frequent loss of KIT in cutaneous melanoma by promoter hypermethylation suggests that distinct KIT signaling pathways have opposing roles in the pathogenesis of melanoma subtypes.

PRL2/PTP4A2 phosphatase is important for hematopoietic stem cell self-renewal.

Hematopoietic stem cell (HSC) self-renewal is tightly controlled by cytokines and other signals in the microenvironment. While stem cell factor (SCF) is an early acting cytokine that activates the receptor tyrosine kinase KIT and promotes HSC maintenance, how SCF/KIT signaling is regulated in HSCs is poorly understood. The protein tyrosine phosphatase 4A (PTP4A) family (aka PRL [phosphatase of regenerating liver] phosphatases), consisting of PTP4A1/PRL1, PTP4A2/PRL2, and PTP4A3/PRL3, represents an intriguing group of phosphatases implicated in cell proliferation and tumorigenesis. However, the role of PTP4A in hematopoiesis remains elusive. To define the role of PTP4A in hematopoiesis, we analyzed HSC behavior in Ptp4a2 (Prl2) deficient mice. We found that Ptp4a2 deficiency impairs HSC self-renewal as revealed by serial bone marrow transplantation assays. Moreover, we observed that Ptp4a2 null hematopoietic stem and progenitor cells (HSPCs) are more quiescent and show reduced activation of the AKT and ERK signaling. Importantly, we discovered that the ability of PTP4A2 to enhance HSPC proliferation and activation of AKT and ERK signaling depends on its phosphatase activity. Furthermore, we found that PTP4A2 is important for SCF-mediated HSPC proliferation and loss of Ptp4a2 decreased the ability of oncogenic KIT/D814V mutant in promoting hematopoietic progenitor cell proliferation. Thus, PTP4A2 plays critical roles in regulating HSC self-renewal and mediating SCF/KIT signaling.

Critical role of Jak2 in the maintenance and function of adult hematopoietic stem cells.

Jak2, a member of the Janus kinase family of nonreceptor protein tyrosine kinases, is activated in response to a variety of cytokines, and functions in survival and proliferation of cells. An activating JAK2V617F mutation has been found in most patients with myeloproliferative neoplasms, and patients treated with Jak2 inhibitors show significant hematopoietic toxicities. However, the role of Jak2 in adult hematopoietic stem cells (HSCs) has not been clearly elucidated. Using a conditional Jak2 knockout allele, we have found that Jak2 deletion results in rapid loss of HSCs/progenitors leading to bone marrow failure and early lethality in adult mice. Jak2 deficiency causes marked impairment in HSC function, and the mutant HSCs are severely defective in reconstituting hematopoiesis in recipient animals. Jak2 deficiency also causes significant apoptosis and loss of quiescence in HSC-enriched LSK (Lin(-)Sca-1(+)c-Kit(+)) cells. Jak2-deficient LSK cells exhibit elevated reactive oxygen species levels and enhanced p38 MAPK activation. Mutant LSK cells also show defective Stat5, Erk, and Akt activation in response to thrombopoietin and stem cell factor. Gene expression analysis reveals significant downregulation of genes related to HSC quiescence and self-renewal in Jak2-deficient LSK cells. These data suggest that Jak2 plays a critical role in the maintenance and function of adult HSCs.

Inhibitory effects of imatinib mesylate on human epidermal melanocytes.

BACKGROUND: In recent years, increasing attention has been focused on the skin hypopigmentation that develops after the initiation of imatinib mesylate therapy in patients with chronic myeloid leukaemia (CML). AIM: To understand the underlying mechanism of this hypopigmentation effect, and to explore the possibility of using imatinib in the treatment of pigmentation disorders. METHODS: We examined the effects of imatinib on the proliferation, apoptosis, melanin content and melanogenic activity of human primary epidermal melanocytes. The responsible molecular events were also investigated in a mechanism study. RESULTS: We found that imatinib led to a dramatic decrease in total melanin content in cultured melanocytes, by affecting melanocyte number and/or melanogenesis in a dose-dependent manner. This inhibition of melanogenesis was due to suppressed expression of tyrosinase and microphthalmia-associated transcription factor (MiTF). Furthermore, stem cell factor (SCF)-stimulated c-Kit activation and melanocyte proliferation were completely abrogated by imatinib. CONCLUSIONS: Inactivation of c-Kit signalling by imatinib has inhibitory effects on melanocyte survival, proliferation and melanogenesis, which explains the clinical hypopigmentation seen in patients with CML. These results also support using imatinib as a clinical depigmentation agent when dosage being carefully determined.

Inhibition of c-Kit signaling by diosmetin isolated from Chrysanthemum morifolium.

The interaction of stem cell factor (SCF) with its cognate receptor c-Kit is closely associated with the survival and maturation of melanocytes. To investigate novel depigmentation agents, we screened 2,000 plant extracts for c-Kit inhibitors to identify active small molecules by using time-resolved fluorescence enzyme assays. For the active extracts identified as inhibitors of c-Kit enzyme, we evaluated the effects of the active extracts and isolated flavonoids on c-Kit phosphorylation in MO7e/melanocytes. Anti-melanogenic activity was also examined in melanocytes and melanoderm model. The flavonoids such as diosmetin, apigenin, acacetin and luteolin isolated from Chrysanthemum morifolium were found to be active in inhibiting c-Kit both at enzyme and cellular levels. In addition, these flavonoids attenuated SCF-induced proliferation of human primary melanocytes without toxicity and suppressed ultraviolet (UV) B irradiation-mediated melanin synthesis significantly. Among the active flavonoids, diosmetin was found to inhibit SCF-induced melanogenesis in a human melanoderm model. These results strongly suggest that C. morifolium extract and diosmetin have potential to suppress SCF-/UVB-induced melanogenesis, and could be developed as anti-pigmentation agents.