OPPOSING EFFECTS OF TYROSINE KINASE INHIBITORS ON MINERALIZATION OF NORMAL AND TUMOR BONE-CELLS

Induction of matrix maturation and mineralization in calcified tissues is important for patients with primary bone tumors and other bone def iciencies, e.g., osteoporosis. For the former it signifies a better pr ognosis in osteosarcoma, and for the latter it might improve bone remo deling. In the present study eve exposed osteosarcoma cells (Saos2), n ormal bone cells, and marrow stroma to two different tyrosine kinase ( TK) inhibitors: AG-555 and AG-1478. These tyrphostins differ in their effect on signal transduction downstream to the TK receptor (RTK): AG- 1478 inhibits src family TKs whereas AG-555 inhibits nuclear TKs. We f ound that both tyrphostins at 50 mu M increased specific alkaline phos phatase (ALP) activity in Saos2 cells. AG-555 abrogated mineralization whereas AG-1478 increased it. Similarly, in human bone-derived cell c ultures the same dose of tyrphostins had an opposing effect on mineral ization but, in contrast to AG-555, AG-1478 positively selected cells with ALP activity. These tyrphostins also differed in their effect on rat marrow stromal cells. AG-555 decreased cell counts unselectively, whereas the decreased cell counts by AG-1478 resulted in selection of osteoprogenitor cells as indicated by a concordant increase in specifi c ALP activity. The effect of a lower dose of AG-1478, 5 mu M, on the increase in mineralization exceeded its own efficiency in selecting ce lls with specific ALP activity. Our results indicate that AG-1478 sele cts and preserves the osteoblastic phenotype, at doses moderately high er than those required to induce mineralization, and substantially hig her than the doses required for RTK inhibition. Identification of down stream molecular targets for AG-1478, in marrow stromal cells, might p rove useful in designing more selective drugs, capable of separating p roliferative from differentiation-inducing activities. (C) 1997 Wiley- Liss, Inc.