E. Monsonego et al., GENERATION OF GROWTH-HORMONE BINDING-PROTEIN BY AVIAN GROWTH-PLATE CHONDROCYTES IS DEPENDENT ON CELL-DIFFERENTIATION, Molecular and cellular endocrinology, 135(1), 1997, pp. 1-10
Growth hormone receptor (GH-R) gene expression was evaluated in avian
growth-plates in situ and in cultured chondrocytes. In the epiphyseal
growth-plate, chondrocytes at different stages of differentiation loca
ted at the proliferative and upper hypertrophic zones express the GH-R
gene. In culture, addition of ascorbic acid facilitated chondrocyte d
ifferentiation as evaluated by decrease in collagen type II gene expre
ssion and increase in alkaline phosphatase activity and osteopontin ge
ne expression. Both the ascorbic acid-treated and untreated chondrocyt
es expressed the gene coding for the chicken growth-hormone receptor (
cGH-R), but only the undifferentiated cells were capable of binding th
e hormone. This reduction in GH-binding resulted in alteration in GH-d
ependent regulation of the GH-R gene expression: only the undifferenti
ated chondrocytes responded to chicken GH (cGH) by down-regulation of
the cGH-R gene expression. Chondrocyte differentiation induced by eith
er ascorbic acid or retinoic acid was associated with the appearance o
f two growth hormone binding-proteins (GHBPs) in the culture medium wi
th estimated MWs of 32 and 70 kDa, respectively. These GHBPs differ in
their MW from the major GHBP found in chicken plasma. Chondrocyte GHB
Ps specifically bind [I-125]CGH, which can be displaced by an excess o
f unlabeled cGH. The differentiation-dependent increase in the 70 kDa
GHBP was observed also using specific chicken GHBP antiserum. Our data
suggest that the reduction of the differentiated chondrocytes respons
e to GH is due to differentiation-dependent loss of the extracellular
domain of the GH-R, resulting in a lack of functional receptors on the
cell surface and generation of GHBP. (C) 1997 Elsevier Science Irelan
d Ltd.