Browsing by Author "Porter, Ryan Michael"

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    Chondrocyte Regulation by IL-I and IGF-I: Interconnection Between Anabolic and Catabolic Factors
    Porter, Ryan Michael (Virginia Tech, 2005-10-14)
    Articular cartilage functions to reduce the mechanical stresses associated with diarthrodial joint movement, protecting these joints over a lifetime of use. Tissue function is maintained through the balance between synthesis and resorption (i.e., metabolism) of extracellular matrix (ECM) by articular chondrocytes (ACs). Two important hormonal regulators of cartilage metabolism are interleukin-1 (IL-1) and insulin-like growth factor-I (IGF-I). These factors have antagonistic effects on chondrocyte activity, and during the progression of osteoarthritis, IL-1 is thought to promote chondrocyte hyporesponsiveness to IGF-I. To better understand how the anabolic (IGF-I) and catabolic (IL-1) stimuli are linked within articular cartilage, we examined the mechanisms by which IL-1 regulates the IGF-I signaling system of ACs. Equine chondrocytes from non-arthritic stifle joints were multiplied over serial passages, re-differentiated in alginate beads, and stimulated with recombinant equine IL-1β. Chondrocytes were assayed for type I IGF receptor (IGF-IR), IGF binding proteins (IGFBPs), and endogenously-secreted IGF-I. Our experimental findings solidify the significance of IL-1 as a key regulator of IGF-I signaling within articular cartilage, demonstrating that regulation of the IGF-I system occurs through both direct (transcription) and indirect (proteolysis) mechanisms. These results have implications for molecular therapies (e.g., gene transfer) directed at reversing osteoarthritic cartilage deterioration. The presented research concerns not only cartilage biology but also tissue engineering strategies for cartilage repair. Alginate hydrogel culture has been reported to re-establish chondrocytic phenotype following monolayer expansion, but studies have not addressed effects on the signaling systems responsible for chondrocyte metabolism. We investigated whether chondrocyte culture history influences the IGF-I system and its regulation by IL-1. ACs expanded by serial passaging were either encapsulated in alginate beads or maintained on tissue culture plastic (TCP). Bead and TCP cells were plated at high-density, stimulated with IL-1β, and assayed for expression of IGF-I signaling mediators. Intermediate alginate culture yielded disparate basal levels of IGF-IR and IGFBP-2, which were attributed to differential transcription. The distinct mediator profiles coincided with varied effects of exogenous IL-1β and IGF-I on collagen Ia1 expression and cell growth rate. This study demonstrates that culture strategy impacts the IGF-I system of ACs, likely impacting their capability to mediate cartilage repair.
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    Examination of Glucocorticoid Treatment on Bone Marrow Stroma: Implications for Bone Disease and Applied Bone Regeneration
    Porter, Ryan Michael (Virginia Tech, 2002-10-29)
    Long-term exposure to pharmacological doses of glucocorticoids has been associated with the development of osteopenia and avascular necrosis. Bone loss may be partially attributed to a steroid-induced decrease in the osteoblastic differentiation of multipotent progenitor cells found in the bone marrow. In order to determine if there is a change in the osteogenic potential of the bone marrow stroma following glucocorticoid treatment, Sprague-Dawley rats were administered methylprednisolone for up to six weeks, then sacrificed at 0, 2, 4, or 6 weeks during treatment or 4 weeks after cessation of treatment. Femurs were collected and analyzed for evidence of steroid-induced osteopenia and bone marrow adipogenesis. Although glucocorticoid treatment did inhibit bone growth, differences in ultimate shear stress and mineral content were not detected. The volume of marrow fat increased with increasing duration of treatment, but returned to near control levels after cessation of treatment. Marrow stromal cells were isolated from tibias, cultured in the presence of osteogenic supplements, and analyzed for their capacity to differentiate into osteoblast-like cells in vitro. Glucocorticoid treatment diminished the absolute number of isolated stromal cells, but did not inhibit the relative levels of bone-like mineral deposition or osteocalcin expression and secretion. Although pharmacological glucocorticoid levels induce bone loss in vivo, physiologically equivalent concentrations have been shown to enhance the formation of bone-like tissue in vitro. However, glucocorticoids have also been reported to inhibit proliferation and type I collagen synthesis in marrow stromal cell cultures. In order to assess the effects of intermittent dexamethasone treatment on the progression of osteogenesis in rat marrow stromal cell culture, this synthetic glucocorticoid was removed from the culture medium after a variable period of initial supplementation. Cell layers were analyzed for total cell number, collagen synthesis, phenotypic marker expression, and matrix mineralization. Prolonged supplementation with dexamethasone decreased proliferation, but did not significantly affect collagen synthesis. Furthermore, increased treatment duration was found to increase bone sialoprotein expression and mineral deposition. The duration of glucocorticoid treatment may be a key factor for controlling the extent of differentiation in vitro.