Prinicipal Investigator:  Michele R.  Hutchison, M.D., Ph.D.
Bibliography

Examination of IGF-I and GH-Regulated Intracellular Signaling Pathway Relevant to Chondrocyte Differentiation in the Mammalian Growth Plate

Long bone growth in children occurs at the growth plate, and is dependent on highly organized proliferation and differentiation of chondrocytes therein. The orderly pattern of growth is regulated by a long list of both systemic and locally derived factors, including growth hormone (GH) and insulin-like growth factor-I (IGF-I). We have isolated chondrocytes from juvenile bovine metacarpals, believing that bovine growth plate physiology approximates the human better than currently used rodent models. The chondrocytes are separated by density gradient centrifugation into fractions that correspond to the recognized zones of the growth plate, based on cell size, glycogen content and expression patterns of genes known to be differentially expressed in histologic growth plate zones (collagen X, alkaline phosphatase, Indian hedgehog, and IGFBP5, etc). In short-term primary cultures, the fractions display a gradient of proliferative response to IGF-I, such that the least differentiated cells respond the most robustly to IGF-I. None of the fractions proliferate in response to GH. Also, IGF-I levels in bovine chondrocytes are extremely low, and are not increased by GH. These results support the original "Somatomedin Hypothesis" of GH action.

We have completed microarray analysis comparing the least-differentiated reserve zone to the hypertrophic zones cells. A number of cell surface receptors and secreted factors have been identified, none of which have been reported in growth plate chondrocytes. We are currently examining the role of several factors in both the modulation of IGF-I-dependent proliferation, as well as in the regulation of chondrocyte differentiation.

Our overall aim is to support the claim that GH has little direct effect at the growth plate, that systemic IGF-I is the main signal for proliferation, differentiation and hypertrophy in growth plate chondrocytes, and that many local signal operate by modulating IGF-I activity at the growth plate. The relevance to public health is to better understand mechanisms regulating bone growth in children, by using cells from young non-rodent animals for in vitro studies.