Nongenotropic, Sex-Nonspecific Signaling through the Estrogen or Androgen Receptors

Risk Factors for Surgical Treatment of Lumbar Degenerative Disc Disease in Middle-aged and Older Women: A Prospective Case-Control Study of 2370 Subjects

OBJECTIVE

Given the distinct physiological and societal traits between women and men, we propose that there are distinct risk factors for lumbar degenerative disc disease surgeries, including lumbar disc herniation (LDH) and lumbar spinal stenosis (LSS), in middle-aged and older populations. However, few studies have focused on middle-aged and older women. This study aims to identify these risk factors specifically in this population.

METHODS

In this case-control study, the study group comprised 1202 women aged ≥ 45 years who underwent operative treatment of lumbar degenerative disc disease (LDH, n = 825; LSS, n = 377), and the control group comprised 1168 women without lumbar disease who visited a health examination clinic during the same period. The study factors included demographics (age, body mass index [BMI], smoking, labor intensity, and genetic history), female-specific factors (menopausal status, number of deliveries, cesarean section, and simple hysterectomy), surgical history (number of abdominal surgeries, hip joint surgery, knee joint surgery, and thyroidectomy), and systemic diseases (hypercholesterolemia, hypertriglyceridemia, hyper-low-density lipoprotein cholesterolemia, hypertension, diabetes, cardiovascular disease, and cerebrovascular disease). Multivariate binary logistic regression analysis was used to calculate the odds ratio (OR) and 95% confidence interval (95% CI) of associated factors.

RESULTS

The risk factors for surgical treatment of LDH in middle-aged and older women included BMI (OR = 1.603), labor intensity (OR = 1.189), genetic history (OR = 2.212), number of deliveries (OR = 1.736), simple hysterectomy (OR = 2.511), hypertriglyceridemia (OR = 1.932), and hyper-low-density lipoprotein cholesterolemia (OR = 2.662). For surgical treatment of LSS, the risk factors were age (OR = 1.889), BMI (OR = 1.671), genetic history (OR = 2.134), number of deliveries (OR = 2.962), simple hysterectomy (OR = 1.968), knee joint surgery (OR = 2.527), hypertriglyceridemia (OR = 1.476), hyper-low-density lipoprotein cholesterolemia (OR = 2.413), and diabetes (OR = 1.643). Cerebrovascular disease was a protective factor against surgery for LDH (OR = 0.267).

CONCLUSIONS

BMI, genetic history, number of deliveries, simple hysterectomy, hypertriglyceridemia, and hyper-low-density lipoprotein cholesterolemia were independent risk factors for surgical treatment of both LDH and LSS in middle-aged and older women. Two disparities were found: labor intensity was a risk factor for LDH patients, and knee joint surgery and diabetes were risk factors for LSS patients.

Osteocyte Estrogen Receptor β (Ot-ERβ) Regulates Bone Turnover and Skeletal Adaptive Response to Mechanical Loading Differently in Male and Female Growing and Adult Mice

Age-related bone loss is a failure of balanced bone turnover and diminished skeletal mechanoadaptation. Estrogen receptors, ERα and ERβ, play critical roles in osteoprotective regulation activated by estrogen and mechanical signals. Previous studies mainly focused on ERα and showed that osteocyte-ERα (Ot-ERα) regulated trabecular, but not cortical bone, and played a minor role in load-induced cortical adaptation. However, the role of Ot-ERβ in bone mass regulation remains unrevealed. To address this issue, we characterized bone (re)modeling and gene expression in male and female mice with Ot-ERβ deletion (ERβ-dOT) and littermate control (LC) at 10 weeks (young) or 28 weeks (adult) of age, as well as their responses to in vivo tibial compressive loading. Increased cancellous bone mass appeared in the L4 vertebral body of young male ERβ-dOT mice. At the same time, femoral cortical bone gene expression showed signs consistent with elevated osteoblast and osteoclast activities (type-I collagen, Cat K, RANKL). Upregulated androgen receptor (AR) expression was observed in young male ERβ-dOT mice relative to LC, suggesting a compensatory effect of testosterone on male bone protection. In contrast, bone mass in L4 decreased in adult male ERβ-dOT mice, attributed to potentially increased bone resorption activity (Cat K) with no change in bone formation. There was no effect of ERβ-dOT on bone mass or gene expression in female mice. Sex-dependent regulation of Ot-ERβ also appeared in load-induced cortical responsiveness. Young female ERβ-dOT mice showed an enhanced tibial cortical anabolic adaptation compared with LC. In contrast, an attenuated cortical anabolic response presented at the proximal tibia in male ERβ-dOT mice at both ages. For the first time, our findings suggest that Ot-ERβ regulates bone (re)modeling and the response to mechanical signals through different mechanisms in males and females. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

A synthetic peptide from Sipunculus nudus promotes bone formation via Estrogen/MAPK signal pathway based on network pharmacology

The tripeptide Leu-Pro-Lys (LPK), derived from the Sipunculus nudus protein, was synthesized and studied to investigate its potential protective effect on bone formation. The effect and mechanism of LPK were analyzed through network pharmacology, bioinformatics, and experimental pharmacology. The study found that LPK at concentrations of 25 μg/mL and 50 μg/mL significantly increased ALP activity and mineralization in C3H10 cells. LPK also increased the expression of COL1A1 and promoted bone formation in zebrafish larvae. Network pharmacology predicted 148 interaction targets between LPK and bone development, and analysis of the protein-protein interaction network identified 13 hub genes, including ESR1, MAPK8, and EGFR, involved in bone development. Through KEGG enrichment pathways analysis, it was determined that LPK promotes bone development by regulating endocrine resistance, the relaxin signaling pathway, and the estrogen signaling pathway. Molecular docking results showed direct interactions between LPK and ESR1, MAPK8, and MAPK14. Additional verification experiments using western blot assay revealed that LPK significantly upregulated the expression of genes related to bone formation, including COL1A1, OPG, RUNX2, ESR1, phosphorylated MAPK14, and phosphorylated MAPK8 in C3H10 cells. These results suggest that LPK promotes bone formation by activating the estrogen/MAPK signaling pathway.

Targeting the non-classical estrogen pathway in neurodegenerative diseases and brain injury disorders

Estrogens can alter the biology of various tissues and organs, including the brain, and thus play an essential role in modulating homeostasis. Despite its traditional role in reproduction, it is now accepted that estrogen and its analogues can exert neuroprotective effects. Several studies have shown the beneficial effects of estrogen in ameliorating and delaying the progression of neurodegenerative diseases, including Alzheimer's and Parkinson's disease and various forms of brain injury disorders. While the classical effects of estrogen through intracellular receptors are more established, the impact of the non-classical pathway through receptors located at the plasma membrane as well as the rapid stimulation of intracellular signaling cascades are still under active research. Moreover, it has been suggested that the non-classical estrogen pathway plays a crucial role in neuroprotection in various brain areas. In this mini-review, we will discuss the use of compounds targeting the non-classical estrogen pathway in their potential use as treatment in neurodegenerative diseases and brain injury disorders.

The osteocyte as a signaling cell

Osteocytes, former osteoblasts encapsulated by mineralized bone matrix, are far from being passive and metabolically inactive bone cells. Instead, osteocytes are multifunctional and dynamic cells capable of integrating hormonal and mechanical signals and transmitting them to effector cells in bone and in distant tissues. Osteocytes are a major source of molecules that regulate bone homeostasis by integrating both mechanical cues and hormonal signals that coordinate the differentiation and function of osteoclasts and osteoblasts. Osteocyte function is altered in both rare and common bone diseases, suggesting that osteocyte dysfunction is directly involved in the pathophysiology of several disorders affecting the skeleton. Advances in osteocyte biology initiated the development of novel therapeutics interfering with osteocyte-secreted molecules. Moreover, osteocytes are targets and key distributors of biological signals mediating the beneficial effects of several bone therapeutics used in the clinic. Here we review the most recent discoveries in osteocyte biology demonstrating that osteocytes regulate bone homeostasis and bone marrow fat via paracrine signaling, influence body composition and energy metabolism via endocrine signaling, and contribute to the damaging effects of diabetes mellitus and hematologic and metastatic cancers in the skeleton.

Konjac Oligosaccharides Modulate the Gut Environment and Promote Bone Health in Calcium-Deficient Mice

This study aimed to investigate the beneficial effect of konjac oligosaccharides (KOS) on bone health in calcium-deficient mice. During the experimental period, low-calcium diet-fed mice were administered with calcium chloride to simulate daily calcium supplementation. Meanwhile, different levels of KOS intervened by adding them into the diet. After 8 weeks, the calcium balance status, bone mass parameters, and gut environment modulation were evaluated. The results showed that dietary KOS intervention alleviated the negative calcium balance, significantly promoted the trabecular number and cortical thickness, and remarkably enhanced the skeletal mechanical strength. Moreover, Pearson's correlation analysis among significantly changed gut microbiota, gut metabolites, and relevant physiological indexes showed that the microbial genera of Lactobacillus, Bifidobacterium, Mucispirillum, Alistipes, and unidentified Clostridia and gut metabolites of kynurenine and testosterone were significantly associated with increased bone mass. These findings provided a new insight into the effect of prebiotics on bone health.

Toward a Mathematical Modeling of Diseases' Impact on Bone Remodeling: Technical Review

A wide variety of bone diseases have hitherto been discovered, such as osteoporosis, Paget’s disease, osteopetrosis, and metastatic bone disease, which are not well defined in terms of changes in biochemical and mechanobiological regulatory factors. Some of these diseases are secondary to other pathologies, including cancer, or to some clinical treatments. To better understand bone behavior and prevent its deterioration, bone biomechanics have been the subject of mathematical modeling that exponentially increased over the last years. These models are becoming increasingly complex. The current paper provides a timely and critical analysis of previously developed bone remodeling mathematical models, particularly those addressing bone diseases. Besides, mechanistic pharmacokinetic/pharmacodynamic (PK/PD) models, which englobe bone disease and its treatment’s effect on bone health. Therefore, the review starts by presenting bone remodeling cycle and mathematical models describing this process, followed by introducing some bone diseases and discussing models of pathological mechanisms affecting bone, and concludes with exhibiting the available bone treatment procedures considered in the PK/PD models.

Denosumab for cancer-related bone loss

INTRODUCTION

Prolonged use of anti-cancer treatments in breast and prostate tumors alters physiological bone turnover leading to adverse skeletal related events, such as osteoporosis, loss of bone mass, and increased risk of fractures. These complications known as cancer treatment-induced bone loss (CTIBL) should be managed with bone targeting agents such as the bisphosphonates and denosumab. The latter is a monoclonal antibody against the receptor activator of nuclear factor-kB ligand (RANKL) that suppresses osteoclasts function and survival increasing bone mass.

AREAS COVERED

This review will focus on the mechanisms associated with bone loss induced by cancer treatments and the most recent evidence about the use of denosumab as preventive and therapeutic strategy to protect bone health. Moreover, we will discuss several key aspects regarding the clinical practical use of denosumab to optimize the management of CTLIB in breast and prostate cancer.

EXPERT OPINION

Denosumab treatment strongly prevents cancer therapies-related skeletal issues in breast and prostate cancer with a good safety profile. Adjuvant six-monthly denosumab delays the time to first fracture onset in early stage breast cancer patients with normal or altered bone mineral density (BMD). Similarly, denosumab treatment is able to prevent fractures and BMD loss in nonmetastatic prostate cancer patients.

Integrative analysis of AR-mediated transcriptional regulatory network reveals IRF1 as an inhibitor of prostate cancer progression

BACKGROUND

Androgen receptor (AR) is crucial for prostate cancer (PCa) initiation and malignant progression. Only half of androgen-responsive genes have been identified as having androgen-responsive elements, suggesting that AR regulates downstream genes through other transcriptional factors. However, whether and how AR regulates the progression via regulating these androgen-responsive genes remains unclear.

METHODS

Androgen-responsive and activity-changed (AC) transcriptional factors (TFs) were identified based on the time-course gene-expression array and gene promoter regions analysis. The intersection of androgen-responsive and AC TFs was selected the core TFs, which were used to construct the core transcriptional regulatory network. GO enrichment analysis, cell proliferation assays, glycolysis experiments, and reverse transcription polymerase chain reaction analysis were used to analyze and validate the functions of the network. As one of the core TFs, the function and mechanism of IRF1 have been further explored.

RESULTS

We devised a new integrated approach to select core TFs and construct core transcriptional regulatory network in PCa. The 24 core TFs and core transcriptional regulatory network participate in regulating PCa cell proliferation, RNA splicing, and cancer metabolism. Further validations showed that AR signaling could promote glycolysis via inducing glycolytic enzymes in PCa cells. IRF1, a novel target of AR, served as a tumor suppressor by inhibiting PCa proliferation, cell cycle, and glycolysis.

CONCLUSIONS

It is the first time to demonstrate the regulating role of the AR-mediated transcriptional regulatory network in a series of important biological processes in PCa cells. IRF1, an AR-regulated TF, acts as tumor suppressor in this core transcriptional regulatory network, which highlights the therapeutic potential of targeting this regulatory network for PCa.

Secondary alterations in bone mineralisation and trabecular thickening occur after long-term estrogen deficiency in ovariectomised rat tibiae, which do not coincide with initial rapid bone loss

This study delineates the time sequence of changes in bone tissue mineralisation in ovariectomised rats. We report that changes in bone mineral distribution arise secondary to the initial rapid bone loss but coincide with trabecular thickening. We propose that these changes compensate for elevated stresses in remaining trabeculae after bone resorption.

INTRODUCTION

Recent studies have shown that osteoporosis is not simply a disease of bone loss and microarchitectural degradation but that important changes in tissue composition also occur. Such changes may be a secondary response to early bone loss, but the time sequence of changes in bone mineral distribution is not fully understood. The objective of this study was to quantify the temporal effects of estrogen deficiency on trabecular mineral distribution in the tibia of ovariectomised (OVX) rats.

METHODS

Weekly in vivo micro-CT scans and morphometric and bone mineral density distribution analyses of the proximal tibia were conducted for the first 4 weeks of estrogen deficiency and then at 8, 14 and 34 weeks.

RESULTS

Here we report that although trabecular bone volume and architecture are significantly deteriorated within the first 4 weeks of estrogen deficiency, there is no change in the distribution of bone mineral within trabeculae during this initial period. The rate of bone loss in OVX animals dramatically reduced between week 4 and week 14, which coincided with the initiation of increases in trabecular thickness and mineralisation in the OVX group.

CONCLUSIONS

Together this study reveals for the first time that alterations in bone mineralisation and trabecular thickening arise secondary to the initial rapid bone loss. We propose that these secondary mineralisation changes act to reinforce the trabecular network in an attempt to compensate for the increased loading that ensues after severe bone loss. This study provides an insight into temporal changes in bone mineral distribution in estrogen deficiency.

Osteoimmunology: A Current Update of the Interplay Between Bone and the Immune System

Immunology, already a discipline in its own right, has become a major part of many different medical fields. However, its relationship to orthopedics and trauma surgery has unfortunately, and perhaps unjustly, been developing rather slowly. Discoveries in recent years have emphasized the immense breadth of communication and connection between both systems and, importantly, the highly promising therapeutic opportunities. Recent discoveries of factors originally assigned to the immune system have now also been shown to have a significant impact on bone health and disease, which has greatly changed how we approach treatment of bone pathologies. In case of bone fracture, immune cells, especially macrophages, are present throughout the whole healing process, assure defense against pathogens and discharge a complex variety of effectors to regulate bone modeling. In rheumatoid arthritis and osteoporosis, the immune system contributes to the formation of the pathological and chronic conditions. Fascinatingly, prosthesis failure is not at all solely a mechanical problem of improper strain but works in conjunction with an active contribution of the immune system as a reaction to irritant debris from material wear. Unraveling conjoined mechanisms of the immune and osseous systems heralds therapeutic possibilities for ailments of both. Contemplation of the bone as merely an unchanging support pillar is outdated and obsolete. Instead it is mandatory that this highly diverse network be incorporated in our understanding of the immune system and hematopoiesis.

Osteocytes and Bone Metastasis

Bone is the most frequent site of breast cancer and prostate cancer metastasis, and one of the most common sites of metastasis for many solid tumors. Once cancer cells colonize in the bone, it imposes a major clinical challenge for the treatment of the disease, and fatality rates increase drastically. Bone, the largest organ in the body, provides a fertile microenvironment enriched with nutrients, growth factors and hormones, a generous reward for cancer cells. Dependent on cancer type, cancer cells can cause osteoblastic (bone forming) or osteolytic lesions to promote the net resorption and/or release of growth factors from the bone extracellular matrix. These processes activate a "vicious cycle", leading to disruption of bone integrity and promoting cancer cell growth and migration. Cancer cells influence the bone microenvironment favoring their colonization and growth. In order to metastasize to the bone, cancer cells must first migrate from the site of origin, and once established within the bone, they must overcome the dormant inducing effects of resident cells. If successful, cancer cells can then colonize and continually disrupt bone homeostasis that is primarily maintained by osteocytes, the most abundant bone cell type. For example, it has been shown that exercise induces osteocytes to release anabolic factors that inhibit osteoclast resorptive activity, promote dormancy and the release of anti-cancer factors that inhibit breast cancer cell metastasis. In this review, we will summarize recent research findings and provide mechanistic insights related to the role of osteocytes in osteolytic metastasis.

In Situ Fluorescence Tracking Toxic Metabolite Mono-2-ethylhexyl phthalate (MEHP) of Di-(2-ethylhexyl) phthalate (DEHP) in HeLa Cells

In this study, we synthesized a small molecule fluorescent probe for detecting mono-2-ethylhexyl phthalate (MEHP) named MEHP-AF, which formed by MEHP cross-linked with 5-aminofluorescein (5-AF) through amide bond. MEHP-AF had been purified based on the different physicochemical properties of 5-AF with MEHP. MEHP-AF showed fluorescence characteristics coming from 5-AF and liposoluble property coming from MEHP. After physicochemical characterization, a series of biological studies of its action in cells were carried out. The results indicated that MEHP-AF was a fluorescent probe with strong specificity and high sensitivity. It can visibly track the location of MEHP in HeLa cell or subcellular levels under confocal laser scanning microscopy in situ. This novel fluorescent probe is expected to use for studying its intracellular behavior at the cell level, especially for investigating the interaction between MEHP and cellular molecules.

High mobility group box 1 protein regulates osteoclastogenesis through direct actions on osteocytes and osteoclasts in vitro

Old age and Cx43 deletion in osteocytes are associated with increased osteocyte apoptosis and osteoclastogenesis. We previously demonstrated that apoptotic osteocytes release elevated concentrations of the proinflammatory cytokine, high mobility group box 1 protein (HMGB1) and apoptotic osteocyte conditioned media (CM) promotes osteoclast differentiation. Further, prevention of osteocyte apoptosis blocks osteoclast differentiation and attenuates the extracellular release of HMGB1 and RANKL. Moreover, sequestration of HMGB1, in turn, reduces RANKL production/release by MLO-Y4 osteocytic cells silenced for Cx43 (Cx43^def ), highlighting the possibility that HMGB1 promotes apoptotic osteocyte-induced osteoclastogenesis. However, the role of HMGB1 signaling in osteocytes has not been well studied. Further, the mechanisms underlying its release and the receptor(s) responsible for its actions is not clear. We now report that a neutralizing HMGB1 antibody reduces osteoclast formation in RANKL/M-CSF treated bone marrow cells. In bone marrow macrophages (BMMs), toll-like receptor 4 (TLR4) inhibition with LPS-RS, but not receptor for advanced glycation end products (RAGE) inhibition with Azeliragon attenuated osteoclast differentiation. Further, inhibition of RAGE but not of TLR4 in osteoclast precursors reduced osteoclast number, suggesting that HGMB1 produced by osteoclasts directly affects differentiation by activating TLR4 in BMMs and RAGE in preosteoclasts. Our findings also suggest that increased osteoclastogenesis induced by apoptotic osteocytes CM is not mediated through HMGB1/RAGE activation and that direct HMGB1 actions in osteocytes stimulate pro-osteoclastogenic signal release from Cx43^def osteocytes. Based on these findings, we propose that HMGB1 exerts dual effects on osteoclasts, directly by inducing differentiation through TLR4 and RAGE activation and indirectly by increasing pro-osteoclastogenic cytokine secretion from osteocytes.

Impairments in the reproductive axis of female mice lacking estrogen receptor β in GnRH neurons

The effect of estrogen on the differentiation and maintenance of reproductive tissues is mediated by two nuclear estrogen receptors (ERs), ERα, and ERβ. Lack of functional ERα and ERβ genes in vivo significantly affects reproductive function; however, the target tissues and signaling pathways in the hypothalamus are not clearly defined. Here, we describe the generation and reproductive characterization of a complete-ERβ KO (CERβKO) and a GnRH neuron-specific ERβKO (GERβKO) mouse models. Both ERβKO mouse models displayed a delay in vaginal opening and first estrus. Hypothalamic gonadotropin-releasing hormone (GnRH) mRNA expression levels in both ERβKO mice were similar to control mice; however female CERβKO and GERβKO mice had lower basal and surge serum gonadotropin levels. Although a GnRH stimulation test in both female ERβKO models showed preserved gonadotropic function in the same animals, a kisspeptin stimulation test revealed an attenuated response by GnRH neurons, suggesting a role for ERβ in normal GnRH neuron function. No alteration in estrogen-negative feedback was observed in either ERβKO mouse models after ovariectomy and estrogen replacement. Further, abnormal development of ovarian follicles with low serum estradiol levels and impairment of fertility were observed in both ERβKO mouse models. In male ERβKO mice, no differences in the timing of pubertal onset or serum luteinizing hormone and follicle-stimulating hormone levels were observed as compared with controls. Taken together, these data provide in vivo evidence for a role of ERβ in GnRH neurons in modulating puberty and reproduction, specifically through kisspeptin responsiveness in the female hypothalamic-pituitary-gonadal axis.

Protective effect of porcine placenta in a menopausal ovariectomized mouse

Menopause is a significant physiological phase that occurs as women's ovaries stop producing ovum and the production of estrogen declines. Human placenta and some amino acids are known to improve menopausal symptoms. In this study, we investigated that porcine placenta extract (PPE) and arginine (Arg), a main amino acid of PPE, would have estrogenic activities in ovariectomized (OVX) mice as a menopause mouse model, human breast cancer cell line (MCF-7) cells, and human osteoblast cell line (MG-63) cells. PPE or Arg significantly inhibited the body weight and increased the vagina weight compared to the OVX mice. PPE or Arg ameliorated the vaginal atrophy in the OVX mice. The levels of 17β-estradiol and the activities of alkaline phosphatase (ALP) were significantly increased by PPE or Arg in the serum of OVX mice. Trabecular bone parameters such as bone mineral density and porosity were also improved by PPE or Arg in the OVX mice. In the MCF-7 and MG-63 cells, PPE or Arg significantly increased the cell proliferation, estrogen receptor β mRNA expression, and estrogen-response elements luciferase activity. Finally, PPE or Arg increased the activations of ALP and extracellular signal-regulated kinase 1/2 in the MG-63 cells. These results indicate that PPE or Arg would have estrogenic and osteoblastic activity. Therefore, PPE or Arg may be useful as new pharmacological tools for treating menopausal symptoms including osteoporosis. Free Korean abstract: A Korean translation of this abstract is freely available at http://www.reproduction-online.org/content/150/3/173/suppl/DC1.

Nongenomic thyroid hormone signaling occurs through a plasma membrane-localized receptor

Thyroid hormone (TH) is essential for vertebrate development and the homeostasis of most adult tissues, including bone. TH stimulates target gene expression through the nuclear thyroid receptors TRα and TRβ; however, TH also has rapid, transcription-independent (nongenomic) effects. We found a previously uncharacterized plasma membrane-bound receptor that was necessary and sufficient for nongenomic TH signaling in several cell types. We determined that this receptor is generated by translation initiation from an internal methionine of TRα, which produces a transcriptionally incompetent protein that is palmitoylated and associates with caveolin-containing plasma membrane domains. TH signaling through this receptor stimulated a pro-proliferative and pro-survival program by increasing the intracellular concentrations of calcium, nitric oxide (NO), and cyclic guanosine monophosphate (cGMP), which led to the sequential activation of protein kinase G II (PKGII), the tyrosine kinase Src, and extracellular signal-regulated kinase (ERK) and Akt signaling. Hypothyroid mice exhibited a cGMP-deficient state with impaired bone formation and increased apoptosis of osteocytes, which was rescued by a direct stimulator of guanylate cyclase. Our results link nongenomic TH signaling to a previously uncharacterized membrane-bound receptor, and identify NO synthase, guanylate cyclase, and PKGII as TH effectors that activate kinase cascades to regulate cell survival and proliferation.

Crosstalk between steroid receptors and the c-Src-receptor tyrosine kinase pathways: implications for cell proliferation

Both steroids and growth factors stimulate proliferation of steroid-dependent tumor cells, and interaction between these signaling pathways may occur at several levels. Steroid receptors are typically classified as ligand-activated transcription factors, and steps by which they bind ligand, dimerize, recruit coregulatory molecules, and activate target gene transcription are well understood. Several steroid responses are functionally linked to c-Src or tyrosine kinase receptors, and the physiological impact and the precise molecular pathways involved in these responses are under intensive investigation. Ligand-independent stimulation of steroid receptor-mediated transcription by growth factors is now believed to occur through activated protein kinases that phosphorylate the receptors and receptor coregulators. Recently, steroid hormones themselves have been shown to rapidly activate intracellular signaling cascades, via binding to cognate cytoplasmic or membrane-associated receptors. In some contexts, steroid receptors interact directly with c-Src and other cytoplasmic signaling molecules, such as Shc, PI3K, and p130 Cas. Crosstalk between growth factors and steroids in both the cytoplasm and nucleus could have profound impact on complex biological processes such as cell growth, and play a significant role in the treatment of steroid-dependent cancers. The potential roles of progesterone and estrogen receptors in this crosstalk are discussed in this review.

Tyrosine-317 of p52(Shc) mediates androgen-stimulated proliferation signals in human prostate cancer cells

The involvement of tyrosine phosphorylation signaling pathways in steroid-induced cell proliferation has received much attention. However, the adaptor molecule that mediates this interaction remains to be identified. In this communication, we identify p52(Shc) as the mediator between tyrosine phosphorylation signaling and steroid signaling in steroid-responsive cell proliferation. Although the different LNCaP prostate cancer cells, C-33, C-51 and C-81, express similar levels of functional androgen receptor (AR), they exhibit different levels of androgen sensitivity. C-33 cell proliferation is highly responsive to the presence of androgens, whereas C-51 cell proliferation is comparatively less responsive to androgens. In contrast, C-81 cell proliferation is independent of androgens. In these cells, tyrosine phosphorylation levels of both p52(Shc) and ErbB-2 were greatest in C-81 cells, comparatively less in C-51 cells and weaker in C-33 cells. The levels and activity of protein tyrosine phosphatase, cellular prostatic acid phosphatase, decreased correspondingly in those cells. In both androgen-independent, rapidly growing C-81 and ErbB-2 cDNA-transfected C-33 cells, p52(Shc) was hyperphosphorylated at Tyr317 (Y317). Conversely, p52(Shc) tyrophosphorylation was decreased in prostatic acid phosphatase cDNA-transfected stable subclones of C-81 cells, which restore androgen-sensitive proliferation and leads to slow growth rates. In C-33 cells, androgen-stimulated cell proliferation correlated with tyrophosphorylation of ErbB-2 and increased phosphorylation of p52(Shc) at Y317, but not at Y239, differing from phosphorylation patterns associated with epidermal growth factor (EGF) stimulation. Furthermore, overexpression of a mutant of p52(Shc), that is Y317F, blocks Y317 phosphorylation of endogenous p52(Shc) and abolishes androgen-stimulated proliferation, but not EGF-stimulated proliferation. Thus, Y317 of p52(Shc) serves as an important regulatory site that allows tyrosine phosphorylation pathways to moderate androgen sensitivity in human prostate cancer cells.

Induction of Androgen Receptor-Dependent Apoptosis in Prostate Cancer Cells by the Retinoblastoma Protein

Re-expression of a tumor suppressor in tumor cells that lack it is an effective way to study its functional activities. However, because tumor cells contain multiple mutations, tumor suppressor functions that are dependent on (an)other regulators are unlikely to be identified by its re-expression alone if the other regulators are also mutated. In this study, we show that re-expression of retinoblastoma (RB) together with the androgen receptor (AR) in RB- and AR-deficient prostate cancer DU-145 cells resulted in an apoptotic activity, acting through the mitochondria damage-initiated caspase activation pathway, which was not present when RB, or the AR, was re-expressed alone. The ability of RB + AR to induce mitochondria damage was dependent on the proapoptotic proteins Bax and Bak and could be blocked by the antiapoptotic protein Bcl-x(L). Coexpressed AR did not detectably change RB's regulation of E2F and cell cycle progression in culture. On the other hand, coexpressed RB could activate the transactivation activity of the AR in an androgen-depleted media. Although androgen induced greater AR transactivation activity in this condition, it did not induce apoptosis in the absence of coexpressed RB. Analysis of mutants of RB and the AR indicated that intact pocket function of RB and the transactivation activity of the AR were required for RB + AR-induced apoptosis. These results provide direct functional data for an AR-dependent apoptosis-inducing activity of RB and highlight the importance of cell type-specific regulators in obtaining a more complete understanding of RB.

Reversal of bone loss in mice by nongenotropic signaling of sex steroids

We show that sex steroids protect the adult murine skeleton through a mechanism that is distinct from that used to preserve the mass and function of reproductive organs. The classical genotropic actions of sex steroid receptors are dispensable for their bone protective effects, but essential for their effects on reproductive tissues. A synthetic ligand (4-estren-3alpha,17beta-diol) that reproduces the nongenotropic effects of sex steroids, without affecting classical transcription, increases bone mass and strength in ovariectomized females above the level of the estrogen-replete state and is at least as effective as dihydrotestosterone in orchidectomized males, without affecting reproductive organs. Such ligands merit investigation as potential therapeutic alternatives to hormone replacement for osteoporosis in both women and men [corrected].

Rapid actions of steroid receptors in cellular signaling pathways

Steroid hormones regulate cellular processes by binding to intracellular receptors that, in turn, interact with discrete nucleotide sequences to alter gene expression. Because most steroid receptors in target cells are located in the cytoplasm, they need to get into the nucleus to alter gene expression. This process typically takes at least 30 to 60 minutes. In contrast, other regulatory actions of steroid hormones are manifested within seconds to a few minutes. These time periods are far too rapid to be due to changes at the genomic level and are therefore termed nongenomic or rapid actions, to distinguish them from the classical steroid hormone action of regulation of gene expression. The rapid effects of steroid hormones are manifold, ranging from activation of mitogen-activated protein kinases (MAPKs), adenylyl cyclase (AC), protein kinase C (PKC), and heterotrimeric guanosine triphosphate-binding proteins (G proteins). In some cases, these rapid actions of steroids are mediated through the classical steroid receptor that can also function as a ligand-activated transcription factor, whereas in other instances the evidence suggests that these rapid actions do not involve the classical steroid receptors. One candidate target for the nonclassical receptor-mediated effects are G protein-coupled receptors (GPCRs), which activate several signal transduction pathways. One characteristic of responses that are not mediated by the classical steroid receptors is insensitivity to steroid antagonists, which has contributed to the notion that a new class of steroid receptors may be responsible for part of the rapid action of steroids. Evidence suggests that the classical steroid receptors can be localized at the plasma membrane, where they may trigger a chain of reactions previously attributed only to growth factors. Identification of interaction domains on the classical steroid receptors involved in the rapid effects, and separation of this function from the genomic action of these receptors, should pave the way to a better understanding of the rapid action of steroid hormones.

Signaling via Shc family adapter proteins

The adapter protein Shc was initially identified as an SH2 containing proto-oncogene involved in growth factor signaling. Since then a number of studies in multiple systems have implicated a role for Shc in signaling via many different types of receptors, such as growth factor receptors, antigen receptors, cytokine receptors, G-protein coupled receptors, hormone receptors and integrins. In addition to the ubiquitous ShcA, two other shc gene products, ShcB and ShcC, which are predominantly expressed in neuronal cells, have also been identified. ShcA knockout mice are embryonic lethal and have clearly suggested an important role for ShcA in vivo. Based on dominant negative studies and mouse embryos deficient in ShcA, a clear role for Shc in leading to mitogen activated protein kinase (MAPK) activation has been established. However MAPK activation may not be the sole function of Shc proteins. Although Shc has also been linked to other signaling events such as c-Myc activation and cell survival, the mechanistic understanding of these signaling events remains poorly characterized. Given the apparently central role that Shc plays signaling via many receptors, delineating the precise mechanism(s) of Shc-mediated signaling may be critical to our understanding of the effects mediated through these receptors.

Manipulating Programmed Cell Death for Better Living!

Manolagas provides a succinct review of the book Apoptosis in Health and Disease. He praises the book for the clarity with which it covers the topics, but he also points out two missing health issues for which regulating apoptosis may prove clinically important: One is the health and regulation of the female reproductive system and the other is the treatment of osteoporosis.