Gene expression changes accompanying p53 activity during estrogen treatment of osteoblasts

17β-Estradiol regulates rat growth plate chondrocyte apoptosis through a mitochondrial pathway not involving nitric oxide or MAPKs

Estrogens cause growth plate closure in both males and females, by decreasing proliferation and inducing apoptosis of postproliferative growth plate chondrocytes. In vitro studies using 17β-estradiol (E(2)) conjugated to bovine serum albumin (E(2)-BSA) show that rat costochondral growth plate resting zone chondrocytes also respond to E(2). Moreover, they are regulated by E(2)-BSA via a protein kinase C and ERK MAPK signaling pathway that is functional only in female cells. To better understand how E(2) regulates apoptosis of growth plate chondrocytes, rat resting zone chondrocytes cells were treated with E(2) or E(2)-BSA. E(2) caused apoptosis in male and female resting zone and growth zone chondrocytes in a dose-dependent manner, based on elevated DNA fragmentation, terminal deoxynucleotidyl transferase dUTP nick end labeling staining and caspase-3 activation. E(2) also up-regulated p53 and Bax protein (Bcl-2-associated X protein) levels and induced release of cytochrome C from the mitochondria, indicating a mitochondrial apoptotic pathway. The apoptotic effect of E(2) did not involve elevated nitric oxide production or MAPKs. It was reduced by ICI 182780, which is an estrogen receptor (ER) antagonist and blocked by antibodies to Erα36, a membrane-associated ER. E(2)-BSA reduced cell viability and increased caspase-3 activity; ICI 182780 had no effect, but anti-ERα36 antibodies blocked the effect. The results indicate that estrogen is able to directly affect the cell population kinetics of growth plate chondrocytes by regulating apoptosis, as well as proliferation and differentiation in both resting zone and growth zone cells. They also have provided further information about the physiological functions of estrogen on longitudinal bone growth.

Biological mechanisms of bone and cartilage remodelling--genomic perspective

Rapid advancements in the field of genomics, enabled by the achievements of the Human Genome Project and the complete decoding of the human genome, have opened an unimaginable set of opportunities for scientists to further unveil delicate mechanisms underlying the functional homeostasis of biological systems. The trend of applying whole-genome analysis techniques has also contributed to a better understanding of physiological and pathological processes involved in homeostasis of bone and cartilage tissues. Gene expression profiling studies have yielded novel insights into the complex interplay of osteoblast and osteoclast regulation, as well as paracrine and endocrine control of bone and cartilage remodelling. Mechanisms of new bone formation responsible for fracture healing and distraction osteogenesis, as well as healing of joint cartilage defects, have also been extensively studied. Microarray experiments have been especially useful in studying pathological processes involved in diseases such as osteoporosis or bone tumours. Existing results show that microarrays hold great promise in areas such as identification of targets for novel therapies or development of new biomarkers and classifiers in skeletal diseases.

Effect of overexpression of estrogen receptors in osteoblasts

Our study focused on investigating the mechanism of action of estrogen in regulating p53 levels within osteoblasts. In the studies reported here, we attempted to understand the role of estrogen receptors, ER-alpha and ER-beta, in the regulation of p53 and osteoblast differentiation. We stably expressed ER-alpha and ER-beta in ROS 17/2.8 cells and isolated several single cell clones. These clones were initially characterized for expression of the exogenous receptors, and representative clones from each type were chosen for further analyses. Cell proliferation, alkaline phosphatase activity, and the viability of these clones in culture were tested. The cells expressing exogenous ER-alpha exhibited more differentiated characteristics than cells expressing ER-beta. Morphologically, ER-beta-overexpressing cells were more rounded than the ER-alpha-overexpressing cells, which were more elongated and fibroblastic in appearance. The ER-beta-expressing cells had a higher survival and growth rate when compared with ER-alpha cells. The ER-alpha clones were not as viable as ER-beta clones, and some of the ER-alpha cell lines showed signs of senescence, with an increase in senescence-associated (SA) galactosidase activity. The basal levels of p53 functional activity were higher in cells expressing ER-alpha as was protein expression of the p53-regulated gene p21. The significance of these receptors to osteoblast differentiation and p53 regulation is discussed.

Role of endocrine-immune dysregulation in osteoporosis, sarcopenia, frailty and fracture risk

Osteoporosis, a key predictor of hip fractures can be treated using a variety of safe and effective interventions. Nevertheless, optimally effective strategies for the prevention of hip fractures must also incorporate efforts to address a broad range of other potentially reversible factors. Hyperthyroidism, anticonvulsants, caffeine and smoking may decrease bone mass and increase fracture risk at any age. In older individuals it is important to also consider additional risk factors, including long-acting benzodiazepines, poor vision and sarcopenia. The presence of sarcopenia, an age-related decline in muscle bulk and quality enhances the risk of frailty and possibly also hip fracture, particularly if associated with diminished functional mobility, lower quadriceps strength and poor balance or body sway. In this review we examine evidence which indicates the presence of endocrine-immune dysregulation in both osteoporosis and sarcopenia. Post-menopausal declines in serum estrogen and androgen levels contribute to increases in local bone levels of cytoclastic cytokines, followed by increased osteoclastogenesis and bone loss. Similarly, the presence of decreased gonadal hormones and IGF-1, combined with unusually high peripheral levels of cytokines, inflammatory mediators and coagulation markers all enhance the risk of sarcopenia and frailty. We propose that a translational research approach which emphasizes common pathophysiologic mechanisms in osteoporosis and sarcopenia could accelerate the speed of discovery of effective strategies for both frailty and hip fracture prevention.

P53 and beta-catenin activity during estrogen treatment of osteoblasts

Background

This study was undertaken to examine the relationship between the tumor suppressor gene p53 and the nuclear signaling protein beta-catenin during bone differentiation. Cross talk between p53 and beta-catenin pathways has been demonstrated and is important during tumorigenesis and DNA damage, where deregulation of beta catenin activates p53. In this study, we used estrogen treatment of osteoblasts as a paradigm to study the relationship between the two proteins during osteoblast differentiation.

Results

We exposed osteoblast-like ROS17/2.8 cells to 17-beta estradiol (E2), in a short term assay, and studied the cellular distribution and expression of beta-catenin. We found beta-catenin to be up regulated several fold following E2 treatment. Levels of p53 and its functional activity mirrored the quantitative changes seen in beta-catenin. Alkaline phosphatase, an early marker of osteoblast differentiation, was increased in a manner similar to beta-catenin and p53. In order to determine if there was a direct relationship between alkaline phosphatase expression and beta-catenin, we used two different approaches. In the first approach, treatment with LiCl, which is known to activate beta-catenin, caused a several fold increase in alkaline phosphatase activity. In the second approach, transient transfection of wild type beta-catenin into osteoblasts increased alkaline phosphatase activity two fold over basal levels, showing that beta catenin expression can directly affect alkaline phosphatase expression. However increase in beta catenin activity was not associated with an increase in its signaling activity through TCF/LEF mediated transcription. Immunofluorescence analyses of p53 and beta-catenin localization showed that E2 first caused an increase in cytosolic beta-catenin followed by the accumulation of beta-catenin in the nucleus. Nuclear p53 localization was detected in several cells.

Expression of p53 was accompanied by distribution of beta-catenin to the cytoplasm and cell borders. A sub population of cells staining strongly for both proteins appeared to be apoptotic.

Conclusion

These results suggest that interactions between p53 and beta-catenin signaling pathways may play a key role in osteoblast differentiation and maintenance of tissue homeostasis.