Jane B. Lian

MOLECULAR MECHANISMS MEDIATING DEVELOPMENTAL AND HORMONE-REGULATED EXPRESSION OF GENES IN OSTEOBLASTS: An Integrated Relationship of Cell Growth and Differentiation

Publisher Summary This chapter discusses the molecular mechanisms mediating developmental and hormone-regulated expression of genes in osteoblasts. The development of methods for culture of normal diploid calvarial-derived osteoblasts under conditions that support development of a tissue-like organization similar to embryonic bone has provided the basis for experimentally addressing the proliferation–differentiation relationship as reflected by structural, biochemical, and molecular parameters within the context of physiological regulation. The sequential and stringently regulated expression of genes that defines three principal periods of osteoblast phenotype development, proliferation, extracellular matrix development and maturation, and mineralization is schematically illustrated in the chapter as a reciprocal and functionally coupled relationship between proliferation and differentiation. Two transition points are key elements of this temporal expression of genes that support cell growth and differentiation: The first transition point is at the completion of the proliferation period, when genes for cell cycle and cell growth control are downregulated and expression of genes encoding proteins for extracellular matrix maturation and organization is initiated, and the second is at the onset of extracellular matrix mineralization.

Integrin alphavbeta6 promotes an osteolytic program in cancer cells by upregulating MMP2

The molecular circuitries controlling osseous prostate metastasis are known to depend on the activity of multiple pathways, including integrin signaling. Here, we demonstrate that the αvβ6 integrin is upregulated in human prostate cancer bone metastasis. In prostate cancer cells, this integrin is a functionally active receptor for fibronectin and latency associated peptide-TGFβ1; it mediates attachment and migration upon ligand binding and is localized in focal contacts. Given the propensity of prostate cancer cells to form bone metastatic lesions, we investigated whether the αvβ6 integrin promotes this type of metastasis. We show for the first time that αvβ6 selectively induces matrix metalloproteinase 2, MMP2, in vitro in multiple prostate cancer cells, and promotes osteolysis in vivo in an immunodeficient mouse model of bone metastasis through upregulation of MMP2, but not MMP9. The effect of αvβ6 on MMP2 expression and activity is independent of androgen receptor in the analyzed prostate cancer cells. Increased levels of PTHrP, known to induce osteoclastogenesis, were also observed in αvβ6 expressing cells. However, using MMP2 shRNA, we demonstrate that the αvβ6 effect on bone loss is due to upregulation of soluble MMP2 by the cancer cells, not to changes in tumor growth rate. Another related αv-containing integrin, αvβ5, fails to show similar responses, underscoring the significance of αvβ6 activity. Overall, these mechanistic studies establish that expression of a single integrin, αvβ6, contributes to the cancer cell -mediated program of osteolysis by inducing matrix degradation through MMP2. Our results open new prospects for molecular therapy of metastatic bone disease.The molecular circuitries controlling osseous prostate metastasis are known to depend on the activity of multiple pathways, including integrin signaling. Here, we demonstrate that the αvβ6 integrin is upregulated in human prostate cancer bone metastasis. In prostate cancer cells, this integrin is a functionally active receptor for fibronectin and latency-associated peptide-TGF-β1; it mediates attachment and migration upon ligand binding and is localized in focal contacts. Given the propensity of prostate cancer cells to form bone metastatic lesions, we investigated whether the αvβ6 integrin promotes this type of metastasis. We show for the first time that αvβ6 selectively induces matrix metalloproteinase 2 (MMP2) in vitro in multiple prostate cancer cells and promotes osteolysis in vivo in an immunodeficient mouse model of bone metastasis through upregulation of MMP2, but not MMP9. The effect of αvβ6 on MMP2 expression and activity is independent of androgen receptor in the analyzed prostate cancer cells. Increased levels of parathyroid hormone-related protein (PTHrP), known to induce osteoclastogenesis, were also observed in αvβ6-expressing cells. However, by using MMP2 short hairpin RNA, we demonstrate that the αvβ6 effect on bone loss is due to upregulation of soluble MMP2 by the cancer cells, not due to changes in tumor growth rate. Another related αv-containing integrin, αvβ5, fails to show similar responses, underscoring the significance of αvβ6 activity. Overall, these mechanistic studies establish that expression of a single integrin, αvβ6, contributes to the cancer cell-mediated program of osteolysis by inducing matrix degradation through MMP2. Our results open new prospects for molecular therapy for metastatic bone disease.

Osteoblasts and their Signaling Pathways: New Frontiers for Linkage to the Immune System

Publisher Summary The goal of this chapter is to review the basic biology of osteoblasts and the regulation of their differentiation in the context of analogous pathways in immune cells. By identifying mechanisms regulating the differentiation of mesenchymal and hematopoietic cell lineages, we have discovered the essential regulatory factors and activation signals that commit cells to distinct phenotypes, as well as negative regulators that suppress differentiation to alternative phenotypes. It is reasonable to postulate that mechanisms exist to regulate crosstalk between osteoblast lineage and immune cells in the bone microenvironment that are mediated by the secretion of factors from these cells. This chapter describes the current understanding of the regulatory roles that osteoblast lineage cells play in supporting activities of the hematopoietic stem cell niche from which immune cells originate. Finally, examples of bone diseases that are models for understanding osteoblast responses to immune cells are provided.

Cell Cycle Control of Transcription at the G1/S Phase Transition

Competency for proliferation and cell cycle progression are functionally linked to the activities of regulatory factors that modulate the cell division cycle in response to the multi-directional signals of cell type specific signaling cascades. The parameters that mediate growth control are complex and interdependent, and whether cells grow or cease division reflects the integration of a broad spectrum of positive and negative growth regulatory signals that operate within distinct biological contexts. Equally important from a clinical perspective, fidelity of cell cycle regulatory mechanisms is compromised in transformed and tumor cells and in nonmalignant disorders, and reflects deregulation of cell growth or abrogation of cell death.