Abstract
Osteoporosis is characterized by low BMD resulting from bone resorption (by osteoclasts) exceeding bone formation (by osteoblasts). In postmenopausal women, estrogen deficiency evokes increased osteoclastic activity and bone loss. Studies showed that B cells may participate in osteoclastogenesis via expression of osteoclast-related factors, such as receptor activator of NF-kappaB ligand (RANKL), transforming growth factor beta (TGF-~), and osteoprotegerin (OPG). In addition, Bcell precursors are able to differentiate into osteoclasts in vitro. However, the role of B cells in bone metabolism and osteoporosis is still largely unknown, particularly at the systematic expression level in vivo. Therefore, in the present project, I conducted a powerful microarray study to identify genes contributing to the etiology of osteoporosis in human B cells in vivo. In this project, I recruited 20 unrelated postmenopausal Caucasian females aged 54 - 60, 10 with high (spine or hip Z-score > 0.84) and 10 with low BMD (spine or hip Z-score < -0.84). Total RNA of the freshly isolated blood B cells from those subjects were extracted and hybridized individually to Affymetrix HG-U133A GeneChip@ arrays to identify genes differentially expressed between low and high BMD females. Significance of differential expression was tested by t-test and adjusted with Benjamini and Hochberg (BH) procedure for multiple testing. Adjusted p S; 0.05 was considered as significant. Twenty-nine genes were down-regulated in the low BMD group compared with their expression In the high BMD group. Those genes were further analyzed using Ingenuity Pathways Analysis (Ingenuity® Systems, www.ingenuity.com) and a network involving estrogen receptor 1 (ESRl) and mitogen-activated protein kinase 3 (MAPK3) was identified. Real-time RT-PCR confirmed the differential expression of 8 genes, including ESRl, MAPK3, MECP2 (methly CpG binding protein 2), PSTPIPI (proline-serine-threonine phosphatase interacting protein 1), SLA (Src-like-adaptor), STK11 (serine/threonine kinase 11), WNKI (WNK lysine deficient protein kinase 1), and ZNF446 (zinc finger protein 446). This is the first in vivo genome-wide expression study on human B cells for osteoporosis. My results suggest a novel mechanism for postmenopausal bone loss that downregulation of ESRI and MAPK3 in B cells further regulates the secretion of factors leading to increased osteoclastogenesis or decreased osteoblastogenesis.