INSULIN-LIKE GROWTH-FACTOR (IGF) BINDING-PROTEINS AND INSULIN-LIKE GROWTH-FACTOR SECRETION BY CULTURED CHONDROCYTE CELLS - IDENTIFICATION, CHARACTERIZATION AND ONTOGENY DURING CELL-DIFFERENTIATION

Insulin-like growth factor binding proteins (IGF BP) and insulin-like growth factors (IGF) secretion by differentiating chondrocytes, derive d from mouse embryonic limb bud and responsive to both IGF-I and -II [ 23], was investigated. The Western ligand blot analysis of the conditi oned medium (CM) from days 1, 3, 5 and 7 of culture revealed the secre tion of IGF BP of approx. 35-40, 28-30 and 24-26 kDa. The 35-40 kDa pr otein which comigrated with the 40 kDa protein in CM of trophoblast ce lls identified as IGF BP-3. The 28-30 kDa protein was identified as IG F BP-2 by Western immunoblotting with alpha-IGF BP-2 antisera. The 24- 26 kDa protein was consistent with the nonglycosylated form of IGF BP- 4. Secretion of three IGF BPs were increased with the age of the cultu re. This suggested that the major IGF BP secreted by differentiating c hondrocytes in culture are IGF BP-2, -3 and -4. All three of these IGF BPs were stimulated by both IGF-I and -II. IGF-I was approx. 2-fold m ore potent than IGF-II. The investigation of the localized production of IGF revealed that chondrocytes, similar to IGF BP, secreted IGF-II in differentiation dependent manner. No IGF-I secretion was identified . Examination of the secretion of solubilized IGF-II receptor by the c hondrocytes, in contrast to trophoblasts, failed to reveal the presenc e of IGF-II receptor in the CM. This suggested that, unlike many other cells, including trophoblasts, chondrocytes do not secrete solubilize d IGF-II receptor. In summary, the present results suggested an intera ctive autocrine/paracrine action of IGF BP and IGF-II in the chondrocy tes, while the IGF-I action is predominantly endocrine.