Heparanase is an
enzyme that cleaves
heparan sulfate and through this activity promotes
tumor growth, angiogenesis, invasion, and
metastasis in several
tumor types. In human
breast cancer patients,
heparanase expression is associated with sentinel lymph node
metastases. However, the precise role of
heparanase in the malignant progression of
breast cancer is unknown. To examine this, a variant of MDA-MB-231 cells was transfected with the
cDNA for human
heparanase (HPSE cells) or with vector alone as a control (NEO cells). Transfection produced a 6-fold increase in
heparanase activity in HPSE cells relative to NEO cells. When injected into the mammary fat pads of severe combined immunodeficient mice, the
tumors formed by HPSE cells initially grow significantly faster than the
tumors formed by NEO cells. The rapid growth is due in part to increased angiogenesis, as microvessel densities are substantially elevated in primary HPSE
tumors compared with NEO
tumors. Although
metastases to bones are not detected, surprisingly vigorous
bone resorption is stimulated in animals bearing
tumors formed by the HPSE cells. These animals have high serum levels of the
C-telopeptide derived from
type I collagen as well as significant elevation of the active form of
tartrate-resistant acid phosphatase (TRAP)-5b. In contrast, in animals having a high
tumor burden of Neo cells, the serum levels of
C-telopeptide and TRAP-5b never increase above the levels found before
tumor injection. Consistent with these findings, histologic analysis for TRAP-expressing cells reveals extensive osteoclastogenesis in animals harboring HPSE
tumors. In vitro osteoclastogenesis assays show that the osteoclastogenic activity of HPSE cell
conditioned medium is significantly enhanced beyond that of NEO
conditioned medium. This confirms that a soluble factor or factors that stimulate osteoclastogenesis are specifically produced when
heparanase expression is elevated. These factors exert a distal effect resulting in resorption of bone and the accompanying enrichment of the bone microenvironment with growth-promoting factors that may nurture the growth of metastatic
tumor cells. This novel role for
heparanase as a promoter of
osteolysis before
tumor metastasis suggests that
therapies designed to block
heparanase function may disrupt the early progression of bone-homing
tumors.