Responsiveness of gene expression markers of osteoblastic and osteoclastic activity to calcitonin in the appendicular and axial skeleton of the rat in vivo

Type I diabetic bone phenotype is location but not gender dependent

Bone is highly dynamic and responsive. Bone location, bone type and gender can influence bone responses (positive, negative or none) and magnitude. Type I diabetes induces bone loss and increased marrow adiposity in the tibia. We tested if this response exhibits gender and location dependency by examining femur, vertebrae and calvaria of male and female, control and diabetic BALB/c mice. Non-diabetic male mice exhibited larger body, muscle, and fat mass, and increased femur BMD compared to female mice, while vertebrae and calvarial bone parameters did not exhibit gender differences. Streptozotocin-induced diabetes caused a reduction in BMD at all sites examined irrespective of gender. Increased marrow adiposity was evident in diabetic femurs and calvaria (endochondrial and intramembranous formed bones, respectively), but not in vertebrae. Leptin-deficient mice also exhibit location dependent bone responses and we found that serum leptin levels were significantly lower in diabetic compared to control mice. However, in contrast to leptin-deficient mice, the vertebrae of T1-diabetic mice exhibit bone loss, not gain. Taken together, our findings indicate that TI-diabetic bone loss in mice is not gender, bone location or bone type dependent, while increased marrow adiposity is location dependent.

A scrutiny of matrix metalloproteinases in osteoclasts: evidence for heterogeneity and for the presence of MMPs synthesized by other cells

Genetic diseases and knockout mice stress the importance of matrix metalloproteinases (MMPs) in skeletal turnover. Our study aims at clarifying which MMPs are expressed by osteoclasts. Previous analyses of this basic question led to conflicting reports in the literature. In the present study, we used a variety of approaches: PCR, Northern blots, Slot blots, in situ hybridization, and immunohistochemistry. We analyzed osteoclasts in culture as well as osteoclasts in native bone at different locations and compared mouse and rabbit osteoclasts. Osteoclasts express MMP-9 and -14 in all conditions, although to a variable extent, and they are able to synthesize MMP-3, -10, and -12, at least under some circumstances. The induction of a given MMP in osteoclasts is influenced by its environment (e.g., osteoclast culture vs. native bone, and various sites within the same bone) and depends on the species (e.g., mouse vs. rabbit). Osteoclasts show high amounts of MMP-2 and -13 protein presumably made to a large extent by other cells, thereby documenting how proteinases of nonosteoclastic origin may contribute to osteoclast activities and giving insight in why the resorptive activity of purified osteoclasts appears insensitive to MMP inhibitors. Our study shows that the confusion about osteoclastic MMPs in the literature reflects the remarkable ability of osteoclasts to adapt to their environment, as required by the structural or functional diversity of bone tissue. Our observations provide basic information needed for understanding the emerging role of MMPs in controlling cell signaling and bone resorption.

Expression of the CT/CGRP gene and its regulation by dibutyryl cyclic adenosine monophosphate in human osteoblastic cells

There is general agreement that calcitonin (CT) inhibits bone resorption by its effects on osteoclast function. CT was also found to have direct effects on osteoblast-like cells. In this study, we investigated the expression of CT and calcitonin gene-related peptide (CGRP), the two peptides encoded by the CT/CGRP gene, in human osteosarcoma cell lines and in normal human trabecular osteoblastic cells (HOB), and we studied the modulation of CT/CGRP gene expression by dibutyryl cyclic adenosine monophosphate ((Bu)2, cAMP), a cAMP analog. We first detected by Northern blot hybridization the presence of CT and CGRP mRNAs in different osteosarcoma cell lines (OHS-4, MG-63, Saos-2, HOS-TE85) and HOB cells. In the steady state, OHS-4 cells express slightly more CT and CGRP mRNAs than other cell lines or normal human osteoblasts, in parallel with messengers of differentiated osteoblasts, such as osteocalcin (OC) and alkaline phosphatase (ALP). OHS-4 cells also express CT and CGRP proteins, as demonstrated by immunocytochemistry. Stimulation of OHS-4 cells with 1 mM (Bu)2 cAMP induced a significant increase in mRNA levels for CT (x 2.5) and CGRP (x 3), as determined by a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) procedure. The involvement of a transcriptional mechanism in this effect was evidenced by nuclear run-off transcription assay. In addition, (Bu)2 cAMP increased OC (x 4) and ALP (x 3) mRNA levels in OHS-4 cells. These effects were observed at 24 h and were maximal at 48 h, indicating that (Bu)2, cAMP induced cell differentiation and increased the transcription of the CT/CGRP gene in OHS-4 osteoblast-like cells. The results indicate that human osteosarcoma cells and primary human osteoblastic cells express CT and CGRP mRNA and proteins, and that (Bu)2 cAMP, an activator of protein kinase A, induces up-regulation of osteoblastic phenotypic genes and enhances CT and CGRP gene transcription, indicating that induction of osteoblastic differentiation by (Bu)2 cAMP is associated with enhanced expression of CT and CGRP in human osteoblastic cells.

17 beta-estradiol suppresses gene expression of tartrate-resistant acid phosphatase and carbonic anhydrase II in ovariectomized rats

Tartrate-resistant acid phosphatase (TRACP) and carbonic anhydrase II (CA II) are key enzymes responsible for osteoclastic bone resorption. In this study, we proposed that estrogen loss in postmenopausal osteoporosis may enhance gene expression of TRACP and CA II, and subsequently increase osteoclastic bone resorption. We have, therefore, used the ovariectomized rat model of postmenopausal bone loss to investigate changes at the gene transcriptional level in osteoclastic bone-resorbing enzymes in ovariectomized (OVX) rats, sham ovariectomized (S-OVX) rats, and estrogen-treated ovariectomized (E-OVX) rats. We have demonstrated for the first time that ovariectomy in rats enhances gene expression of TRACP, and CA II. The mRNA levels in OVX were approximately three- and four-fold higher, respectively, than those in S-OVX. Enhancement was observed 1 week after ovariectomy and transcripts remain high during the experimental period of 8 weeks. Administration of 17 beta-estradiol to OVX (E-OVX) reduced gene expression of these osteoclastic bone-resorbing enzymes 18 hours after injection. It appeared that the suppression of the osteoclastic bone-resorbing enzymes by 17 beta-estradiol was most effective during the first 1-2 weeks but the degree of suppression was reduced at 8 weeks after ovariectomy. In conclusion, our results suggest that estrogen prevents bone loss by reducing the mRNA levels of osteoclastic bone-resorbing enzymes in bone tissue.