Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) by rosiglitazone suppresses components of the insulin-like growth factor regulatory system in vitro and in vivo.

Rosiglitazone (Rosi) belongs to the class of thiazolidinediones (TZDs) that are ligands for peroxisome proliferator-activated receptor gamma (PPARgamma). Stimulation of PPARgamma suppresses bone formation and enhances marrow adipogenesis. We hypothesized that activation of PPARgamma down-regulates components of the IGF regulatory system, leading to impaired osteoblast function. Rosi treatment (1 microm) of a marrow stromal cell line (UAMS-33) transfected with empty vector (U-33/c) or with PPARgamma2 (U-33/gamma2) were analyzed by microarray. Rosi reduced IGF-I, IGF-II, IGFBP-4, and the type I and II IGF receptor (IGF1R and IGF2R) expression at 72 h in U-33/gamma2 compared with U-33/c cells (P < 0.01); these findings were confirmed by RT-PCR. Rosi reduced secreted IGF-I from U-33/gamma2 cells by 75% (P < 0.05). Primary marrow stromal cells (MSCs) extracted from adult (8 months) and old (24 months) C57BL/6J (B6) mice were treated with Rosi (1 microm) for 48 h. IGF-I, IGFBP-4, and IGF1R transcripts were reduced in Rosi-treated MSCs compared with vehicle (P < 0.01) and secreted IGF-I was also suppressed (P < 0.05). B6 mice treated with Rosi (20 mg/kg.d) for short duration (i.e. 4 d), and long term (i.e. 7 wk) had reduced serum IGF-I; this was accompanied by markedly suppressed IGF-I transcripts in the liver and peripheral fat of treated animals. To determine whether Rosi affected circulating IGF-I in humans, we measured serum IGF-I, IGFBP-2, and IGFBP-3 at four time points in 50 postmenopausal women randomized to either Rosi (8 mg/d) or placebo. Rosi-treated subjects had significantly lower IGF-I at 8 wk than baseline (-25%, P < 0.05), and at 16 wk their levels were reduced 14% vs. placebo (P = 0.15). We conclude that Rosi suppresses IGF-I expression in bone and liver; these changes could affect skeletal acquisition through endocrine and paracrine pathways.

Congenic mice provide in vivo evidence for a genetic locus that modulates serum insulin-like growth factor-I and bone acquisition.

We identified quantitative trait loci (QTL) that determined the genetic variance in serum IGF-I through genome-wide scanning of mice derived from C57BL/6J(B6) x C3H/HeJ(C3H) intercrosses. One QTL (Igf1s2), on mouse chromosome 10 (Chr10), produces a 15% increase in serum IGF-I in B6C3 F2 mice carrying c3 alleles at that position. We constructed a congenic mouse, B6.C3H-10 (10T), by backcrossing c3 alleles from this 57-Mb region into B6 for 10 generations. 10T mice have higher serum and skeletal IGF-I, greater trabecular bone volume fraction, more trabeculae, and a higher number of osteoclasts at 16 wk, compared with B6 (P < 0.05). Nested congenic sublines generated from further backcrossing of 10T allowed for recombination and produced four smaller sublines with significantly increased serum IGF-I at 16 wk (i.e. 10-4, 10-7, 10-10, and 10-13), compared with B6 (P < 0.0003), and three smaller sublines that showed no differences in IGF-I vs. age- and gender-matched B6 mice. Like 10T, the 10-4 nested sublines at 16 wk had higher femoral mineral (P < 0.0001) and greater trabecular connectivity density with significantly more trabeculae than B6 (P < 0.01). Thus, by comprehensive phenotyping, we were able to narrow the QTL to an 18.3-Mb region containing approximately 148 genes, including Igf1 and Elk-3(ETS domain protein). Allelic differences in the Igf1s2 QTL produce a phenotype characterized by increased serum IGF-I and greater peak bone density. Congenic mice establish proof of concept of shared genetic determinants for both circulating IGF-I and bone acquisition.

Expression of terminal differentiation proteins defines stages of mouse mammary gland development.

Immunohistochemical analysis using paraffin-embedded specimens is the method of choice to evaluate protein expression at a cellular level while preserving tissue architecture in normal and neoplastic tissues. Current knowledge of the expression of terminal differentiation markers in the mouse mammary gland relies on the evaluation of frozen tissues by use of immunofluorescence. We assessed changes in patterns of expression of terminal differentiation markers throughout the development of the mouse mammary gland in paraffin-embedded tissues. The expression of alpha-smooth muscle actin (SMA) and keratins (K) 5, 8/18, and 14 was influenced by the development stage of the mammary gland. Expression of K5 and SMA was restricted to basal cells. Keratin 14 was consistently expressed by mammary basal cells, and was detected in scattered luminal cells from 13.5 days after conception through puberty. Labeling for K8/18 of luminal cells was heterogeneous at all times. Heterogeneous expression patterns in luminal cells suggest this layer has cells with a variety of biological functions. The absence of K6 expression at any stage of the development of the mammary gland was confirmed by use of reverse transcriptase-polymerase chain reaction analysis, which indicates that this intermediate filament is not a marker of the mammary gland stem cell. Finally, consistent with results of earlier studies, keratins 1, 10, 13, and 15, and filaggrin, involucrin, and loricrin were not detected at any stage of mammary gland development.

Genetic differences in the IGF-I gene among inbred strains of mice with different serum IGF-I levels.

There is significant heterogeneity in serum IGF-I concentrations among normal healthy individuals across all ages and among inbred strains of mice. C3H/HeJ (C3H) mice have 30% higher serum IGF-I concentrations over a lifetime than C57BL/6J (B6), even though body size and length are identical. The underlying mechanism for this disparity remains unknown although several possibilities exist including altered GH secretion, resistance to GH action, or impaired IGF-I secretion from the liver or peripheral tissues. To study this further, we evaluated mRNA levels of pituitary GH, and of IGF-I, GH receptor (GHR) and acid-labile subunit (ALS) in liver and skeletal muscle of male C3H and B6 strains. mRNA levels of hepatic IGF-I paralleled serum IGF-I levels, whereas pituitary GH mRNA expression was significantly lower in C3H than B6. In addition, reduced hepatic mRNA levels of ALS and GHR in B6 suggests hepatic GH resistance in B6. In contrast, mRNA levels of IGF-I and GHR in skeletal muscle were not different between B6 and C3H. There was a single sequence repeat polymorphism (SSR) in the promoter region of both GHR and IGF-I genes in mice; the SSR in the IGF-I gene was significantly different between the two strains. The SSR in the IGF-I gene corresponds to the E2F binding site, which is critical for regulating IGF-I gene expression. These results suggest that the SSR in the promoter region of the IGF-I gene may be partially responsible for differences in serum IGF-I levels between B6 and C3H strains.

IGF-I regulates osteoprotegerin (OPG) and receptor activator of nuclear factor-kappaB ligand in vitro and OPG in vivo.

IGF-I, a ubiquitous polypeptide, plays a key role in longitudinal bone growth and acquisition. The most predominant effect of skeletal IGF-I is acceleration of the differentiation program for osteoblasts. However, in vivo studies using recombinant human (rh) IGF-I and/or rhGH have demonstrated stimulation of both bone formation and resorption, thereby potentially limiting the usefulness of these peptides in the treatment of osteoporosis. In this study, we hypothesized that IGF-I modulates bone resorption by regulating expression of osteoprotegerin (OPG) and receptor activator of nuclear factor-kappaB (RANK) ligand (RANKL) in bone cells. Using Northern analysis in ST2 cells, we found that human IGF-I suppressed OPG mRNA in a time- and dose-dependent manner: 100 micro g/LIGF-I (13 nM) decreased OPG expression by 37.0 +/- 1.8% (P < 0.002). The half maximal inhibitory dose of IGF-I was reached at 50 micro g/liter ( approximately 6.5 nM) with no effect of IGF-I on OPG message stability. Conditioned media from ST2 cells confirmed that IGF-I decreased secreted OPG, reducing levels by 42%, from 12.1-7 ng/ml at 48 h (P < 0.05). Similarly, IGF-I at 100 micro g/liter (13 nM) increased RANKL mRNA expression to 353 +/- 74% above untreated cells as assessed by real-time PCR. In vivo, low doses of rhGH when administered to elderly postmenopausal women only modestly raised serum IGF-I (to concentrations of 18-26 nM) and did not affect circulating OPG concentrations; however, administration of rhIGF-I (30 micro g/kg.d) for 1 yr to older women resulted in a significant increase in serum IGF-I (to concentrations of 39-45 nM) and a 20% reduction in serum OPG (P < 0.05). In summary, we conclude that IGF-I in a dose- and time-dependent manner regulates OPG and RANKL in vitro and in vivo. These data suggest IGF-I may act as a coupling factor in bone remodeling by activating both bone formation and bone resorption; the latter effect appears to be mediated through the OPG/RANKL system in bone.

Mouse genetic model for bone strength and size phenotypes: NZB/B1NJ and RF/J inbred strains.

The relationships of bone size, bone strength, and bone formation were investigated in two strains of mice, NZB/B1NJ and RF/J. Measurement of the femur midshaft size by peripheral quantitative computed tomography (pQCT) showed that the RF/J mice had a 32% greater cross-sectional area than NZB/B1NJ mice at 10 weeks of age, and a 38% greater cross-sectional area at 22 weeks of age. Body weight in the RF/J mice was 10% higher at 10 weeks but 9% lower at 22 weeks. Bone strength was determined by a three-point bending method. In agreement with the difference in bone cross-sectional area, the femurs of the RF/J mice were stronger (80% greater) and stiffer (80% greater) than the bones of the NZB/B1NJ mice. To determine whether periosteal bone formation played a role in the greater size of the RF/J mice, the mice were injected with tetracycline to label areas of new bone formation. Histomorphometrical analysis of the femur diaphysis demonstrated higher rates of periosteal bone formation (131% greater) and of periosteal forming surface (81% greater) in RF/J than in NZB/B1NJ mice. We conclude that a high rate of periosteal bone formation increases bone size and strength in RF/J mice when compared with NZB/B1NJ mice. The NZB/B1NJ and RF/J mice should be an excellent model to investigate the genes that regulate femur size and strength.