Normal Bone Growth and the Pathogenesis of Osteoporosis

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Normal Bone Growth and the Pathogenesis of
Osteoporosis
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Outline

• Basic Skeletal Structures Functions
• Mechanisms of Cellular Genesis
• Pathogenesis of Osteoporosis
• Pharmacological Therapy for Osteoporosis

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Osteoporosis
Reduction in the quantity of bone or atrophy of
skeletal tissues occurs in postmenopausal women
and elderly men, resulting in bone trabeculae
that are scanty, thin and without osteoclastic
resorption
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Women vs. Men

• Begin bone loss in their 40s
• Rapid loss of bone mass for 5-10 years after
  menopause onset
• Smaller bone cortices and diameter from growth
  phase, especially during puberty

• Begin bone loss in their 40s
• Bone loss remains linear and slow as sex steroid
  production progressively declines
• Larger bone cortices and diameter from growth
  phase

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Other Risk Factors

• Chronic glucocorticoid excess
• Hyperthyroidism
• Inappropriately high T4 replacement
• Alcoholism
• Prolonged immobilization
• Gastrointestinal disorders
• Hypercalciuria
• Malignancies
• Cigarette smoking

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Cells, Connective Tissue and Spaces

• Highly specialized cells
• Mineralized connective tissue matrix
• Unmineralized connective tissue matrix
• Spaces bone marrow cavity vascular
  canals canaliculi lacunae

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Development/Growth

• Modeling- During growth phase, the removal of
  bone at one site and desposition to another site
• Purpose- Achieve skeletal size and shape

• Remodeling- Once mature, a periodic replacement
  of old bone with new at the same location
• Purpose- Repair fatigue damage and prevent
  excess aging on load bearing bones

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BMU
MOVEMENT
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Vital Statistics

• Lifespan of BMU 6-9 months
• Speed 25 mm/day
• Bone volume replaced 0.025 mm3
  by a single BMU
• Lifespan of osteoclast 2 weeks
• Lifespan of osteoblast (active) 3 months
• Interval between successive 2-5 years
  remodeling events at the same location
• Rate of turnover of whole 10 per yr.
  skeleton

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Osteoblastogenesis

• Multipotent mesenchymal stem cell precursors
• Reach bone by migration from neighboring
  connective tissue
• Initiated by bone morphogenenetic proteins (BMP)
• BMP activates core binding factor a1 (Cbfa1)

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Osteoblastogenesis

• Cbfa1 activates osteoblast genes osteopontin,
  bone sialoprotein type 1 collagen,
  osteocalcin
• BMP also stimulates distal-less 5 (Dlx5)
• Dlx5 regulates osteocalcin, alkaline phosphatase
  and mineralization
• Osteoblast differentiation stimulated by TGFb,
  PDGF, FGF and IGF

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Osteoblasts

• Produce and secrete proteins for bone matrix
• Also produce osteocalcin and osteonectin
• Mineralization, the desposition of
  hydroxyapatite
• Matrix synthesis bone volume
• Mineralization bone density

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Osteocytes

• Osteoblasts buried in lacunae of matrix
• Communicate inter- and extracellularly by
  extensions of the plasma membrane
• Mechanosensory cells for bone growth or reduction
  and repair of microdamage
• Sense changes in interstitial fluid and detect
  changes in hormone levels

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Lining Cells

• Osteoblasts that have become flat and elongated
  cells on the bone surface over a layer of
  unmineralized collagen
• Possible functions secrete
  collagenase produce homing signal for
  osteoclast precursors by direction of
  osteocytes

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Osteoclastogenesis

• Hematopoietic cell precursor
• Reach the bone by circulation
• Cytokines and colony-stimulating factors IL-1,
  IL-3, IL-6, IL-11, LIF, OSM, TNF, GM-CSF, M-CSF,
  c-kit ligand
• Inhibiting cytokines IL-4, IL-10, IL-18,
  INF-g
• Stimulating hormones- PTH and 1,25-dihydroxyvitam
  in D3

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Interleukin-6

• Produced by cells of the osteoblastic lineage
• Stimulates osteoclastogenesis and bone
  resorption
• Binding recruits gp130, which initiates a
  tyrosine phosphorylation pathway
• Also effects the differentiation of osteoblasts
  when in the presence of an IL-6 receptor

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Interleukin-6

• Requires IL-6 membrane-bound receptors in order
  to begin osteoclastogenesis
• Can initiate osteoclastogenesis in the absence
  of IL-6R if exogenous dexamethasone is added

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Interleukin-6

• Osteoblastic stromal cells express very little
  IL-6R mRNA without dexamethasone treatment
• Splenic and bone marrow cells do not require
  dexamethasone in order to express IL-6R mRNA
• IL-11 does not require dexamethasone treatment
  to express IL-11R mRNA

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Interleukin-6

• Requires pretreatment with dexamethasone in
  order to initiate a tyrosine phosphorylation
  pathway using gp130 signal transducing protein of
  osteoblastic cells

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Interleukin-6

• Osteoclasts can be formed when human IL-6 is
  added to a coculture of splenic cells from a
  normal mouse and osteoblastic cells from a
  transgenic mouse overexpressing human IL-6R but
  not from a coculture of splenic cells from a
  transgenic mouse overexpressing human IL-6R and
  osteoblastic cells from a normal mouse

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Conclusion

• gp130 needs to be stimulated in order to produce
  osteoclasts
• Cytokine-specific receptors are required to
  initiate a response to the cytokine
• Dexamethasone allowed IL-6 to bind and initiate
  differentiation in osteoclasts in the absence of
  the receptor

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Osteoclasts

• Ruffled border erosion
• Clear zone of suitable microenvironment
• Degrade bone matrix
• Endocytose matrix components and pass them
  through the cell to be exocytosed into the bone
  membrane

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Apoptosis

• 100 of osteoclasts die vs. 65 of osteoblasts
• Osteocytes also undergo apoptosis, though they
  have longer lifespans than osteoclasts and
  osteoblasts
• Growth factors and cytokines can initiate cell
  apoptosis for example, TGFb stimulates
  osteoclastic apoptosis but inhibits osteoblastic
  apoptosis

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Sex Steroid Deficiency

• Up-regulating the production and action of
  cytokines that are responsible for
  osteoclastogenesis and osteoblastogenesis due to
  a loss of sex steroids results in up-regulation
  of osteoclast and osteoblast formation in the
  marrow
• Stimulation of osteoclastogenesis exceeds that
  of osteoblastogenesis and results in bone loss

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Estrogen Deficiency

• Specifically, normal levels of estrogen suppress
  IL-6 activity with control of both IL-6 and IL-6R
  genes
• Increase in bone resorption is due to increased
  osteoclastogenesis
• Does osteoblastogenesis increase as well?

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Ovariectomized Mice

• Osteoblastic progenitors (CFU-F and CFU-OB)
  increase as a result of ovary removal
• Larger colonies of osteoblastic precursors are
  less differentiated than smaller colonies
• Smaller colonies are more committed to
  osteoblastogenesis

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Ovariectomized Mice

• Does ovary removal have an effect on progenitor
  proliferation?
• What is the effect of ovariectomy on the
  expression of alkaline phosphatase?

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Ovariectomized Mice

• Initial volume of CFU-F increases and gradually
  falls to the control level
• CFU-F and CFU-OB react in the same prolific
  manner, though there are many more CFU-F
• Osteocalcin serum level initially increases and
  gradually falls to the control level

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Ovariectomized Mice

• Osteoclasts and osteoclastic precursors also
  initially increase but rapidly fall to the
  control level eariler after ovary removal than
  osteoblasts
• Bone volume decreased more in ovariectomized
  mice than in the control group due to a 3-4 fold
  increase in osteoclastogenesis

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Ovariectomized Mice

• Is osteoblastogenesis dependent on factors
  released from the bone matrix during resorption?

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Pharmacotherapy

• A promising drug therapy for osteoporosis would
  be an anabolic drug that increases skeletal mass
  through rebuilding
• PTH does increase bone mass, but is the
  mechanism by stimulation of osteoblast
  differentiation or prevention of osteoblast
  apoptosis?

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Pharmacotherapy

• PTH stimulated an increase of bone mineral
  density (BMD) in both normal mice and mice with
  osteopenia
• The increase in the two strains occurred at an
  equal interval from their respective base-line
  values

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Pharmacotherapy

• Is the action of PTH directly toward the
  osteoblasts and osteocytes or indirectly by other
  secondary actions?

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References

• Jilka, RL, Takahashi, K, Munshi, M, Williams, DC,
  Roberson, PK, Manolagas SC. 1998. Loss of
  Estrogen Upregulates Osteobastogenesis in the
  Murine Bone Marrow. Journal of Clinical
  Investigation 101(5) 1942-1950.
• Jilka, RL, Weinstein, RS, Bellido, T, Roberson,
  P, Parfitt, AM, Manolagas, SC. 1999. Increased
  Bone Formation by Prevention of Osteoblast
  Apoptosis with Parathyroid Hormone. Journal of
  Clinical Investigation 104(4) 439-446.
• Manolagas, SC. 2000. Birth and Death of Bone
  Cells Basic Regulatory Mechanisms and
  Implications for the Pathogenesis and Treatment
  of Osteoporosis. Endocrine Reviews 21(2)
  115-137
• Roodman, GD. 1996. Advances in Bone Biology the
  osteoclast. Endocrine Reviews 17(4) 66-80.
• Udagawa, N, Takahashi, N, Katagiri, T, Tamura, T,
  Wada, S, Findlay, DM, Martin, TJ, Hirota, H,
  Taga, T, Kishimoto, T, Suda, T. 1995.
  Interleukin-6 Induction of Osteoclast
  Differentiation Depends on IL-6 Receptors
  Expressed on Osteoblastic Cells But Not on
  Osteoclast Progenitors. Journal of Experimental
  Medicine 182(11) 1461-1468.