Our laboratory undertakes research in two areas: 1) elucidating the structural biology of the interactions of parathyroid hormone (PTH) with its G protein-coupled receptor (PTH1-Rc); and 2) understanding the mechanisms responsible for breast cancer spreading to bone.

In the first project, we are using several approaches to study the nature of the bimolecular hormone--receptor complex in the PTH/PTH1-Rc system. Formation of the complex between PTH and the hPTH1-Rc leads to a sequence of events, namely hormone binding, PTH1-Rc activation, and signal transduction, which culminate in expression of hormonal bioactivity. The impetus for this research program comes from a desire to understand: 1) the fundamental basis of molecular recognition between a peptide hormone and its G protein-coupled Rc; 2) the differences in Rc states (conformations) which translate into hormone agonism, antagonism, inverse agonism, etc.; and 3) the mechanisms of action of the hormone (PTH) responsible for minute-to-minute regulation of calcium levels in blood. The introduction of PTH as a major new agent for treatment of osteoporosis also focuses attention on the mechanism of anabolic action of this hormone. By gaining insight into the nature of the hormone-Rc complex, the discovery of PTH analogs may be facilitated by structure-guided design in the future. By integrating photoaffinity scanning, molecular biology, pharmacology, and structural biology (conformational studies of hormone and Rc, and molecular modeling), we are succeeding in generating an advanced experimentally derived model of the PTH/PTH1-Rc bimolecular complex that provides structural detail and reveals some of the dynamics of hormone-Rc interaction. We are now positioned to take the next major step in mapping the interface of PTH and its Rc, and to extend our studies to identify the shifts in Rc conformation associated with Rc activation.

The second project is directed at identifying the genes responsible for breast cancer (BrCa) spreading from its primary site in breast to the skeleton. This homing is termed osteotropism. Approximately 200,000 new cases of BrCa are diagnosed in the U.S. each year. In women who develop skeletal metastases (>70% of those with BrCa), the disease is essentially incurable. In addition to mortality, skeletal metastasis is a major cause of complications of BrCa. While important advances have been made in the treatment of primary BrCa, little progress has been made in developing therapies specific for metastasis to bone.

We recently created a model of human BrCa osteotropism, critically needed to identify new targets for metastasis-specific therapy. Fragments of human bone are implanted into immunodeficient (NOD/SCID) mice. Then, human BrCa cells are administered orthotopically into the mammary pad. Several weeks after injection of cancer cells, metastases appear in the human bone implant, but not in the mouse skeleton. This model reflects the process of osteotropism: it replicates the metastatic sequence occurring in patients (by which a primary BrCa migrates to the skeleton and establishes a secondary tumor).

This osteotropism model is all-human. The mouse is merely a conduit. Hence, it has advantages for discovery of novel gene targets. We are undertaking: 1) Gene expression profiling of cancer cells taken from the skeletal metastases and comparing to the original primary BrCa. By this means, we will identify an ensemble of genes that are up- or down-regulated in skeletal metastases. 2) Once candidate genes are identified, we will build an osteotropic BrCa cell line from an otherwise poorly metastatic line by inserting one or more of the metastasis-associated genes, thus validating the role of these genes in gain of function for osteotropism. 3) Bone constituents (e.g. collagen, bone cells, marrow endothelial cells, and combinations) will be implanted in engineered bone (using a silk fiber platform) to create a minimally sufficient target for osteotropism. The components of this artificial homing-site contain essential targets on the bone-side of the tumor--bone interaction. 4) The model will be refined into a reliable assay in which metastatic tumor burden is quantified. Availability of a human BrCa osteotropism assay will enable evaluation of potential metastasis-specific therapeutic agents.

In this multidisciplinary approach, we continue to successfully integrate a number of methods of investigation – from peptide chemistry to molecular and cellular biology to mouse models of human BrCa metastasis to bone.