The Role of PTHrP in Skeletal Metastases and Hypercalcemia of Malignancy

Since its discovery as the principal mediator of humoral hypercalcemia of malignancy, parathyroid hormone-related protein (PTHrP) has emerged as a key player in skeletal complications associated with solid tumor metastasis to bone. In addition to functioning as an endocrine factor, this pleiotropic peptide mediates its actions locally on tumor and stromal cells in paracrine, autocrine, and intracrine fashion when cancer metastasizes to the bone compartment. Multiple splice variants and newly described PTHrP fragments confer diverse functions to PTHrP that extend beyond binding to its common receptor with parathyroid hormone. Here, we summarize the causal role of PTHrP in humoral hypercalcemia of malignancy and its local involvement in the progression of osteolytic and osteoblastic cancer bone metastases. Clinical and preclinical findings describing how PTHrP regulates tumor and stromal cell interactions are summarized, with emphasis on emerging evidence of PTHrP’s role in tumorigenesis and cancer cell proliferation. Finally, we examine therapeutic opportunities and limitations to targeting PTHrP directly and indirectly in cancer patients.