Cancer cachexia anorexia syndrome (CACS) is a disease of disordered energy metabolism leading to the pronounced loss of body mass. It afflicts 80% of patients with advanced malignancies and is a direct cause of 20% of cancer deaths. Metabolic phenotypes in patients with tumor-induced cachexia are heterogeneous but all suffer from negative energy balance associated with active catabolism of fat and muscle tissue. It is thought that cancers secrete factors that trigger tissue catabolism to increase the availability of circulating nutrients for tumor cells. Parathyroid hormone-related protein (PTHrP) is a growth factor that is structurally related to parathyroid hormone (PTH) and, when secreted by tumor cells, causes a paraneoplastic syndrome known as humoral hypercalcemia of malignancy (HHM). Some studies have suggested that PTHrP may also contribute to CACS by inducing ?browning? of white adipose tissue (WAT). We created a tetracycline-regulated transgenic model of PTHrP overexpression in breast cancer (Tet-PTHrP;PyMT mice) and found that overexpression of PTHrP in mammary tumors causes hypercalcemia, severe fat wasting, anorexia and pronounced weight loss. However metabolic studies in these mice did not find evidence of WAT browning or increased energy expenditure but suggested that PTHrP and hypercalcemia may act together to trigger lipolysis, anorexia and progressive fat loss. Therefore, the central premise of this application is that PTHrP and calcium synergize to disrupt energy metabolism and to contribute to CACS in patients with humoral hypercalcemia of malignancy (HHM). In order to explore the effects of PTHrP in CACS further we propose the following Aims. First, in Aim 1, we will study whether PTHrP activates lipolysis and muscle catabolism in vivo and will characterize whether inflammatory cytokines and/or anorexia contribute to this response. Second, in Aim 2, we will examine whether hypercalcemia acts on the calcium-sensing receptor (CaSR) to augment the lipolytic effects of PTHrP using cultured adipocytes in vitro and by pharmacologically and genetically inhibiting CaSR signaling in Tet-PTHrP;PyMT mice in vivo. Our experiments are designed to explore and extend exciting preliminary data demonstrating that PTHrP contributes to CACS and to determine whether hypercalcemia augments the metabolic effects of elevated levels of PTHrP. We believe these studies will pave the way for further mechanistic studies in patients with CACS and for the development of therapeutic strategies to block PTHrP and or CaSR signaling in cancer patients with cachexia.
Humoral hypercalcemia of malignancy (HHM) is caused by tumor secretion of parathyroid hormone-related protein (PTHrP), a growth factor that is secreted into the circulation by tumor cells. Although prior studies have suggested that PTHrP contributes to the cancer anorexia cachexia syndrome (CACS) by inducing ?browning? of white adipose, our preliminary data suggest that fat wasting and cachexia may result from the combination of PTHrP- and calcium-induced lipolysis and anorexia. We will use a new transgenic mouse model of HHM as well as adipocytes in culture to explore changes in energy and fat metabolism in response to hypercalcemia and elevated levels of PTHrP.
