The role of calcium-sensing receptor and signalling pathways in the pathophysiology in two in vitro models of malignant hypercalcemia: the rat rice H-500 Leydig testis cancer and prostate cancer (PC-3) cells. Expression and regulation of pituitary tumor transforming gene in Leydig testis cancer and astrocyte and astrocytoma cells.

The calcium-sensing receptor (CaR) is a seven transmembrane receptor incorporated into the cell membrane that is sensitive to extracellular calcium and other cations. The finding that the CaR is expressed on cancer cells has opened the door to a new understanding of the role of extracellular calcium as a promalignant stimulus through the CaR and its signaling apparatus as demonstrated in this thesis. I found, in a model of humoral hypercalcemia of malignancy (HHM), that stimulation of the CaR worsens the promalignant features of the testicular H-500 Leydig cancer cells that were used in my studies. The CaR upregulated the release of parathyroid hormone-related peptide (PTHrP), the main mediator of hypercalcemia in HHM. The growth rate of the tumor was also increased by stimulation of the CaR, as DNA synthesis and protection against apoptosis were enhanced. The oncogene, pituitary tumor-transforming gene (PTTG), was found to be upregulated by the CaR in the H-500 cells, whereas calcium had no effect on PTTG expression in the U-87 astrocytoma cell line, but other proproliferative agents did upregulate PTTG in the U-87 cells. This makes PTTG a potential marker of malignancy and a therapeutic target in cancer, where the CaR is promalignant. Nitric oxide synthase (NOS) exists in three isoforms, and I found that the CaR upregulated the inducible NOS but not the two other isoforms. This upregulation was accompanied by an increased production of NO. NO has been shown to be potentially promalignant, although such a role was not established in the H-500 cells. Therefore, the CaR stimulates several promalignant features in the H-500 cells. In turn, blocking these effects by targeting a proximal downstream signaling molecule of the CaR may be a future clinical approach, since blocking the CaR might have too many adverse effects on calcium homeostasis. In conclusion, the CaR plays diverse roles in cancer-acting as an inhibitor of cell proliferation in the colon crypt cells giving rise to colon cancer but as a promalignant receptor in most other cancer types, including Leydig cell cancers.