Jeffrey D. Zajac

Lrp5 Controls Bone Formation by Inhibiting Serotonin Synthesis in the Duodenum

Loss- and gain-of-function mutations in the broadly expressed gene Lrp5 affect bone formation, causing osteoporosis and high bone mass, respectively. Although Lrp5 is viewed as a Wnt coreceptor, osteoblast-specific disruption of beta-Catenin does not affect bone formation. Instead, we show here that Lrp5 inhibits expression of Tph1, the rate-limiting biosynthetic enzyme for serotonin in enterochromaffin cells of the duodenum. Accordingly, decreasing serotonin blood levels normalizes bone formation and bone mass in Lrp5-deficient mice, and gut- but not osteoblast-specific Lrp5 inactivation decreases bone formation in a beta-Catenin-independent manner. Moreover, gut-specific activation of Lrp5, or inactivation of Tph1, increases bone mass and prevents ovariectomy-induced bone loss. Serotonin acts on osteoblasts through the Htr1b receptor and CREB to inhibit their proliferation. By identifying duodenum-derived serotonin as a hormone inhibiting bone formation in an Lrp5-dependent manner, this study broadens our understanding of bone remodeling and suggests potential therapies to increase bone mass.

Low testosterone levels are common and associated with insulin resistance in men with diabetes.

CONTEXT Low testosterone levels are common in men with type 2 diabetes and may be associated with insulin resistance. OBJECTIVE We investigated prevalence of testosterone deficiency and the relationship between testosterone and insulin resistance in a large cohort of men with type 2 and type 1 diabetes. DESIGN The study was a cross-sectional survey of 580 men with type 2 diabetes and 69 men with type 1 diabetes. A subgroup of 262 men with type 2 diabetes was then reassessed after a median of 6 months. RESULTS Forty-three percent of men with type 2 diabetes had a reduced total testosterone, and 57% had a reduced calculated free testosterone. Only 7% of men with type 1 diabetes had low total testosterone. By contrast, 20.3% of men with type 1 diabetes had low calculated free testosterone, similar to that observed in type 2 diabetes (age-body mass index adjusted odds ratio = 1.4; 95% confidence interval = 0.7-2.9). Low testosterone levels were independently associated with insulin resistance in men with type 1 diabetes as well as type 2 diabetes. Serial measurements also revealed an inverse relationship between changes in testosterone levels and insulin resistance. CONCLUSIONS Testosterone deficiency is common in men with diabetes, regardless of the type. Testosterone levels are partly influenced by insulin resistance, which may represent an important avenue for intervention, whereas the utility of testosterone replacement remains to be established in prospective trials.

Falls Relate to Vitamin D and Parathyroid Hormone in an Australian Nursing Home and Hostel

OBJECTIVES: To determine whether falling relates to serum levels of vitamin D and parathyroid hormone.

Characterization of an osteoblast-like clonal cell line which responds to both parathyroid hormone and calcitonin

SummaryThe clonal cell line UMR 106, which was originally derived from a rat transplantable osteogenic sarcoma with an osteoblastic phenotype, was subcloned after the emergence of a calcitonin-responsive adenylate cyclase was noted in late passages. Detailed studies on the stimulation of adenylate cyclase and activation profile of the cyclic AMP-dependent protein kinase isoenzymes in response to parathyroid hormone (PTH) and salmon calcitonin (SCT) were conducted on two subclones (UMR 106-01 and UMR 106-06). Both subclones responded in an identical manner to PTH, which stimulated adenylate cyclase and activated both isoenzyme I and isoenzyme II of cyclic AMP-dependent protein kinase. In contrast, only UMR 106-06 cells responded to calcitonin. At 3×10−8M SCT, there was a sevenfold stimulation of adenylate cyclase, 84% activation of isoenzyme I, and 44% activation of isoenzyme II. The activation profiles of the isoenzymes to PTH and SCT in UMR 106-06 were similar. Furthermore, their response to SCT correlates with the presence of specific, saturable binding of125I-labeled SCT. Binding parameters indicate apparent Kd=0.8 nM and 6,000 receptors/cell. These data point to a significant phenotypic change having taken place in this clonal cell line with prolonged maintenance in culture, with the emergence of a calcitonin receptor linked to adenylate cyclase and protein kinase activation.

Amylin inhibits bone resorption while the calcitonin receptor controls bone formation in vivo

Amylin is a member of the calcitonin family of hormones cosecreted with insulin by pancreatic β cells. Cell culture assays suggest that amylin could affect bone formation and bone resorption, this latter function after its binding to the calcitonin receptor (CALCR). Here we show that Amylin inactivation leads to a low bone mass due to an increase in bone resorption, whereas bone formation is unaffected. In vitro, amylin inhibits fusion of mononucleated osteoclast precursors into multinucleated osteoclasts in an ERK1/2-dependent manner. Although Amylin +/− mice like Amylin-deficient mice display a low bone mass phenotype and increased bone resorption, Calcr +/− mice display a high bone mass due to an increase in bone formation. Moreover, compound heterozygote mice for Calcr and Amylin inactivation displayed bone abnormalities observed in both Calcr +/− and Amylin +/− mice, thereby ruling out that amylin uses CALCR to inhibit osteoclastogenesis in vivo. Thus, amylin is a physiological regulator of bone resorption that acts through an unidentified receptor.

Impaired skeletal muscle development and function in male, but not female, genomic androgen receptor knockout mice

To identify mechanisms of anabolic androgen action in muscle, we generated male and female genomic androgen receptor (AR) knockout (ARKO) mice, and characterized muscle mass, contractile function, and gene expression. Muscle mass is decreased in ARKO males, but normal in ARKO females. The levator ani muscle, which fails to develop in normal females, is also absent in ARKO males. Force production is decreased from fast‐twitch ARKO male muscle, and slow‐twitch muscle has increased fatigue resistance. Microarray analysis shows up‐regulation of genes encoding slow‐twitch muscle contractile proteins. Realtime PCR confirms that expression of genes encoding polyamine biosynthetic enzymes, ornithine decarboxylase (Odc1), and S‐adenosylmethionine decarboxylase (Amd1), is reduced in ARKO muscle, suggesting androgens act through regulation of polyamine biosynthesis. Altered expression of regulators of myoblast progression from proliferation to terminal differentiation suggests androgens also promote muscle growth by maintaining myoblasts in the proliferate state and delaying differentiation (increased Cdkn1c and Igf2, decreased Itg1bp3). A similar pattern of gene expression is observed in orchidectomized male mice, during androgen withdrawal‐dependent muscle atrophy. In conclusion, androgens are not required for peak muscle mass in females. In males, androgens act through the AR to regulate multiple gene pathways that control muscle mass, strength, and fatigue resistance.—MacLean, H. E., Maria Chiu, W. S., Notini, A. J., Axell, A.‐M., Davey, R. A., McManus, J. F., Ma, C., Plant, D. R., Lynch, G. S., Zajac, J. D. Impaired skeletal muscle development and function in male, but not female, genomic androgen receptor knockout mice. FASEB J. 22, 2676–2689 (2008)

Localization of functional domains in the androgen receptor

Functional domains of the androgen receptor (AR) have been localized through a combination of studies on naturally occurring AR gene mutations, in vitro mutagenesis studies and comparison with the structure of other members of the steroid/nuclear receptor superfamily. Two activation domains exist within the amino-terminal domain, and a ligand-dependent activation domain is present in the ligand binding domain. The poly(Gln) stretch within the amino-terminal domain may inhibit the transactivation function of the receptor. Different ligands or binding to different promoters may recruit the use of different activation domains, which may provide promoter-specific effects of receptor action. Co-activator proteins that modulate or enhance AR action have been identified, many of which interact with the ligand binding domain of the AR. Tissue-specific expression of such co-activators, and promoter-specific protein interactions, may also help control the specificity of androgen action. Target Ser residues for phosphorylation have been identified, which may be the site of action for cross-talk from protein kinase signalling pathways. However, the role of phosphorylation in AR function in general is still unclear. It is now clear that interactions occur between receptor domains, modulating functions including ligand dissociation, dimerization and transactivation. By studying the functional domains of the AR, and how they control receptor function in response to different activation signals, we are beginning to understand the mechanisms controlling the specificity of receptor action.

Osteoblast deletion of exon 3 of the androgen receptor gene results in trabecular bone loss in adult male mice.

The mechanism of androgen action on bone was studied in male mice with the AR deleted in mature osteoblasts. These mice had decreased trabecular bone volume associated with a decrease in trabecular number, suggesting that androgens may act directly on osteoblasts to maintain trabecular bone.

Increase in visceral and subcutaneous abdominal fat in men with prostate cancer treated with androgen deprivation therapy.

Objective  Androgen deprivation therapy (ADT) for prostate cancer is associated with increases in fat mass and risk of type 2 diabetes; however, the relationship between sex steroid deficiency and abdominal fat distribution remains controversial.

Reproductive status in long-term bone marrow transplant survivors receiving busulfan-cyclophosphamide (120 mg/kg)

There are few published data on the recovery of fertility after ‘little’ bu-cy (busulfan 16 mg/kg, cyclophosphamide 120 mg/kg) conditioning for bmt. to address this, we identified 19 females aged less than 40 years at transplant and 47 males from a single centre who were alive a minimum of 2 years after bmt with little bu-cy as conditioning and who were evaluable for testing. fsh, lh, testosterone and inhibin b levels were measured in males. twenty-six also had semen analysis, a median of 5 years post transplant; 21 had detectable sperm, with 11 having counts >20 × 106/ml. There was an association between prolonged chronic graft-versus-host disease and low sperm counts. FSH and inhibin B levels correlated with sperm counts but not to the extent that they could reliably predict counts in individual patients. An additional six of seven males attempting to father children did so, a median of 3.2 years post transplant. Low testosterone levels were noted in 12% of males, most of whom had symptoms consistent with androgen deficiency. FSH, LH and oestradiol levels in the absence of hormone replacement therapy were measured in females; all remained amenorrheic with endocrine evidence of ovarian failure. These results have implications for fertility counselling and hormone replacement therapy both pre- and post BMT. Bone Marrow Transplantation (2000) 26, 1089–1095.