Decoupling the Functional Pleiotropy of Stem Cell Factor by Tuning c-Kit Signaling

Most secreted growth factors and cytokines are functionally pleiotropic because their receptors are expressed on diverse cell types. While important for normal mammalian physiology, pleiotropy limits the efficacy of cytokines and growth factors as therapeutics. Stem cell factor (SCF) is a growth factor that acts through the c-Kit receptor tyrosine kinase to elicit hematopoietic progenitor expansion but can be toxic when administered in vivo because it concurrently activates mast cells. We engineered a mechanism-based SCF partial agonist that impaired c-Kit dimerization, truncating downstream signaling amplitude. This SCF variant elicited biased activation of hematopoietic progenitors over mast cells in vitro and in vivo. Mouse models of SCF-mediated anaphylaxis, radioprotection, and hematopoietic expansion revealed that this SCF partial agonist retained therapeutic efficacy while exhibiting virtually no anaphylactic off-target effects. The approach of biasing cell activation by tuning signaling thresholds and outputs has applications to many dimeric receptor-ligand systems.

The role of gene mutations and gene products in intestinal tissue reactions from ionising radiation

The response of the intestine to (low linear-energy-transfer) ionising radiation is reviewed regarding the cellular basis to the reactions, the regenerative processes which restore the tissue, and external agents which aid its recovery. In the steady-state, it is generally considered that the crypt cell lineages in both small and large intestine are maintained by a small number of stem cells, but there are differences for example in the composition of their niche residence and in the numbers of transit cell generations. Various cell surface markers are now available to indentify particular lineage cell types. Radiation doses up to 1Gy cause apoptotic stem-cell death in particular locations, at higher doses to >6Gy Lgr5⁺ stem cells are required for normal intestinal recovery, and at >8Gy some crypts are sterilised and the probability of animal death from intestinal injury increases with higher doses. Mutations in repair genes, tumour suppressor genes, and survival genes cause various degrees of stem cell and clonogenic cell radiosensitisation. Recent evidence is suggesting much plasticity in the crypt cell lineage, potentially contributing to flexibility in the hierarchical lineage, clonogen number variations and the sensitisation differences. Knockout mice for many different genes have been used to detect their role in both steady state and in irradiated conditions, expected to lead to further insight to the damage and restorative processes. Many different external agents have been used to ameliorate intestinal reactions, including prostaglandins, interleukins, angiogenic and epithelial growth factors, other cytokines, and intraluminal factors.

Prevention and mitigation of acute radiation syndrome in mice by synthetic lipopeptide agonists of Toll-like receptor 2 (TLR2)

Bacterial lipoproteins (BLP) induce innate immune responses in mammals by activating heterodimeric receptor complexes containing Toll-like receptor 2 (TLR2). TLR2 signaling results in nuclear factor-kappaB (NF-κB)-dependent upregulation of anti-apoptotic factors, anti-oxidants and cytokines, all of which have been implicated in radiation protection. Here we demonstrate that synthetic lipopeptides (sLP) that mimic the structure of naturally occurring mycoplasmal BLP significantly increase mouse survival following lethal total body irradiation (TBI) when administered between 48 hours before and 24 hours after irradiation. The TBI dose ranges against which sLP are effective indicate that sLP primarily impact the hematopoietic (HP) component of acute radiation syndrome. Indeed, sLP treatment accelerated recovery of bone marrow (BM) and spleen cellularity and ameliorated thrombocytopenia of irradiated mice. sLP did not improve survival of irradiated TLR2-knockout mice, confirming that sLP-mediated radioprotection requires TLR2. However, sLP was radioprotective in chimeric mice containing TLR2-null BM on a wild type background, indicating that radioprotection of the HP system by sLP is, at least in part, indirect and initiated in non-BM cells. sLP injection resulted in strong transient induction of multiple cytokines with known roles in hematopoiesis, including granulocyte colony-stimulating factor (G-CSF), keratinocyte chemoattractant (KC) and interleukin-6 (IL-6). sLP-induced cytokines, particularly G-CSF, are likely mediators of the radioprotective/mitigative activity of sLP. This study illustrates the strong potential of LP-based TLR2 agonists for anti-radiation prophylaxis and therapy in defense and medical scenarios.

ICRP publication 118: ICRP statement on tissue reactions and early and late effects of radiation in normal tissues and organs--threshold doses for tissue reactions in a radiation protection context

This report provides a review of early and late effects of radiation in normal tissues and organs with respect to radiation protection. It was instigated following a recommendation in Publication 103 (ICRP, 2007), and it provides updated estimates of 'practical' threshold doses for tissue injury defined at the level of 1% incidence. Estimates are given for morbidity and mortality endpoints in all organ systems following acute, fractionated, or chronic exposure. The organ systems comprise the haematopoietic, immune, reproductive, circulatory, respiratory, musculoskeletal, endocrine, and nervous systems; the digestive and urinary tracts; the skin; and the eye. Particular attention is paid to circulatory disease and cataracts because of recent evidence of higher incidences of injury than expected after lower doses; hence, threshold doses appear to be lower than previously considered. This is largely because of the increasing incidences with increasing times after exposure. In the context of protection, it is the threshold doses for very long follow-up times that are the most relevant for workers and the public; for example, the atomic bomb survivors with 40-50years of follow-up. Radiotherapy data generally apply for shorter follow-up times because of competing causes of death in cancer patients, and hence the risks of radiation-induced circulatory disease at those earlier times are lower. A variety of biological response modifiers have been used to help reduce late reactions in many tissues. These include antioxidants, radical scavengers, inhibitors of apoptosis, anti-inflammatory drugs, angiotensin-converting enzyme inhibitors, growth factors, and cytokines. In many cases, these give dose modification factors of 1.1-1.2, and in a few cases 1.5-2, indicating the potential for increasing threshold doses in known exposure cases. In contrast, there are agents that enhance radiation responses, notably other cytotoxic agents such as antimetabolites, alkylating agents, anti-angiogenic drugs, and antibiotics, as well as genetic and comorbidity factors. Most tissues show a sparing effect of dose fractionation, so that total doses for a given endpoint are higher if the dose is fractionated rather than when given as a single dose. However, for reactions manifesting very late after low total doses, particularly for cataracts and circulatory disease, it appears that the rate of dose delivery does not modify the low incidence. This implies that the injury in these cases and at these low dose levels is caused by single-hit irreparable-type events. For these two tissues, a threshold dose of 0.5Gy is proposed herein for practical purposes, irrespective of the rate of dose delivery, and future studies may elucidate this judgement further.

Polyamine depletion inhibits irradiation-induced apoptosis in intestinal epithelia

Our group has previously shown that polyamine depletion delays apoptosis in rat intestinal epithelial (IEC-6) cells (Ray RM, Viar MJ, Yuan Q, and Johnson LR, Am J Physiol Cell Physiol 278: C480-C489, 2000). Here, we demonstrate that polyamine depletion inhibits gamma-irradiation-induced apoptosis in vitro and in vivo. Pretreatment of IEC-6 cells with 5 mM alpha-difluoromethylornithine (DFMO) for 4 days significantly reduced radiation-induced caspase-3 activity and DNA fragmentation. This protective effect was prevented by the addition of 10 muM exogenous putrescine. Radiation exposure to mice resulted in a high frequency of apoptosis over cells positioned fourth to seventh in crypt-villus units. Pretreatment of mice with 2% DFMO in drinking water significantly reduced apoptotic cells from approximately 2.75 to 1.61 per crypt-villus unit, accompanied by significant decreases in caspase-3 levels. Further examination showed that DFMO pretreatment inhibited the radiation-induced increase in the proapoptotic protein Bax. Moreover, DFMO pretreatment significantly enhanced the intestinal crypt survival rate by 2.1-fold subsequent to radiation and ameliorated mucosal structural damage. We conclude that polyamine depletion by DFMO inhibits gamma-irradiation-induced apoptosis of intestinal epithelial cells both in vitro and in vivo through inhibition of Bax and caspase-3 activity, which leads to attenuation of radiation-inflicted intestinal injury. These data indicate that DFMO may be therapeutically useful to counteract the gastrointestinal toxicity caused by chemoradiotherapy. This is the first demonstration that polyamines are required for apoptosis in vivo.

Mechanism and modification of gastrointestinal soft tissue response to radiation: role of growth factors

PURPOSE

The negative effects of radiation on the bowel critically limit the treatment doses possible for tumors in the abdomen. The purpose of the present study was to measure mRNA levels of inflammatory cytokines in abdominally irradiated mouse bowel.

METHODS AND MATERIALS

Eight- to 12-week-old DBA mice were irradiated to the whole bowel in single fractions of 0 (mock irradiation), 12.5, or 13.5 Gy, and sacrificed 18-25 weeks thereafter. Gross bowel reactions were scored for bowel retraction, bowel wall thickening, mesenteric telangiectasia, and petechia. Tissues were snap frozen and processed for RNase protection assay or reverse transcription polymerase chain reaction assay, or both. Transforming growth factor beta1 (TGFbeta1), TGFbeta2, TGFbeta3, tumor necrosis factor alpha, interleukin-6, and interferon gamma mRNA were measured.

RESULTS

Radiation at 12.5 Gy and at 13.5 Gy produced significant bowel damage. Levels of all cytokines in irradiated mice were significantly increased (p < 0.05).

CONCLUSIONS

Late radiation-related bowel fibrovascular toxicity includes cytokine signal pathways that parallel those of many other normal tissues. These cytokine responses include elevations of tumor necrosis factor alpha, TGFbeta1, and interleukin-6. There exist approaches for lowering these cytokine levels that do not also protect tumor, and thus a therapeutic gain is expected. Opportunities to use these cytokine measurements both to predict clinical toxicity and to develop interventions are discussed.

Activation of c-kit by stem cell factor induces radioresistance to apoptosis through ERK-dependent expression of survivin in HL60 cells

We investigated the effect of SCF, a c-kit ligand, on the radiosensitivity of HL60 cells. X-ray-induced apoptosis in HL60 cells was significantly lower in the presence of SCF than in the absence of SCF. This attenuation of X-ray-induced apoptosis by SCF was abolished by PD98059 (an ERK inhibitor), but not by wortmannin (a PI3-K inhibitor) or GF109203X (a PKC inhibitor). The expression of phospho-ERK1/2 (active form) and the ERK1/2-regulated expression of survivin were found to increase in cells treated with X irradiation and SCF. However, X irradiation alone induced down-regulation of the expression of phospho-ERK1/2. Our findings suggest that activation of c-kit by SCF confers radioresistance through up-regulation of ERK-dependent survivin expression in HL60 cells.

HST-1/FGF-4 plays a critical role in crypt cell survival and facilitates epithelial cell restitution and proliferation

The fibroblast growth factor-4 (HST-1/FGF-4) is a heparin-binding growth factor that influences on epithelial and many other cells through interaction with FGF receptors. It has been demonstrated that the HST-1/FGF-4 gene protects mice from lethal irradiation by preventing bone marrow damage and intestinal tract damage. However, the radioprotective mechanism is unknown. In this study, we have investigated the expression of Hst-1/Fgf-4 in mouse small intestine after irradiation, and determined the role of HST-1/FGF-4 in mouse intestinal crypt cell survival and epithelial cell proliferation and restitution. We found the induction of endogenous Hst-1/Fgf-4 expression in intestine when mice are exposed to 9.0 Gy irradiation. Laser-captured microdissection (LCM) coupled with RT-PCR analysis revealed that expression of Hst-1/Fgf-4 was found in epithelial cell of the villi and crypt cells. Pretreatment of HST-1/FGF-4 caused an increase in the number of surviving crypt cells, and clearly suppresses the radiation-induced apoptosis of the crypt cells. Moreover, exogenous HST-1/FGF-4 enhances epithelial cell restitution and proliferation in an in vitro model. These data suggest that HST-1/FGF-4 is induced by irradiation injury, and that HST-1/FGF-4 will find a therapeutic role in the prevention of intestinal cell toxicity following intensive chemotherapy and radiation therapy protocols, and in allogenic cell transplantation.

Stem cell factor attenuates liver damage in a murine model of acetaminophen-induced hepatic injury

Acute liver injury is a common cause of intensive care unit visits. In these studies, we used a murine model of acetaminophen poisoning to examine the role of stem cell factor (SCF) on liver damage. In the initial studies, we identified that the liver produces relatively high constitutive levels of SCF. Upon administration of acetaminophen, the levels of SCF fell dramatically, correlating to damage within the liver. When the liver was allowed to regenerate, the levels of SCF again correlated with the liver regeneration. We next treated mice with anti-SCF before sublethal doses of acetaminophen and significantly increased lethality in anti-SCF-treated animals. When exogenous SCF was given to mice, the lethality was significantly reduced compared with the control acetaminophen-treated animals and the damage within the liver tissue was attenuated. The administration of rSCF reduced the level of steady-state mRNA for cytochrome P450 cyp2E1 enzyme both in vitro and in vivo. These data suggest that SCF functions as an important factor that protects livers from acute damage.

Kit signaling inhibits the sphingomyelin-ceramide pathway through PLCγ1: implication in stem cell factor radioprotective effect

Previous studies demonstrated that Kit activation confers radioprotection. However, the mechanism by which Kit signaling interferes with cellular response to ionizing radiation (IR) has not been firmly established. Based on the role of the sphingomyelin (SM) cycle apoptotic pathway in IR-induced apoptosis, we hypothesized that one of the Kit signaling components might inhibit IR-induced ceramide production or ceramide-induced apoptosis. Results show that, in both Ba/F3 and 32D murine cell lines transfected with wild-type c-kit, stem cell factor (SCF) stimulation resulted in a significant reduction of IR-induced apoptosis and cytotoxicity, whereas DNA repair remained unaffected. Moreover, SCF stimulation inhibited IR-induced neutral sphingomyelinase (N-SMase) stimulation and ceramide production. The SCF inhibitory effect on SM cycle was not influenced by wortmannin, a phosphoinositide-3 kinase (PI3K) inhibitor. The SCF protective effect was maintained in 32D-KitYF719 cells in which the PI3K/Akt signaling pathway is abolished due to mutation in Kit docking site for PI3K. In contrast, phospholipase C γ (PLCγ) inhibition by U73122 totally restored IR-induced N-SMase stimulation, ceramide production, and apoptosis in Kit-activated cells. Moreover, SCF did not protect 32D-KitYF728 cells (lacking a functional docking site for PLCγ1), from IR-induced SM cycle. Finally, SCF-induced radioprotection of human CD34+ bone marrow cells was also inhibited by U73122. Altogether, these results suggest that SCF radioprotection is due to PLCγ1-dependent negative regulation of IR-induced N-SMase stimulation. Beyond the scope of Kit-expressing cells, it suggests that PLCγ1 status could greatly influence the post-DNA damage cellular response to IR, and perhaps, to other genotoxic agents.

Kit signaling and negative regulation of daunorubicin-induced apoptosis: role of phospholipase Cgamma

Previous studies have demonstrated that activation of Kit by stem cell factor (SCF), its natural ligand, or by gain-of-function point mutation in its intracellular domain, confers significant protection against apoptosis induced by growth factor deprivation or radiation. However, the effects of Kit activation on the cellular response to anti-tumor agents have not been so extensively documented. This study shows that daunorubicin-induced apoptosis and cytotoxicity were reduced in the murine Ba/F3 cells transfected with Kit (Ba/F3-Kit) in the presence of SCF and in Ba/F3 cells transfected with a constitutively active Kit variant (Ba/F3-KitDelta27), compared to either parental Ba/F3 (Ba/F3-wt) or unstimulated Ba/F3-Kit cells. In Ba/F3-wt and in Ba/F3-Kit cells, daunorubicin stimulated within 8-15 min neutral sphingomyelinase and ceramide production but not in SCF-stimulated Ba/F3-Kit or in Ba/F3-KitDelta27 whereas all Ba/F3 cells were equally sensitive to exogenous cell-permeant C6-ceramide. In Ba/F3-Kit, SCF-induced Kit activation resulted in a rapid phospholipase Cgamma (PLCgamma) tyrosine phosphorylation followed by diacylglycerol release and protein kinase C (PKC) stimulation. U-73122, a PLCgamma inhibitor, not only blocked diacylglycerol production and PKC stimulation but also restored daunorubicin-induced sphingomyelinase stimulation, ceramide production, and apoptosis. These results suggest that Kit activation results in PLCgamma-mediated PKC-dependent sphingomyelinase inhibition which contributes to drug resistance in Kit-related malignancies.

Gastrointestinal cells of IL-7 receptor null mice exhibit increased sensitivity to irradiation

IL-7 is a critical cytokine in the development of T and B cells but little is known about its activity on nonhematopoietic cells. An unexpected finding was noted in allogeneic bone marrow transplant studies using IL-7 receptor null (IL-7R alpha(-/-)) mice as recipients. These mice exhibited a significantly greater weight loss after total body irradiation compared with wild type, IL-7R alpha(+/+), mice. Pathological assessment indicated greater intestinal crypt damage in IL-7R alpha(-/-) recipients, suggesting these mice may be predisposed to gut destruction. Therefore, we determined the effect of the conditioning itself on the intestinal tract of these mice. IL-7R alpha(-/-) mice and IL-7R alpha(+/+) mice were irradiated and examined for lesions and apoptosis within the small intestine. In moribund animals, IL-7R alpha(-/-) mice had extensive damage in the small intestine, including marked ablation of the crypts and extreme shortening of villi following 1500 cGy total body irradiation. In contrast, by 8 days after irradiation, the small intestines of IL-7R alpha(+/+) mice had regenerated as distinguished by normal villus length and hyperplastic crypts. Following 750 cGy irradiation, IL-7R alpha(-/-) mice had a higher proportion of apoptotic cells in the crypts and an accompanying increase in the pro-apoptotic protein Bak was expressed in intestinal epithelial cells. These results demonstrate the increased radiosensitivity of intestinal stem cells within the crypts in IL-7R alpha(-/-) mice and a role for IL-7 in the protection of radiation-induced apoptosis in these same cells. This study describes a novel role of IL-7 in nonhematopoietic tissues.

Effects of stem cell factor and granulocyte macrophage-colony stimulating factor on survival of porcine type A spermatogonia cultured in KSOM

Spermatogenesis is initiated with the divisions of the type A spermatogonial stem cells; however, the regulation of this stem cell population remains unknown. In order to obtain a better understanding of the biology of these cells, type A spermatogonia were isolated from 80-day-old pig testes by sedimentation velocity at unit gravity. The cells were cultured for up to 120 h in Dulbecco's modified Eagle's medium/Ham's F-12 medium (DMEM/F12) or a potassium-rich medium derived by the simplex optimization method (KSOM). At the end of the 120-h culture period, 30-50% of the spermatogonia were viable in KSOM, whereas in DMEM/F12 very few cells survived. Using KSOM as the culture medium, the effects of stem cell factor (SCF) and granulocyte macrophage-colony stimulating factor (GM-CSF) were studied. SCF significantly enhanced the percentage of cell survival at 100 ng/ml but not at lower concentrations. In comparison, GM-CSF promoted survival at relatively low concentrations (0.01, 0.1, and 1 ng/ml). At a higher dose (10 ng/ml), a significant reduction in percentage of cell survival was observed. The combination of SCF with GM-CSF had no significant effect on the percentage survival of type A spermatogonial cells. These data indicate that SCF and GM-CSF play a role in the regulation of survival and/or proliferation of type A spermatogonia.