Several
cytokines including members of the
transforming growth factor-beta (
TGF-beta) and
tumor necrosis factor (TNF) families have been implicated in the homing mechanism of
breast cancer metastasis. We hypothesize that primary
breast tumor tissues differentially express modulators of bone cell function and that this expression pattern contributes to their aggressive and metastatic potential and to their capacity to establish and grow in bone. We, therefore, examined the gene expression pattern of the
TGF-beta family members (
inhibin/
activin betaA subunit (
activin betaA),
inhibin alpha subunit, and
bone morphogenetic protein-2 (BMP-2)), the TNF family members (receptor activator of
NF-KB ligand (RANKL) and
osteoprotegerin (OPG)), and
osteopontin (OPN) in normal, non-invasive, invasive, and metastatic human
breast cancer specimens. The
mRNA transcript levels of these genes were quantified by reverse transcription (RT) and fluorescent-based kinetic PCR in 18 normal breast tissues, five
ductal carcinoma in situ (
DCIS). 24 primary
breast tumor tissue, and five distant
metastases. The
mRNA transcript level of each gene was normalized to the amount of
beta-actin present in the samples. We observed differential gene expression of the selected
TGF-beta family members as well as OPN in
breast cancer progression. The average gene expression of the putative
tumor suppressor,
inhibin alpha, did not significantly change in any of the
tumor tissues examined compared to normal breast tissue. The
mRNA level of BMP-2, a
protein with anti-proliferative effects in
breast cancer cell lines and involved in bone formation, significantly decreased in non-invasive, invasive, and liver metastatic
breast tumor tissue compared to normal breast tissue. The gene expression of
activin betaA, a
protein involved in cell proliferation and osteoclast induction, increased in invasive and bone metastatic
tumor tissue compared to normal breast tissue. The
mRNA level of OPN, a bone matrix
protein associated with enhanced
malignancy, increased in non-invasive, invasive, and liver and bone metastatic
breast tumor tissue compared to normal breast tissue. In contrast, the average gene expressions of the TNF family members, RANKL and OPG,
proteins involved in the regulation of osteoclastogenesis, were only slightly if at all changed in the different stage
breast tumor tissues. These results suggest that differential gene expression of bone-related
proteins, especially OPN,
activin betaA, and BMP-2, by primary
breast tumor tissues may play a significant role in the invasiveness and metastatic potential of
breast cancer.