Osteoblast-Derived Proteins

Chapter 26 - Bones, Joints, and Soft Tissue Tumors

Andrew E. Rosenberg MD

Bones

Normal

The skeletal system is as vital to life as any organ system because of its essential roles in mechanical support and mineral homeostasis. Importantly, the skeleton also houses the

hematopoietic elements, protects viscera, and determines body size and shape. The skeletal system is composed of 206 bones that vary in size and shape (tubular, flat, cuboid). The bones

are interconnected by a variety of joints that allow for a wide range of movement while maintaining structural stability.

Bone is a type of connective tissue, and it is unique because it is one of the few tissues that normally undergo mineralization. Biochemically, it is defined by its distinctive admixture of

inorganic elements (65%) and organic matrix (35%). The inorganic component, calcium hydroxyapatite [10Ca:6(PO4 ):(OH)2 ], is the mineral that gives bone strength and hardness, and

is the storehouse for 99% of the body's calcium, 85% of the body's phosphorus, and 65% of the body's sodium and magnesium. The formation of hydroxyapatite crystal in bone is a

phase transformation from liquid to solid analogous to the conversion of water to ice. The process involves the initiation and induction of mineralization by the organic matrix and it is

tightly regulated by numerous factors.[1] The rate of mineralization can vary, but normally there is a 12- to 15-day lag time between the formation of the matrix and its mineralization.

Bone that is unmineralized is known as osteoid.

The organic component includes the cells of bone and the proteins of the matrix. The bone-forming cells include the osteoprogenitor cells, osteoblasts, and osteocytes. The generation

and stimulation of these cells are regulated by cytokines and growth factors such as bone morphogenic proteins (BMPs), fibroblast growth factor (FGF), platelet-derived growth factor

(PDGF), insulin-like growth factor, and transforming growth factor-b (TGF-b).[2]

• Osteoprogenitor cells are pluripotent mesenchymal stem cells that are located in the vicinity of all bony surfaces. When appropriately stimulated by growth factors such as

bone morphogenic proteins, which are members of the TGF-b superfamily, they undergo cell division and produce offspring that differentiate into osteoblasts. The process of

osteoblastic differentiation is initiated and governed by the transcription factor core binding factor a1, which activates osteoblast-specific gene expression.[3] The generation of

osteoblasts from osteoprogenitor cells is vital to growth, remodeling, and repair of bone throughout life.

• Osteoblasts and surface lining cells are located on the surface of bone and synthesize, transport, and arrange

the many proteins of matrix detailed later ( Fig. 26-1). They also initiate the process of mineralization. Osteoblasts express cell-surface receptors that bind many hormones (parathyroid

hormone [PTH], vitamin D, and estrogen), cytokines, growth factors, and extracellular matrix proteins. Recently, the hormone leptin and low-density lipoprotein receptor-related protein

5 have been shown to play an important role in determining osteoblastic activity, and they may represent evidence of central nervous system and cell-autonomous control of bone mass,

respectively.[4][5][6]Metabolically active osteoblasts have a life span of approximately 3 months and then either undergo apoptosis, become surrounded by matrix and transform into

osteocytes, or become quiescent, flattened, bone surface-lining cells.

• Osteocytes are more numerous than any other bone-forming cell and outnumber osteoblasts by about 10:1. Although encased by bone, they communicate with each other and

with surface cells via an intricate network of tunnels through the matrix known as canaliculi. The osteocytic cell processes traverse the canaliculi, and their contacts along gap

junctions allow the transfer of surface membrane potentials and substrates. The large number of osteocytic processes and their distribution throughout bone tissue enable them to

be the key cells in several biologic processes. Studies have shown that this network may be important in controlling the second-to-second fluctuations in serum calcium and

phosphorus levels by altering the concentration of these minerals in the local extracellular fluid compartment. Osteocytes also can detect mechanical forces and translate them

into biologic activity, including the release of chemical mediators by signal transduction pathways, which activate second messengers such as cyclic adenosine monophosphate

(cAMP).[7]

• The osteoclast is the cell responsible for bone resorption. It is derived from hematopoietic progenitor cells that also give rise to monocytes and macrophages. Information

regarding the molecular regulation of osteoclast formation in humans is limited. In mice, a number of transcription factors, including PU.1 and Fos, are essential for developing

an osteoclast phenotype. [8] The cytokines and growth factors crucial for osteoclast differentiation and maturation in humans include interleukin (IL)-1, IL-3, IL-6, IL-11, tumor

necrosis factor (TNF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and macrophage colony-stimulating factor (M-CSF).[9] These factors work by either

stimulating osteoclast progenitor cells or participating in a paracrine system in which osteoblasts and marrow stromal cells play a central role. This paracrine system is essential

to bone metabolism, and its mediators include the molecules RANK (Receptor Activator for Nuclear factor kB), RANK ligand (RANKL), and osteoprotegerin (OPG).[9] [10]

RANK is a member of the TNF family of receptors expressed mainly on cells of macrophage/monocytic lineage such as preosteoclasts. When this receptor binds its specific

ligand (RANKL) through cell-to-cell contact, osteoclastogenesis is initiated. RANKL is produced by and expressed on the cell membranes of osteoblasts and marrow stromal

cells; its major role in bone metabolism is stimulation of osteoclast formation, fusion, differentiation, activation, and survival. The actions of RANKL can be blocked by another

member of the TNF family of receptors, osteoprotegrin (OPG), which is a soluble protein produced by a number of tissues, including bone, hematopoietic marrow cells, and

immune cells. OPG inhibits osteoclastogenesis by acting as a decoy receptor that binds to RANKL, thus preventing the interaction of RANK with RANKL. Therefore, interplay

between bone cells and these molecules permits osteoblasts and stromal cells to control osteoclast development[10] ( Fig. 26-2 ). This ensures the tight coupling of bone

formation and resorption vital to the success of the skeletal system, and provides a mechanism for a wide variety of biologic mediators (hormones, cytokines, growth factors) to

influence the homeostasis of bone tissue.

Mature multinucleated osteoclasts (containing 6 to 12 nuclei) form from fusion of circulating mononuclear precursors and have a limited life span (approximately 2 weeks).

They are intimately related to the bone surface ( Fig. 26-3 ), where their activity is initiated by binding to matrix adhesion proteins. The scalloped resorption pits they produce,

and frequently reside in, are known as Howship lacunae. The portion of the osteoclast cell membrane overlying the resorption surface is modified by numerous villous

extensions, known as the ruffled border, which serve to increase the membrane surface area. The plasmalemma bordering this region is specialized and forms a seal with the

underlying bone, preventing leakage of digestion products. This self-contained extracellular space is analogous to a secondary lysosome, and the osteoclast acidifies it with a

hydrogen pump system that solubilizes the mineral. The osteoclast also releases into this space a multitude of enzymes that help disassemble the matrix proteins into amino acids

and liberate and activate growth factors, cytokines, and enzymes (such as collagenase), which have been previously deposited and bound to the matrix by osteoblasts. Thus, as

bone is broken down to its elemental units, substances are released into the microenvironment that initiate its renewal ( Fig. 26-4 ).

• The proteins of bone include type 1 collagen and a family of noncollagenous proteins that are derived mainly from osteoblasts. Type 1 collagen forms the backbone of matrix

and accounts for 90% of the weight of the organic component. Osteoblasts deposit collagen either in a random weave known as woven bone or in an orderly layered manner

designated

lamellar bone( Fig. 26-5). Normally, woven bone is seen in the fetal skeleton and is formed at growth plates. Its advantages are that it is produced quickly and resists forces equally from

all directions. The presence of woven bone in the adult is always indicative of a pathologic state; however, it is not diagnostic of a particular disease. For instance, in circumstances

requiring rapid reparative stability, such as a fracture, woven bone is produced. It is also formed around sites of infection and composes the matrix of bone-forming tumors. Lamellar

bone, which gradually replaces woven bone during growth, is deposited much more slowly and is stronger than woven bone.There are four different types of lamellar bone. Three are

present only in the cortex—circumferential, concentric, and interstitial ( Fig. 26-6). The fourth type, trabecular lamellae, composes the bone trabeculae in which the lamellae are oriented

parallel to the long axis of the trabeculum.

The noncollagenous proteins of bone are bound to the matrix and grouped according to their function as adhesion proteins, calcium-binding proteins, mineralization proteins, enzymes,

cytokines, and growth factors ( Table 26-1).[11]Of these, only osteocalcin is unique to bone. It is used as a sensitive and specific serum marker for osteoblast activity. Cytokines and

growth factors control bone cell proliferation, maturation, and metabolism.[12]They serve an important messenger function in translating mechanical and metabolic signals into local

bone cell activity and eventual skeletal adaptation. In this fashion the skeleton is uniquely able to change its structure in response to new physical forces; witness the repositioning of

teeth by the forces of braces.

Figure 26-1Active osteoblasts synthesizing bone matrix. The surrounding spindle cells represent osteoprogenitor cells.

Figure 26-2Paracrine molecular mechanisms that regulate osteoclast formation and function. Osteoclasts are derived from the same stem cells that produce macrophages. Osteoblast/

stromal cell membrane-associated RANK ligand (RANKL) binds to its receptor RANK located on the cell surface of osteoclast precursors. This interaction in the background of

macrophage colony-stimulating factor (M-CSF) causes the precursor cells to produce functional osteoclasts. Stromal cells also secrete osteoprotegerin (OPG) which acts as a decoy

receptor for RANKL, preventing it from binding the RANK receptor on osteoclast precursors. Consequently OPG prevents bone resorption by inhibiting osteoclast differentiation.

Figure 26-3Two osteoclasts resorbing bone.

Figure 26-4Bone resorption and formation are coupled processes that are controlled by systemic factors and local cytokines and growth factors, some of which are deposited in the bone

matrix. Cytokines, growth factors, and signal-transducing molecules are key in the communication between osteoblasts and osteoclasts.

Figure 26-5Woven bone (top) deposited on the surface of pre-existing lamellar bone (bottom).

Figure 26-6The schematic of normal bone structure reveals the subperiosteal and endosteal circumferential lamellae, concentric lamellae about vascular cores creating haversian

systems, and the interstitial lamellae that fill the spaces in between the haversian systems. The trabecular lamellae extend from the endosteal surface. The individual lamellae are

punctuated by osteocytic lacunae with their finely ramifying and interconnecting canals, which contain cell processes.

TABLE 26-1-- Proteins of Bone Matrix

Osteoblast-Derived Proteins

Type 1 collagen

Cell adhesion proteins

••Osteopontin, fibronectin, thrombospondin

Calcium-binding proteins

••Osteonectin, bone sialoprotein

Proteins involved in mineralization

••Osteocalcin

Enzymes

••Collagenase, alkaline phosphatase

Growth factors

••IGF-1, TGF-b, PDGF

Cytokines

••Prostaglandins, IL-1, IL-6, RANKL

Date: 2016-04-22; view: 1363