Breast cancer cells (BCC) frequently metastasize to bone where they may cau
se tumor-induced osteolysis (TIO). While the important eroding role of the
osteoclasts in TIO is well admitted, the possibility that BCC and/or osteob
lasts activated by tumoral factors could also directly degrade bone matrix
in this pathology has been much less investigated. We show here that the ne
t collagen amount produced in vitro by normal human osteoblasts and osteobl
ast-like cells was significantly reduced by culture medium conditioned by s
everal BCC lines, including three newly isolated ones. There was no evidenc
e for a decrease in collagen synthesis, as assessed by the production of th
e carboxyterminal propeptide of type I collagen. In contrast, the effect of
BCC-derived medium on collagen amount was attenuated by inhibitors of matr
ix metalloproteinases (MMPs) as well as by tranexamic acid, an inhibitor of
the plasminogen conversion to plasmin, while it was abolished in presence
of the two kinds of proteinase inhibitors. This osteoblastic protein degrad
ation activity appeared to be attributable to factors secreted by the osteo
blasts as well as by BCC. These factors had molecular weights lower as well
as higher than 10 kD. Our data suggest that besides the eroding action of
osteoclasts, BCC- and osteoblast-derived MMPs and serine proteinases might
play a direct role in bone collagen degradation in TIO.