IDENTIFICATION OF FUNCTIONAL INSULIN-LIKE GROWTH FACTOR-II MANNOSE-6-PHOSPHATE RECEPTORS IN ISOLATED BONE-CELLS

The role of the IGF-II/cation independent mannose-6-phosphate (IGF-II/ M6P) receptor in the transduction of cellular effects evoked by IGF-II has been extensively debated in the literature. Many reports suggest that ICF-II transduces its effects through the IGF-I receptor, while o thers show that IGF-II utilizes the type II receptor to affect cellula r activity. This study 1) verifies the presence of the IGF-II/M6P rece ptor in rat calvarial osteoblasts, and 2) evaluates the ability of the receptor to initiate intracellular signals. Using conventional recept or binding assays, it was found that osteoblasts bind IGF-II with high affinity. Scatchard analyses indicated that there are 5.08 x 10(4) IG F-II/M6P receptors per osteoblast with a K-d near 2.0 nM). The recepto r protein was further identified by cross-linking with I-125-IGF-II. N orthern analysis was used to identify an mRNA transcript for the IGF-I I/M6P receptor protein. To examine if the IGF-II/M6P receptor can init iate second messenger signals, the ability of IGF-II to evoke Ca2+ tra nsients was evaluated. Osteoblasts pretreated with IGF-I did not lose their ability to respond to IGF-II. Further, a polyclonal antibody aga inst the rat IGF-II/M6P receptor (R-II-PAB1) 1) was able to evoke its own Ca2+ response, and 2) was able to block the generation of Ca2+ tra nsients caused by IGF-II. The data in this report show that the osteob lastic Ca2+ response to IGF-II appears to be caused by an intracellula r release of Ca2+ which is mediated by the IGF-II/M6P receptor making it possible to envision how the receptor may be an important modulator of osteoblast mediated osteogenesis, (C) 1995 Wiley-Liss, Inc.