The role of autocrine growth factors in radiation damage to the epiphyseal growth plate

The normal and fractured physis: an anatomic and physiologic overview

The growth plate (physis) is responsible for enabling and regulating longitudinal growth of upper and lower limbs. This regulation occurs through interaction of the cells in the growth plate with systemic and locally produced factors. This complex interaction leads to precisely controlled changes in chondrocyte size, receptors, and matrix, which ultimately result in endochondral bone formation. With advances in cellular and molecular biology, our knowledge about these complex interactions has increased significantly over the past decade. Deficiency of any of the regulating factors or physeal injury during childhood can alter this well-orchestrated sequence of events and lead to abnormalities in growth. This review highlights the histology of the normal physis, including recent findings at the cellular and molecular levels, mechanics and mechanobiology of the growth plate, pathologies that can affect the physis, and treatment options, including interposition materials.

Effect of Jiaweisinisan on morphology of the gastric mucosa of rats following chronic psychological stress

AIM: To study the protective effect of Jiaweisinisan (JWSNS) on the gastric mucosa of rats following physical and mental stress and to explore the possible mechanisms involved.

METHODS: Rats were randomly divided into normal group, model group, low-, medium-, and high-dose JWSNS groups, and omeprazole group. Gastric ulcer was induced in rats using a chronic psychological stress method. After JWSNS intervention, gastric juice pH value was measured, mucosal ulcer index (UI) was evaluated, morphological and ultrastructural changes in gastric tissue were observed by light microscopy and transmission electron microscopy. The levels of gastrin (GAS) and vasoactive intestinal peptide (VIP) in gastric tissue were determined by immunoradiometric assay, and mRNA expression of GASR in gastric and VIPR2 in jejunal tissue was detected by RT-PCR.

RESULTS: Compared to the model group, gastric UI was significantly lower in the normal group, low-, medium-, and high-dose JWSNS groups, and omeprazole group (0.32 ± 1.58, 13.70 ± 2.26, 8.50 ± 3.88, 4.67 ± 3.08, 12.00 ± 2.73 vs 25.33 ± 4.33, all P < 0.05). Compared to the omeprazole group, gastric UI was significantly lower in the medium- and high-dose JWSNS groups (both P < 0.05). The gastric mucosa of rats in the model group showed obvious injury compared to the normal group. Varying degrees of relief of gastric mucosa injury was observed in the JWSNS groups. Treatment with JWSNS also significantly improved ultrastructural changes in gastric tissue. Compared to the model group, levels of GAS in the gastric antrum were significantly higher in the normal group, low-, medium-, and high-dose JWSNS groups, and omeprazole group (39.23 ± 5.54, 15.74 ± 4.56, 27.52 ± 4.28, 38.27 ± 4.18, 28.30 ± 5.53 vs 8.33 ± 4.63, all P < 0.05). Compared to the omeprazole group, levels of GAS in the gastric antrum were significantly higher in the high-dose JWSNS group (P < 0.05). Compared to the model group, levels of VIP in the gastric antrum were significantly lower in the normal group, medium-, and high-dose JWSNS groups, and omeprazole group (36.32 ± 1.68, 60.50 ± 3.85, 39.67 ± 2.98, 59.00 ± 2.89 vs 89.33 ± 4.43, all P < 0.05). Compared to the omeprazole group, levels of VIP in the gastric antrum were significantly lower in the high-dose JWSNS group (P < 0.05). Compared to the model group, expression of GASR mRNA in gastric tissue increased significantly (1.00 ± 0.10, 0.25 ± 0.02, 0.53 ± 0.03, 0.62 ± 0.05, 0.43 ± 0.03 vs 0.19 ± 0.05, all P < 0.05) and that of VIPR2 mRNA in jejunal tissue decreased significantly (1.00 ± 0.20, 1.83 ± 0.30, 2.39 ± 0.36, 1.15 ± 0.33, 1.79 ± 0.35 vs 3.40 ± 0.57, all P < 0.05) in the normal group, low-, medium-, and high-dose JWSNS groups, and omeprazole group. Compared to the omeprazole group, expression of GASR mRNA in gastric tissue increased significantly (P < 0.05) and that of VIPR2 mRNA in jejunal tissue decreased significantly (P < 0.01) in the high-dose JWSNS group.

CONCLUSION: JWSNS has a good protective effect on the gastric mucosa of rats following physical and mental stress.

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.

Methotrexate toxicity in growing long bones of young rats: a model for studying cancer chemotherapy-induced bone growth defects in children

The advancement and intensive use of chemotherapy in treating childhood cancers has led to a growing population of young cancer survivors who face increased bone health risks. However, the underlying mechanisms for chemotherapy-induced skeletal defects remain largely unclear. Methotrexate (MTX), the most commonly used antimetabolite in paediatric cancer treatment, is known to cause bone growth defects in children undergoing chemotherapy. Animal studies not only have confirmed the clinical observations but also have increased our understanding of the mechanisms underlying chemotherapy-induced skeletal damage. These models revealed that high-dose MTX can cause growth plate dysfunction, damage osteoprogenitor cells, suppress bone formation, and increase bone resorption and marrow adipogenesis, resulting in overall bone loss. While recent rat studies have shown that antidote folinic acid can reduce MTX damage in the growth plate and bone, future studies should investigate potential adjuvant treatments to reduce chemotherapy-induced skeletal toxicities.

Mitigation effect of an FGF-2 peptide on acute gastrointestinal syndrome after high-dose ionizing radiation

PURPOSE

Acute gastrointestinal syndrome (AGS) resulting from ionizing radiation causes death within 7 days. Currently, no satisfactory agent exists for mitigation of AGS. A peptide derived from the receptor binding domain of fibroblast growth factor 2 (FGF-P) was synthesized and its mitigation effect on AGS was examined.

METHODS AND MATERIALS

A subtotal body irradiation (sub-TBI) model was created to induce gastrointestinal (GI) death while avoiding bone marrow death. After 10.5 to 16 Gy sub-TBI, mice received an intramuscular injection of FGF-P (10 mg/kg/day) or saline (0.2 ml/day) for 5 days; survival (frequency and duration) was measured. Crypt cells and their proliferation were assessed by hematoxylin, eosin, and BrdU staining. In addition, GI hemoccult score, stool formation, and plasma levels of endotoxin, insulin, amylase, interleukin (IL)-6, keratinocyte-derived chemokine (KC) monocyte chemoattractant protein 1 (MCP-1) and tumor necrosis factor (TNF)-alpha were evaluated.

RESULTS

Treatment with FGF-P rescued a significant fraction of four strains of mice (33-50%) exposed to a lethal dose of sub-TBI. Use of FGF-P improved crypt survival and repopulation and partially preserved or restored GI function. Furthermore, whereas sub-TBI increased plasma endotoxin levels and several pro-inflammation cytokines (IL-6, KC, MCP-1, and TNF-alpha), FGF-P reduced these adverse responses.

CONCLUSIONS

The study data support pursuing FGF-P as a mitigator for AGS.

Transforming growth factor-beta and Wnt signals regulate chondrocyte differentiation through Twist1 in a stage-specific manner

We investigated the molecular mechanisms underlying the transition between immature and mature chondrocytes downstream of TGF-beta and canonical Wnt signals. We used two developmentally distinct chondrocyte models isolated from the caudal portion of embryonic chick sternum or chick growth plates. Lower sternal chondrocytes exhibited immature phenotypic features, whereas growth plate-extracted cells displayed a hypertrophic phenotype. TGF-beta significantly induced beta-catenin in immature chondrocytes, whereas it repressed it in mature chondrocytes. TGF-beta further enhanced canonical Wnt-mediated transactivation of the Topflash reporter expression in lower sternal chondrocytes. However, it inhibited Topflash activity in a time-dependent manner in growth plate chondrocytes. Our immunoprecipitation experiments showed that TGF-beta induced Sma- and Mad-related protein 3 interaction with T-cell factor 4 in immature chondrocytes, whereas it inhibited this interaction in mature chondrocytes. Similar results were observed by chromatin immunoprecipitation showing that TGF-beta differentially shifts T-cell factor 4 occupancy on the Runx2 promoter in lower sternal chondrocytes vs. growth plate chondrocytes. To further determine the molecular switch between immature and hypertrophic chondrocytes, we assessed the expression and regulation of Twist1 and Runx2 in both cell models upon treatment with TGF-beta and Wnt3a. We show that Runx2 and Twist1 are differentially regulated during chondrocyte maturation. Furthermore, whereas TGF-beta induced Twist1 in mature chondrocytes, it inhibited Runx2 expression in these cells. Opposite effects were observed upon Wnt3a treatment, which predominates over TGF-beta effects on these cells. Finally, overexpression of chick Twist1 in mature chondrocytes dramatically inhibited their hypertrophy. Together, our findings show that Twist1 may be an important regulator of chondrocyte progression toward terminal maturation in response to TGF-beta and canonical Wnt signaling.

Protective effect of pentoxifylline on growth plate in neonatal rats following long-term phototherapy

We demonstrated previously that receiving long-term phototherapy was associated with early impairment of growth plate structure in neonatal rats, and oxidative stress may be the main risk factor for growth plate injury. The purpose of this study was to examine the histomorphometric effects of pentoxifylline treatment on the growth plate. Sixty weanling Sprague-Dawley rats were randomly separated into three equal groups. Group A, the control group, did not receive phototherapy and pentoxifylline. Groups B and C were exposed to phototherapy for 7 d. In addition to phototherapy, group C was also given pentoxifylline during the study period. Compared with zonal lengths on d 7 after initiation of phototherapy, group B had significantly lower values than group A for all zonal lengths (p < 0.001). Zonal lengths of growth plate were increased significantly with pentoxifylline treatment in group C for 7 d compared with group B (p < 0.001). After phototherapy, group B had significantly higher values than groups A and C for plasma malondialdehyde levels (p < 0.001). The pentoxifylline was found here to have some potential to reduce the effects of phototherapy on growth plate in neonatal rats at a relatively low dose.

Targeting of cell survival genes using small interfering RNAs (siRNAs) enhances radiosensitivity of Grade II chondrosarcoma cells

The main treatment for chondrosarcoma is surgical resection with a wide margin. However, there are certain chondrosarcomas, such as those found in the pelvis and the spine, which cannot be resected adequately with surgery alone. Unfortunately, most chondrosarcomas are resistant to radiation and chemotherapy. Radiation and chemotherapy are thought to kill chondrosarcoma cells by inducing apoptosis, or programmed cell death. In this article, we hypothesize that antiapoptotic gene silencing enhances radiosensitivity in chondrosarcoma cells by facilitating apoptotic pathways. We knocked down antiapoptotic genes in chondrosarcoma cells using small interfering RNAs (siRNAs). Two well-established Grade II human chondrosarcoma cell lines were pretreated with siRNAs that specifically target mRNAs for Bcl-2, Bcl-xL, or XIAP. The cells were then treated with radiation. Cell death was assessed by flow cytometry. Cell survival and proliferation were measured by clonogenic survival assays. Chondrosarcoma cells exhibited radioresistance and increased the expression of Bcl-2, Bcl-xL, and XIAP in response to radiation. When one of the Bcl-2, Bcl-xL, or XIAP genes was silenced with the corresponding siRNA, radiosensitivity increased up to 9.2-fold (p < 0.05). When two out of the three antiapoptotic mRNAs were knocked down simultaneously, there was an 11.3-fold increase in cell death after radiation (p < 0.05). Our findings support a novel therapeutic concept that gene silencing may be used as a molecular adjuvant therapy for radioresistant sarcomas.

Effects of radiation therapy on chondrocytes in vitro

The negative irradiation complications of growth loss leading to limb length asymmetry and pathological fracture incurred following radiation therapy in pediatric patients has led to a renewed interest in understanding the specific effects of irradiation on the growth plate and the surrounding bone. In the present report, we examined the radiation therapy effects on primary rat growth cartilage chondrocytes in order to determine the chondrocyte radiosensitivity relative to other bone cell constituents and tumor cells, the postirradiation temporal progression of radiation-induced alterations in chondrocyte function, and the time course for the functional restoration of chondrocyte pathways that drive the eventual recovery in growth function. We employed an in vitro primary rat costochondral growth cartilage cell culture model system to evaluate the radiation therapy effects on proliferative chondrocytes using serial radiation doses (0-20 Gy) that are well within the clinically relevant range. Following irradiation, all of the following occurred in a dose-dependent manner: proliferation decreased, cytotoxicity increased, several markers of apoptosis increased, markers of radiation-induced cellular differentiation increased, and cell synthetic activity was disturbed. Alterations in proliferation, cell death, and induction of apoptosis are likely due to a transient radiation-induced derangement of the parathyroid hormone-related protein-Indian hedgehog proliferation-maturation pathway. Alterations in cellular differentiation and cell synthetic activity are novel observations for chondrocytes. Further, these results correspond very well to our previous work in an in vivo Sprague-Dawley rat model, making this model particularly relevant to researching the radiation therapy effects on longitudinal growth.

Effects of phototherapy on the growth plate in newborn rats

The aim of the present study was to evaluate the effect of phototherapy and oxidative stress on the growth plate of newborn rats. Forty newborn Sprague-Dawley rats were randomized into a phototherapy group and a control group. Twenty of the rats received phototherapy for 7 days. All zones of the growth plate were assessed with quantitative histomorphometric analysis. Individual zonal lengths were measured for the reserve zone (RZ), the proliferative zone (PZ), the hypertrophic zone (HZ), ossifying cartilage (OC), and total zone (TZ) of the growth plate. Levels of plasma malondialdehyde (MDA), an index of oxidative stress, were also evaluated. Compared with zonal lengths on day 7 after phototherapy between the two groups, the phototherapy group had significantly lower values than those of controls for RZ (5.13 +/- 0.36 vs. 6.4 +/- 0.85 mm x 10(-2); P < 0.001), PZ (20.6 +/- 3.0 vs. 29.25 +/- 1.68 mm x 10(-2); P < 0.001), HZ (15.4 +/- 1.44 vs. 20.87 +/- 1.12 mm x 10(-2); P < 0.001), OC (47.08 +/- 4.25 vs. 62.06 +/- 3.7 mm x 10(-2); P < 0.001), and TZ (88.15 +/- 6.56 vs. 118.48 +/- 4.50 mm x 10(-2); P < 0.001). Plasma MDA levels were correlated with the size of the PZ in the phototherapy group (r = -0.53, P = 0.01). In a multivariate regression model for all rats, being in the phototherapy group was the best predictor of the size of the TZ (beta = -0.94, P < 0.001), with the total variance explained being 88%. These results suggest that in newborn rats, receiving phototherapy is associated with early impairment of growth plate structure, and oxidative stress may be the main risk factor for growth plate injury.

The effects of external beam irradiation on the growth of flat bones in children: modeling a dose-volume effect

PURPOSE

To model the effects of external beam irradiation on the developing flat bones of pediatric patients undergoing radiation therapy (RT) for tumors involving the musculoskeletal system.

METHODS AND MATERIALS

Patients with image-guided RT plans including areas adjacent to facial or pelvic flat bones underwent retrospective contouring of nontumor involved flat bones ipsilateral and contralateral to the treatment side. Radiation dose-volume information and bone volume data (initial and the most recent follow-up) were analyzed in 15 paired flat bones from 10 patients (ages 1.0-17.0 years). The models to predict bone growth after completion of RT (v(post)) were based on initial bone volume (v(pre)), the patient's age, time to follow-up (t), and the dose-volume parameter (v(Int35+)).

RESULTS

We developed a dose effects model as follows: Log (v(post) / v(pre)) = beta(time)t + beta(age group)t + beta(dose)t v(Int35). The dose-volume parameter v(Int35) predicted significantly for alterations in growth in younger patients, but not for older patients. The predictability of the fitted model for relative change in bone growth improved in the younger age group with the addition of the dose-volume term v(Int35) (correlation coefficient of r = 0.5510 to r = 0.6760 with the addition v(Int35)).

CONCLUSIONS

Our model accurately predicted flat bone growth and is notable for the inclusion of radiation dose-volume information, which is now available in the image-guided RT era. Further refinement of this model in a prospective patient population is underway.

Temporal changes in PTHrP, Bcl-2, Bax, caspase, TGF-beta, and FGF-2 expression following growth plate irradiation with or without radioprotectant

This study examined temporal changes in growth plate apoptosis molecules and growth factors in an animal model of radiation injury with and without a radioprotectant. Thirty weanling 5-week Sprague-Dawley rats underwent right knee irradiation with single-fraction 17.5 Gy while the left served as internal control. Six animals each were sacrificed at 0.5, 1, 2, 3, or 4 weeks after irradiation. Half of the animals received pretreatment with amifostine (WR-2721) radioprotectant. Immunohistochemical staining for PTHrP, Bcl-2, Bax, caspase-3, FGF-2, and TGF-beta was performed. PTHrP decreased to a nadir at 1 week after irradiation but rebounded to above control levels at 2 weeks in the reserve and transitional zones. The radioprotectant amifostine blunted the decrease in PTHrP but kept PTHrP expression lower than controls during the rebound phase in untreated irradiated animals. Hypertrophic zone Bax expression was decreased by amifostine in both irradiated and non-irradiated limbs at 1 and 2 weeks. FGF, TGF-beta, Bcl-2, and caspase levels generally decreased at 1 week and returned thereafter toward control levels. These findings underscore the importance of PTHrP in response to growth plate irradiation and show the novel finding of a decrease in Bax expression with amifostine pretreatment.

Physiology and pathophysiology of the growth plate

Longitudinal growth of the skeleton is a result of endochondral ossification that occurs at the growth plate. Through a sequential process of cell proliferation, extracellular matrix synthesis, cellular hypertrophy, matrix mineralization, vascular invasion, and eventually apoptosis, the cartilage model is continually replaced by bone as length increases. The regulation of longitudinal growth at the growth plate occurs generally through the intimate interaction of circulating systemic hormones and locally produced peptide growth factors, the net result of which is to trigger changes in gene expression by growth plate chondrocytes. This review highlights recent advances in genetics and cell biology that are illuminating the important regulatory mechanisms governing the structure and biology of the growth plate, and provides selected examples of how studies of human mutations have yielded a wealth of new knowledge regarding the normal biology and pathophysiology of growth plate cartilage.

Role of pentoxifylline in preventing radiation damage to epiphyseal growth plate chondrocytes

Radiation therapy plays an important role as part of multimodality treatment for a number of childhood malignancies. The damaging effects of radiation on bone formation in children have been well documented. Recent work suggests that the postirradiation increase in cytosolic calcium is probably responsible for the deleterious effects of radiation on growth plate chondrocytes because it causes a specific suppression of the mitogen PTHrP. Using an in vitro model of avian growth plate chondrocytes, this study demonstrates that pentoxifylline is effective in increasing basal PTHrP mRNA levels and partially preventing the radiation-induced decrease in PTHrP mRNA. This effect of pentoxifylline is probably due to its ability to lower basal levels of cytosolic calcium and the radiation-induced increase in cytosolic calcium in chondrocytes. Pentoxifylline also prevented the radiation-induced decreases in [3H]thymidine uptake and BCL2 and PTHrP receptor mRNA levels in chondrocytes. The effects of pentoxifylline appear to be specific for the PTHrP signaling pathway because it did not alter basal TGFB mRNA levels or TGFB mRNA expression in irradiated chondrocytes. The results of the current study suggest that by decreasing basal cytosolic calcium levels and curtailing the radiation-induced increase in cytosolic calcium levels in chondrocytes, pentoxifylline is able to sustain PTHrP signaling in chondrocytes and maintains the proliferative signal that is necessary to prevent chondrocytes from undergoing apoptosis.