Combining Src inhibitors and aromatase inhibitors: a novel strategy for overcoming endocrine resistance and bone loss

Adverse events associated with aromatase inhibitors: An Analysis of Real-World Datasets and drug-gene interaction network

BACKGROUND

Aromatase inhibitors (AIs) are commonly used to treat postmenopausal hormone receptor positive breast cancer, but there is currently a lack of comprehensive safety reports on AIs in large-scale cohorts.

RESEARCH DESIGN AND METHODS

We conducted a retrospective pharmacovigilance survey based on the FDA Adverse Event Reporting System, retrieving relevant reports from the first quarter of 2004 to the third quarter of 2023, aiming to conduct a comprehensive comparative analysis of adverse reactions associated with commonly used AIs in clinical practice. Adverse event signals were assessed using disproportionality analysis. In addition, we elucidated the potential toxicological mechanisms of aromatase inhibitor related adverse events through functional enrichment analysis of human genes that interact with AIs.

RESULTS

A total of 7,933 adverse event reports related to AIs were collected, and there were 642 positive signals at the preferred term level. The top three signal intensities for anastrozole are: antiphospholipid syndrome, plantar fasciitis and autoimmune pancreatitis. The top three signal intensities for letrozole are: androgenetic alopecia and myosclerosis, pneumonic herpes virus. The top three signal intensities for exemestane are: infection reactivation, thyroxine free decreased and dilatation atrial. In terms of onset time, letrozole has the earliest onset time overall, followed by exemestane, and finally anastrozole.

CONCLUSIONS

Our research corroborates the typical adverse events linked to AIs while highlighting potential safety concerns in their real-world clinical application.

Ginsenoside compound-K attenuates OVX-induced osteoporosis via the suppression of RANKL-induced osteoclastogenesis and oxidative stress

Osteoporosis (OP), a systemic and chronic bone disease, is distinguished by low bone mass and destruction of bone microarchitecture. Ginsenoside Compound-K (CK), one of the metabolites of ginsenoside Rb1, has anti-aging, anti-inflammatory, anti-cancer, and hypolipidemic activities. We have demonstrated CK could promote osteogenesis and fracture healing in our previous study. However, the contribution of CK to osteoporosis has not been examined. In the present study, we investigated the effect of CK on osteoclastogenesis and ovariectomy (OVX)-induced osteoporosis. The results showed that CK inhibited receptor activator for nuclear factor-κB ligand (RANKL)-mediated osteoclast differentiation and reactive oxygen species (ROS) activity by inhibiting the phosphorylation of NF-κB p65 and oxidative stress in RAW264.7 cells. In addition, we also demonstrated that CK could inhibit bone resorption using bone marrow-derived macrophages. Furthermore, we demonstrated that CK attenuated bone loss by suppressing the activity of osteoclast and alleviating oxidative stress in vivo. Taken together, these results showed CK could inhibit osteoclastogenesis and prevent OVX-induced bone loss by inhibiting NF-κB signaling pathway.

Metastatic Breast Cancer, Organotropism and Therapeutics: A Review

The final stage of breast cancer involves spreading breast cancer cells to the vital organs like the brain, liver lungs and bones in the process called metastasis. Once the target organ is overtaken by the metastatic breast cancer cells, its usual function is compromised causing organ dysfunction and death. Despite the significant research on breast cancer metastasis, it's still the main culprit of breast cancer-related deaths. Exploring the complex molecular pathways associated with the initiation and progression of breast cancer metastasis could lead to the discovery of more effective ways of treating the devastating phenomenon. The present review article highlights the recent advances to understand the complexity associated with breast cancer metastases, organotropism and therapeutic advances.

SRC Promotes Tamoxifen Resistance in Breast Cancer via Up-Regulating SIRT1

Background

Endocrine therapy plays a key role in estrogen receptor-positive breast cancer patients; but, tamoxifen resistance could be a real difficulty for these patients. Several attempts have been made to explore the mechanism and new therapies for these patients. We intend to clarify the expression change of SRC and SIRT1 in tamoxifen-resistant breast cancer cells and explore their functions on tamoxifen resistance.

Methods

SRC and SIRT1 expressions were analyzed by RNA sequencing, qPCR and Western blotting. Loss and gain of function of SRC and SIRT1 were utilized to indicate their oncogenic roles in tamoxifen resistance in vitro and in vivo. Kaplan–Meier analysis and receiver operating characteristic curve were used to evaluate the survival and the predicted effects of SRC and SIRT1 on patients’ prognosis.

Results

High expressions of SRC and/or SIRT1 were found in tamoxifen-resistant cells and related to poor overall survival (p<0.05 for SRC, p<0.001 for SIRT1, p<0.001 for SRC and SIRT1) and cancer-specific survival (p<0.05 for SRC, p<0.01 for SIRT1, p<0.01 for SRC and SIRT1) of tamoxifen-treated breast cancer patients. Down-regulation of SRC (p<0.01) or SIRT1 (p<0.05) separately reversed the resistance to tamoxifen and the minimal concentration of SRC inhibitor KX-01 (p<0.05) or SIRT1 inhibitor EX527 (p<0.001) could also suppress cell proliferation. The expression level of SIRT1 was positively correlated with that of SRC. Overexpression of SRC significantly promotes the cell resistance to tamoxifen inhibited by SIRT1 (p<0.01). In vivo experiments confirmed the effects of SRC on tumor growth by over- or down-regulating SRC expression (p<0.001 and p<0.001, respectively).

Conclusion

SRC and SIRT1 are both up-regulated in tamoxifen-resistant breast cancer cells and related to a poor prognosis in tamoxifen-treated breast cancer. Moreover, SRC could promote tamoxifen resistance by up-regulating SIRT1. SRC and SIRT1 might be novel therapeutic targets in tamoxifen-resistant breast cancer and the interaction between SRC and SIRT1 needs to be further explored.

The Central Contributions of Breast Cancer Stem Cells in Developing Resistance to Endocrine Therapy in Estrogen Receptor (ER)-Positive Breast Cancer

Breast cancer stem cells (BCSC) play critical roles in the acquisition of resistance to endocrine therapy in estrogen receptor (ER)-positive (ER + ve) breast cancer (BC). The resistance results from complex alterations involving ER, growth factor receptors, NOTCH, Wnt/β-catenin, hedgehog, YAP/TAZ, and the tumor microenvironment. These mechanisms are likely converged on regulating BCSCs, which then drive the development of endocrine therapy resistance. In this regard, hormone therapies enrich BCSCs in ER + ve BCs under both pre-clinical and clinical settings along with upregulation of the core components of “stemness” transcriptional factors including SOX2, NANOG, and OCT4. SOX2 initiates a set of reactions involving SOX9, Wnt, FXY3D, and Src tyrosine kinase; these reactions stimulate BCSCs and contribute to endocrine resistance. The central contributions of BCSCs to endocrine resistance regulated by complex mechanisms offer a unified strategy to counter the resistance. ER + ve BCs constitute approximately 75% of BCs to which hormone therapy is the major therapeutic approach. Likewise, resistance to endocrine therapy remains the major challenge in the management of patients with ER + ve BC. In this review we will discuss evidence supporting a central role of BCSCs in developing endocrine resistance and outline the strategy of targeting BCSCs to reduce hormone therapy resistance.

EGFR participates downstream of ERα in estradiol-17β-D-glucuronide-induced impairment of Abcc2 function in isolated rat hepatocyte couplets

Estradiol-17β-D-glucuronide (E17G) induces acute endocytic internalization of canalicular transporters, including multidrug resistance-associated protein 2 (Abcc2) in rat, generating cholestasis. Several proteins organized in at least two different signaling pathways are involved in E17G cholestasis: one pathway involves estrogen receptor alpha (ERα), Ca(2+)-dependent protein kinase C and p38-mitogen activated protein kinase, and the other pathway involves GPR30, PKA, phosphoinositide 3-kinase/AKT and extracellular signal-related kinase 1/2. EGF receptor (EGFR) can potentially participate in both pathways since it interacts with GPR30 and ERα. Hence, the aim of this study was to analyze the potential role of this receptor and its downstream effectors, members of the Src family kinases in E17G-induced cholestasis. In vitro, EGFR inhibition by Tyrphostin (Tyr), Cl-387785 or its knockdown with siRNA strongly prevented E17G-induced impairment of Abcc2 function and localization. Activation of EGFR was necessary but not sufficient to impair the canalicular transporter function, whereas the simultaneous activation of EGFR and GPR30 could impair Abcc2 transport. The protection of Tyr was not additive to that produced by the ERα inhibitor ICI neither with that produced by Src kinase inhibitors, suggesting that EGFR shared the signaling pathway of ERα and Src. Further analysis of ERα, EGFR and Src activations induced by E17G, demonstrated that ERα activation precedes that of EGFR and EGFR activation precedes that of Src. In conclusion, activation of EGFR is a key factor in the alteration of canalicular transporter function and localization induced by E17G and it occurs before that of Src and after that of ERα.

SRC drives growth of antiestrogen resistant breast cancer cell lines and is a marker for reduced benefit of tamoxifen treatment

The underlying mechanisms leading to antiestrogen resistance in estrogen-receptor α (ER)-positive breast cancer is still poorly understood. The aim of this study was therefore to identify biomarkers and novel treatments for antiestrogen resistant breast cancer. We performed a kinase inhibitor screen on antiestrogen responsive T47D breast cancer cells and T47D-derived tamoxifen and fulvestrant resistant cell lines. We found that dasatinib, a broad-spectrum kinase inhibitor, inhibited growth of the antiestrogen resistant cells compared to parental T47D cells. Furthermore western blot analysis showed increased expression and phosphorylation of Src in the resistant cells and that dasatinib inhibited phosphorylation of Src and also signaling via Akt and Erk in all cell lines. Immunoprecipitation revealed Src: ER complexes only in the parental T47D cells. In fulvestrant resistant cells, Src formed complexes with the Human Epidermal growth factor Receptor (HER)1 and HER2. Neither HER receptors nor ER were co-precipitated with Src in the tamoxifen resistant cell lines. Compared to treatment with dasatinib alone, combined treatment with dasatinib and fulvestrant had a stronger inhibitory effect on tamoxifen resistant cell growth, whereas dasatinib in combination with tamoxifen had no additive inhibitory effect on fulvestrant resistant growth. When performing immunohistochemical staining on 268 primary tumors from breast cancer patients who had received tamoxifen as first line endocrine treatment, we found that membrane expression of Src in the tumor cells was significant associated with reduced disease-free and overall survival. In conclusion, Src was identified as target for treatment of antiestrogen resistant T47D breast cancer cells. For tamoxifen resistant T47D cells, combined treatment with dasatinib and fulvestrant was superior to treatment with dasatinib alone. Src located at the membrane has potential as a new biomarker for reduced benefit of tamoxifen.

Medical treatment of breast cancer bone metastasis: from bisphosphonates to targeted drugs

Breast cancer bone metastasis causing severe morbidity is commonly encountered in daily clinical practice. It causes pain, pathologic fractures, spinal cord and other nerve compression syndromes and life threatening hypercalcemia. Breast cancer metastasizes to bone through complicated steps in which numerous molecules play roles. Metastatic cells disrupt normal bone turnover and create a vicious cycle to which treatment efforts should be directed. Bisphosphonates have been used safely for more than two decades. As a group they delay time to first skeletal related event and reduce pain, but do not prevent development of bone metastasis in patients with no bone metastasis, and also do not prolong survival. The receptor activator for nuclear factor κB ligand inhibitor denosumab delays time to first skeletal related event and reduces the skeletal morbidity rate. Radionuclides are another treatment option for bone pain. New targeted therapies and radionuclides are still under investigation. In this review we will focus on mechanisms of bone metastasis and its medical treatment in breast cancer patients.

Identification of gene regulation patterns underlying both oestrogen- and tamoxifen-stimulated cell growth through global gene expression profiling in breast cancer cells

PURPOSE

A c-Src inhibitor blocks oestrogen (E2)-induced stress and converts E2 responses from inducing apoptosis to growth stimulation in E2-deprived breast cancer cells. A reprogrammed cell line, MCF-7:PF, results in a functional oestrogen receptor (ER). We addressed the question of whether the selective ER modulator 4-hydroxytamoxifen (4-OHT) could target ER to prevent E2-stimulated growth in MCF-7:PF cells.

METHODS

Expression of mRNA was measured through real-time RT-PCR. Global gene expression profile was analysed through microarray. Transcriptome profiles were screened by RNA-sequencing.

RESULTS

Unexpectedly, both 4-OHT and E2 stimulated cell growth in a concentration-dependent manner. Expression profiling showed a remarkable overlap in genes regulated in the same direction by E2 and 4-OHT. Pathway enrichment analysis of the 280 genes commonly deregulated in MCF-7:PF cells by 4-OHT and E2 revealed functions mainly related to membrane, cytoplasm and metabolic processes. Further analysis of 98 genes up-regulated by both 4-OHT and E2 uncovered a significant enrichment in genes associated with membrane remodelling, cytoskeleton reorganisation, cytoplasmic adapter proteins, cytoplasm organelle proteins and related processes. 4-OHT was more potent than E2 in up-regulating some membrane remodelling molecules, such as EHD2, FHL2, HOMER3 and RHOF. In contrast, 4-OHT acted as an antagonist to inhibit expression of the majority of enriched membrane-associated genes in wild-type MCF-7 cells.

CONCLUSIONS

Long-term selection pressure has changed the cell population responses to 4-OHT. Membrane-associated signalling is critical for 4-OHT-stimulated cell growth in MCF-7:PF cells. This study provides a rationale for the further investigation of target therapy for tamoxifen resistant patients.

Advances in mechanisms of resistance to aromatase inhibitors

Clinically, there are two distinct types of aromatase inhibitor (AI) resistance, namely acquired and innate resistance. Because the underlying mechanisms of these two types of resistance may not be mutually exclusive, strategies to tackle these resistances may not be effective when used interchangeably. Activation of growth factor receptor pathways is the hallmark of acquired AI resistance. These pathways can be targeted either at the cell surface receptor level or their downstream signaling cascades. Currently, everolimus in combination with exemestane represents a new standard of care for patients progressing on non-steroidal AIs. HDAC inhibitors have also shown promising results For innate resistance, the combination of fulvestrant and AI in the front line setting represents a new treatment option, particularly for patients who present with de novo metastatic disease. A Phase III trial is currently ongoing to evaluate the benefit of CDK 4/6 inhibitor, palbociclib, in the first line setting in combination with AI.

Treatment and Prevention of Bone Metastases from Breast Cancer: A Comprehensive Review of Evidence for Clinical Practice

Bone is the most common site of metastasis from breast cancer. Bone metastases from breast cancer are associated with skeletal-related events (SREs) including pathological fractures, spinal cord compression, surgery and radiotherapy to bone, as well as bone pain and hypercalcemia, leading to impaired mobility and reduced quality of life. Greater understanding of the pathophysiology of bone metastases has led to the discovery and clinical utility of bone-targeted agents such as bisphosphonates and the receptor activator of nuclear factor kappa-B ligand (RANK-L) antibody, denosumab. Both are now a routine part of the treatment of breast cancer bone metastases to reduce SREs. With regards to prevention, there is no evidence that oral bisphosphonates can prevent bone metastases in advanced breast cancer without skeletal involvement. Several phase III clinical trials have evaluated bisphosphonates as adjuvant therapy in early breast cancer to prevent bone metastases. The current published data do not support the routine use of bisphosphonates in unselected patients with early breast cancer for metastasis prevention. However, significant benefit of adjuvant bisphosphonates has been consistently observed in the postmenopausal or ovarian suppression subgroup across multiple clinical trials, which raises the hypothesis that its greatest anti-tumor effect is in a low estrogen microenvironment. An individual patient data meta-analysis will be required to confirm survival benefit in this setting. This review summarizes the key evidence for current clinical practice and future directions.

Endocrine resistance in breast cancer: molecular pathways and rational development of targeted therapies

Endocrine resistance presents a major challenge in the management of estrogen receptor (ER)-positive breast cancer and is an area under intense investigation. Although the underlying mechanism is still poorly understood, many studies point towards the 'cross-talk' between ER and growth factor receptor signaling pathways as the key in the development of estrogen-independent growth in breast cancer. This review aims to provide the reader our current understanding of various molecular pathways that mediate endocrine resistance and that are being evaluated as therapeutic targets for ER-positive breast cancer. While most of the agents that target these pathways have only been tested in Phase I or small Phase II trials, some have shown encouraging results. A critical issue that remains is the development of research strategies and clinical trials that take into account the molecular heterogeneity of ER-positive breast cancer.

Targeting Src family kinases in anti-cancer therapies: turning promise into triumph

Src is a non-receptor tyrosine kinase that is deregulated in many types of cancer. Decades of research have revealed the crucial role of Src in many aspects of tumor development, including proliferation, survival, adhesion, migration, invasion and, most importantly, metastasis, in multiple tumor types. Despite extensive preclinical evidence that warrants targeting Src as a promising therapeutic approach for cancer, Src inhibitor(s) showed only minimal therapeutic activity in various types of solid tumors when used as a single agent in recent early-phase clinical trials. In this review, we highlight the most recent advances from preclinical studies and clinical trials that shed light on potential clinical use of Src inhibitor-containing combinatorial regimens in overcoming resistance to current anticancer therapies and in preventing metastatic recurrence.

Targeting Src family kinases in anti-cancer therapies: turning promise into triumph

Src is a non-receptor tyrosine kinase that is deregulated in many types of cancer. Decades of research have revealed the crucial role of Src in many aspects of tumor development, including proliferation, survival, adhesion, migration, invasion and, most importantly, metastasis, in multiple tumor types. Despite extensive preclinical evidence that warrants targeting Src as a promising therapeutic approach for cancer, Src inhibitor(s) showed only minimal therapeutic activity in various types of solid tumors when used as a single agent in recent early-phase clinical trials. In this review, we highlight the most recent advances from preclinical studies and clinical trials that shed light on potential clinical use of Src inhibitor-containing combinatorial regimens in overcoming resistance to current anticancer therapies and in preventing metastatic recurrence.

Role of Src in breast cancer cell migration and invasion in a breast cell/bone-derived cell microenvironment

The preferential metastasis of breast cancer cells to bone comprises a complex set of events including homing and preferential growth, which may require unique factors produced by bone or other cells in the immediate microenvironment. In this study, an in vitro co-culture system composed of bone mesenchymal stem cells and breast cancer cell lines is used to examine the role of Src kinase on breast cancer cell migration and invasion in the presence of bone-derived cells. This research shows that Src kinase activity in breast cancer cell lines with either high or low levels of endogenous Src activity is increased by bone-derived cell-conditioned medium but not HS68 fibroblast-conditioned medium. Breast cancer cells exhibit enhanced migration in co-culture with bone-derived cells but not HS68 fibroblasts or no co-cultured cells. Inhibition of Src kinase activity using the inhibitors PP2 or saracatinib or using siRNA abrogates the preferential migration of the breast cancer cell lines in response to bone-derived cells. Inhibition of Src activity with saracatinib does not have any significant effect on breast cancer cell invasion in the presence of bone-derived cells. Factors are identified that are produced preferentially by bone-derived cells over HS68 cells that may impact breast cancer cell behavior. This research implicates Src kinase as an important effector of bone-derived cell signals on breast cancer cell migration.

Understanding the metabolic basis of drug resistance: therapeutic induction of the Warburg effect kills cancer cells

Previously, we identified a form of epithelial-stromal metabolic coupling, in which cancer cells induce aerobic glycolysis in adjacent stromal fibroblasts, via oxidative stress, driving autophagy and mitophagy. In turn, these cancer-associated fibroblasts provide recycled nutrients to epithelial cancer cells, "fueling" oxidative mitochondrial metabolism and anabolic growth. An additional consequence is that these glycolytic fibroblasts protect cancer cells against apoptosis, by providing a steady nutrient stream of to mitochondria in cancer cells. Here, we investigated whether these interactions might be the basis of tamoxifen-resistance in ER(+) breast cancer cells. We show that MCF7 cells alone are Tamoxifen-sensitive, but become resistant when co-cultured with hTERT-immortalized human fibroblasts. Next, we searched for a drug combination (Tamoxifen + Dasatinib) that could over-come fibroblast-induced Tamoxifen-resistance. Importantly, we show that this drug combination acutely induces the Warburg effect (aerobic glycolysis) in MCF7 cancer cells, abruptly cutting off their ability to use their fuel supply, effectively killing these cancer cells. Thus, we believe that the Warburg effect in tumor cells is not the "root cause" of cancer, but rather it may provide the necessary clues to preventing chemo-resistance in cancer cells. Finally, we observed that this drug combination (Tamoxifen + Dasatinib) also had a generalized anti-oxidant effect, on both co-cultured fibroblasts and cancer cells alike, potentially reducing tumor-stroma co-evolution. Our results are consistent with the idea that chemo-resistance may be both a metabolic and stromal phenomenon that can be overcome by targeting mitochondrial function in epithelial cancer cells. Thus, simultaneously targeting both (1) the tumor stroma and (2) the epithelial cancer cells, with combination therapies, may be the most successful approach to anti-cancer therapy. This general strategy of combination therapy for overcoming drug resistance could be applicable to many different types of cancer.

Optimal management of bone metastases in breast cancer patients

Bone metastasis in breast cancer is a significant clinical problem. It not only indicates incurable disease with a guarded prognosis, but is also associated with skeletal-related morbidities including bone pain, pathological fractures, spinal cord compression, and hypercalcemia. In recent years, the mechanism of bone metastasis has been further elucidated. Bone metastasis involves a vicious cycle of close interaction between the tumor and the bone microenvironment. In patients with bone metastases, the goal of management is to prevent further skeletal-related events, manage complications, reduce bone pain, and improve quality of life. Bisphosphonates are a proven therapy for the above indications. Recently, a drug of a different class, the RANK ligand antibody, denosumab, has been shown to reduce skeletal-related events more than the bisphosphonate, zoledronic acid. Other strategies of clinical value may include surgery, radiotherapy, radiopharmaceuticals, and, of course, effective systemic therapy. In early breast cancer, bisphosphonates may have an antitumor effect and prevent both bone and non-bone metastases. Whilst two important Phase III trials with conflicting results have led to controversy in this topic, final results from these and other key Phase III trials must still be awaited before a firm conclusion can be drawn about the use of bisphosphonates in this setting. Advances in bone markers, predictive biomarkers, multi-imaging modalities, and the introduction of novel agents have ushered in a new era of proactive management for bone metastases in breast cancer.

Aromatase inhibitor-associated bone and musculoskeletal effects: new evidence defining etiology and strategies for management

Aromatase inhibitors are widely used as adjuvant therapy in postmenopausal women with hormone receptor-positive breast cancer. While the agents are associated with slightly improved survival outcomes when compared to tamoxifen alone, bone and musculoskeletal side effects are substantial and often lead to discontinuation of therapy. Ideally, the symptoms should be prevented or adequately treated. This review will focus on bone and musculoskeletal side effects of aromatase inhibitors, including osteoporosis, fractures, and arthralgias. Recent advances have been made in identifying potential mechanisms underlying these effects. Adequate management of symptoms may enhance patient adherence to therapy, thereby improving breast cancer-related outcomes.

Breast cancer at bone metastatic sites: recent discoveries and treatment targets

Breast carcinoma is the most common cancer of women. Bones are often involved with breast carcinoma metastases with the resulting morbidity and reduced quality of life. Breast cancer cells arriving at bone tissues mount supportive microenvironment by recruiting and modulating the activity of several host tissue cell types including the specialized bone cells osteoblasts and osteoclasts. Pathologically activated osteoclasts produce osteolytic lesions associated with bone pain, pathological fractures, cord compression and other complications of metastatic breast carcinoma at bone. Over the last decade there has been enormous growth of knowledge in the field of osteoclasts biology both in the physiological state and in the tumor microenvironment. This knowledge allowed the development and implementation of several targeted therapeutics that expanded the armamentarium of the oncologists dealing with the metastases-associated osteolytic disease. While the interactions of cancer cells with resident bone cells at the established metastatic gross lesions are well-studied, the preclinical events that underlie the progression of disseminated tumor cells into micrometastases and then into clinically-overt macrometastases are just starting to be uncovered. In this review, we discuss the established information and the most recent discoveries in the pathogenesis of osteolytic metastases of breast cancer, as well as the corresponding investigational drugs that have been introduced into clinical development.

Aromatase activity and bone loss

Aromatase is a specific component of the cytochrome P450 enzyme system that is responsible for the transformation of C19 androgen precursors into C18 estrogenic compounds. This enzyme is encoded by the CYP19A1 gene located at chromosome 15q21.2, that is expressed in ovary and testis not only but also in many extraglandular sites such as the placenta, brain, adipose tissue, and bone. The regulation of the level and activity of aromatase determines the levels of estrogens that have endocrine, paracrine, and autocrine effects on target issues including bone. Importantly, extraglandular aromatization of circulating androgen precursors is the major source of estrogen not only in men (since only 15% of circulating estradiol is released directly by the testis) but also in women after the menopause. Several lines of clinical and experimental evidence now clearly indicate that aromatase activity and estrogen production are necessary for longitudinal bone growth, attainment of peak bone mass, the pubertal growth spurt, epiphyseal closure, and normal bone remodeling in young individuals. Moreover, with aging, individual differences in aromatase activity and thus in estrogen levels may significantly affect bone loss and fracture risk in both genders.