The role of adrenomedullin in osteolytic breast cancer bone metastases

The human adrenomedullin gene encodes preproadrenomedullin, which is processed into two secreted multifunctional peptides: the 52 amino acid adrenomedullin (AM) and the 20 amino acid proadrenomedullin N-terminal 20 peptide (PAMP). AM is increased by hypoxia and frequently produced by tumors, where it can stimulate angiogenesis and cell proliferation and suppress cancer cell apoptosis. Breast cancer cell lines that metastasize to bone consistently express AM mRNA. AM is also a potent stimulator of osteoblasts and bone formation, suggesting that it could play a specific role in skeletal metastases. A role of PAMP in breast cancer and bone has not been established.;We tested the role of AM in primary breast cancer and bone metastases by generating MDA-MB-231 osteolytic breast cancer clones with stable overexpression (+5-fold) or siRNA knockdown (-90%) of human AM mRNA, altering production of both AM and PAMP peptides. Clones were tested in vitro for changes in proliferation, cell cycle distribution, and expression of other tumor-secreted factors. Cells were tested for growth in the mammary fat pad (MFP) and bone metastasis mouse models.;Overexpression of AM mRNA did not significantly change cell proliferation in vitro, expression of other tumor-secreted factors, apoptosis, or cell cycle distribution. AM overexpression increased MFP tumor growth. It also increased bone metastases and decreased survival.;AM knockdown did not inhibit growth in vitro, but altered progression through the cell cycle. AM knockdown increased sensitivity to the chemotherapeutic agent taxol. It inhibited tumor establishment and growth in the MFP but not in bone; Knockdown increased tumor burden in bone and osteolytic bone destruction but did not affect survival.;The microenvironment affects the importance of AM in tumor growth. AM may function as a tumor survival factor for the initiation of tumor growth in the MFP but not in bone metastases. AM may induce bone metastases by increasing osteoblast numbers and recruitment of cells.;Small molecule AM inhibitors that inhibit AM-induced new bone formation could be developed for breast cancer treatment. Specific inhibition of AM, separate from PAMP, would reveal the potential of these inhibitors for bone metastasis treatment.