A 74-year-old woman presents to the clinic for evaluation of increasing back pain and malaise. The symptoms are insidious in onset and she reports no history of recent trauma to the back. Her past medical history is significant for hypertension and breast cancer which was diagnosed 7 years ago and treated with surgery and adjuvant therapy.On physical examination, the pertinent findings are percussion tenderness over the lower spine. An x-ray of the lumbar spine reveals lytic lesions compatible with metastatic bone disease, and her serum calcium level is elevated. Which of the following mediators is least likely to be involved?