Elham Karimian

The role of estrogen receptor-α and its activation function-1 for growth plate closure in female mice

High estradiol levels in late puberty induce growth plate closure and thereby cessation of growth in humans. In mice, the growth plates do not fuse after sexual maturation, but old mice display reduced longitudinal bone growth and high-dose estradiol treatment induces growth plate closure. Estrogen receptor (ER)-α stimulates gene transcription via two activation functions (AFs), AF-1 and AF-2. To evaluate the role of ERα and its AF-1 for age-dependent reduction in longitudinal bone growth and growth plate closure, female mice with inactivation of ERα (ERα(-/-)) or ERαAF-1 (ERαAF-1(0)) were evaluated. Old (16- to 19-mo-old) female ERα(-/-) mice showed continued substantial longitudinal bone growth, resulting in longer bones (tibia: +8.3%, P < 0.01) associated with increased growth plate height (+18%, P < 0.05) compared with wild-type (WT) mice. In contrast, the longitudinal bone growth ceased in old ERαAF-1(0) mice (tibia: -4.9%, P < 0.01). Importantly, the proximal tibial growth plates were closed in all old ERαAF-1(0) mice while they were open in all WT mice. Growth plate closure was associated with a significantly altered balance between chondrocyte proliferation and apoptosis in the growth plate. In conclusion, old female ERα(-/-) mice display a prolonged and enhanced longitudinal bone growth associated with increased growth plate height, resembling the growth phenotype of patients with inactivating mutations in ERα or aromatase. In contrast, ERαAF-1 deletion results in a hyperactive ERα, altering the chondrocyte proliferation/apoptosis balance, leading to growth plate closure. This suggests that growth plate closure is induced by functions of ERα that do not require AF-1 and that ERαAF-1 opposes growth plate closure.

Catch-up growth after dexamethasone withdrawal occurs in cultured postnatal rat metatarsal bones

Children exposed to systemic glucocorticoids often exhibit growth retardation and after the cessation of therapy catch-up growth occurs in many, but not all patients. The developmental regulation and underlying cellular mechanisms of catch-up growth are not fully understood. To clarify this issue, we established an in vitro model of catch-up growth. Here we present a protocol for the long-term culture (up to 160 days) of fetal (E20) as well as postnatal (P8) rat metatarsal bones which allowed us to characterize ex vivo the phenomenon of catch-up growth without any influence by systemic factors. The relevance of the model was confirmed by the demonstration that the growth of fetal and postnatal bones were stimulated by IGF1 (100 ng/ml) and inhibited by dexamethasone (Dexa; 1 microM). We found that the capacity to undergo catch-up growth was restricted to postnatal bones. Catch-up growth occurred after postnatal bones had been exposed to Dexa for 7 or 12 days but not after a more prolonged exposure (19 days). Incomplete catch-up growth resulted in compromised bone length when assessed at the end of the 4-month period of culture. While exposure to Dexa was associated with decreased chondrocyte proliferation and differentiation, catch-up growth was only associated with increased cell proliferation. We conclude that the phenomenon of catch-up growth after Dexa treatment is intrinsic to the growth plate and primarily mediated by an upregulation of chondrocyte proliferation.

Tamoxifen Impairs Both Longitudinal and Cortical Bone Growth in Young Male Rats

Tamoxifen (Tam) has been used experimentally to treat boys with gynecomastia and girls with McCune‐Albright syndrome. This drug was recently shown to inhibit the growth of cultured fetal rat metatarsal bones and thus might also affect bone growth in vivo. Four‐week‐old Sprague‐Dawley rats were gavaged daily with vehicle alone (peanut oil), Tam (40 mg/kg/d; 1 or 4 wk), or estradiol (40 μg/kg/d; 4 wk). Five of the 10 rats in each group were killed after 4 wk and the other five after 14 wk of recovery. Bone growth was followed by repeat DXA scans, whereas other bone parameters and spine length were evaluated by pQCT and X‐ray at the time of death. Four‐week Tam treatment significantly decreased body weight, nose‐anus distance, spinal and tibial bone lengths, trabecular BMD, cortical periosteal circumference, and bone strength and also reduced serum IGF‐I levels (424 ± 54 versus 606 ± 53 ng/ml in control; p < 0.05). Analysis of the tibial growth plate of treated rats showed elevated chondrocyte proliferation (BrdU) and apoptosis (TUNEL), as well as decreases in the number of hypertrophic chondrocytes and in the size of terminal hypertrophic chondrocytes. Despite a complete catch‐up of body weight after 14 wk of recovery, the tibia was still shorter (p < 0.001) and its cortical region was smaller. We conclude that, when administered at a clinically relevant dose, Tam causes persistent retardation of longitudinal and cortical radial bone growth in young male rats. Our findings suggest that this inhibition results from local effects on the growth plate cartilage and systemic suppression of IGF‐I production. Based on these rat data, we believe that Tam, if given to growing individuals, might compromise cortical bone growth, bone strength, and adult height.

Pro-Inflammatory Cytokines Produced by Growth Plate Chondrocytes May Act Locally to Modulate Longitudinal Bone Growth

Introduction: Interleukin-1β (IL-1β) and tumour necrosis factor-α (TNF-α), both cytokines upregulated during chronic inflammation, are known to suppress bone growth. So far no role of these cytokines in modulation of normal bone growth has been established. Methodology: Applying RT-PCR and immunohistochemistry, expression of IL-1β and TNF-α was studied in cultured fetal (E20) rat metatarsal bones. Anakinra (500 µg/ml; IL-1 receptor antagonist) and/or etanercept (500 µg/ml; soluble TNF-α receptor) were used to block cytokine actions. Results: The local expression of IL-1β and TNF-α was confirmed in the rat metatarsal growth plate. When cultured for 12 days and compared to control, the length of bones exposed to anakinra, etanercept, or anakinra plus etanercept increased by 7.7 ± 2.0 (p < 0.05), 11.7 ± 2.8 (p < 0.01) and 20.3 ± 1.9% (p < 0.001), respectively, while the height of the hypertrophic growth plate zone (collagen X staining) increased by 11.0 ± 6.7, 17.4 ± 7.1 and 43.1 ± 5.0% (p < 0.01), respectively. Moreover, etanercept increased chondrocyte proliferation (BrdU incorporation). Conclusion: Our findings that IL-1β and TNF-α are produced by growth plate chondrocytes and that their antagonists improve growth of cultured metatarsal bones suggest that these cytokines play a physiological role in the normal regulation of longitudinal bone growth.

Genetic regulation of the growth plate.

The epiphyseal growth plate consists of a layer of cartilage present only during the growth period and vanishes soon after puberty in long bones. It is divided to three well-defined zones, from epiphyses; resting, proliferative, and hypertrophic zones. Chondrocyte proliferation and differentiation and subsequent bone formation in this cartilage are controlled by various endocrine, autocrine, and paracrine factors which finally results into elimination of the cartilaginous tissue and promotion of the epiphyseal fusion. As chondrocytes differentiate from round, quiescent, and single structure to flatten and proliferative and then large and terminally differentiated, they experience changes in their gene expression pattern which allow them to transform from cartilaginous tissue to bone. This review summarizes the literature in this area and shortly describes different factors that affect growth plate cartilage both at the local and systemic levels. This may eventually help us to develop new treatment strategies of different growth disorders.

The biologics anakinra and etanercept prevent cytokine-induced growth retardation in cultured fetal rat metatarsal bones.

Background/Aims: Chronic inflammation during childhood often leads to impaired bone growth and reduced adult height. Proinflammatory cytokines interleukin (IL)-1β and tumor necrosis factor (TNF)-α synergistically impair bone growth in vitro. We hypothesized that biologic agents may rescue bones from cytokine-induced growth impairment and that insulin growth factor (IGF)-I may potentiate such an effect. Methodology: Metatarsal bones from fetal Sprague-Dawley rats (19–20 days p.c.) were treated with IL-1β plus TNF-α, or the combination of these cytokines with anakinra (IL-1 receptor antagonist), etanercept (TNF-inhibitor) and/or IGF-I. The bones were measured and growth expressed as percent increase in bone length over the 7-day culture period. Results: When exposed to IL-1β plus TNF-α (10 + 10 ng/ml), bone growth was markedly suppressed (6.6 ± 1.4 vs. 50.6 ± 2.5% in control bones; p < 0.001). The growth of cytokine exposed bones (IL-1β plus TNF-α) was dose-dependently rescued by anakinra (0.05–500 µg/ml) or etanercept (0.5–500 µg/ml); at the highest concentrations, growth was similar as in control bones never exposed to cytokines. Also when combining IGF-I (100 ng/ml) and relatively low concentrations of anakinra (0.05 µg/ml) or etanercept (5 µg/ml), growth was rescued in an additive way. Conclusion: Etanercept and anakinra efficiently and dose-dependently prevent cytokine-induced bone growth impairment, and combination with IGF-I further improves bone growth.

Resveratrol Treatment Delays Growth Plate Fusion and Improves Bone Growth in Female Rabbits

Trans-resveratrol (RES), naturally produced by many plants, has a structure similar to synthetic estrogen diethylstilbestrol, but any effect on bone growth has not yet been clarified. Pre-pubertal ovary-intact New Zealand white rabbits received daily oral administration of either vehicle (control) or RES (200 mg/kg) until growth plate fusion occurred. Bone growth and growth plate size were longitudinally monitored by X-ray imaging, while at the endpoint, bone length was assessed by a digital caliper. In addition, pubertal ovariectomized (OVX) rabbits were treated with vehicle, RES or estradiol cypionate (positive control) for 7 or 10 weeks and fetal rat metatarsal bones were cultured in vitro with RES (0.03 µM–50 µM) and followed for up to 19 days. In ovary-intact rabbits, sixteen-week treatment with RES increased tibiae and vertebrae bone growth and subsequently improved final length. In OVX rabbits, RES delayed fusion of the distal tibia, distal femur and proximal tibia epiphyses and femur length and vertebral bone growth increased when compared with controls. Histomorphometrical analysis showed that RES-treated OVX rabbits had a wider distal femur growth plate, enlarged resting zone, increased number/size of hypertrophic chondrocytes, increased height of the hypertrophic zone, and suppressed chondrocyte expression of VEGF and laminin. In cultured fetal rat metatarsal bones, RES stimulated growth at 0.3 µM while at higher concentrations (10 μM and 50 μM) growth was inhibited. We conclude that RES has the potential to improve longitudinal bone growth. The effect was associated with a delay of growth plate fusion resulting in increased final length. These effects were accompanied by a profound suppression of VEGF and laminin expression suggesting that impairment of growth plate vascularization might be an underlying mechanism.

Estrogen signaling in growth plate cartilage.

It is well known that sex steroids, in particular estrogens, play an important role in longitudinal bone growth during puberty. High doses of estrogen therapy can reduce the final height of an individual, but such treatment is also associated with severe side effects. At the same time, attenuation of estrogen production by aromatase inhibitors increases adult final height, inhibiting bone turnover, which influences bone architecture and may increase the risk for vertebrae fractures. SERMs, which display either estrogenic and/or antiestrogenic effects, bind to ERs with different affinities and subsequently recruit co-modulators of transcription in a tissue-specific manner.