The combined effect of tumor-produced parathyroid hormone-related protein and transforming growth factor-α enhance hypercalcemia in vivo and bone resorption in vitro

Humoral hypercalcemia of malignancy is a multifactorial syndrome caused by the action of tumor-produced factors on target organs of bone, kidney, and intestine to disrupt normal calcium homeostasis. Although parathyroid hormone-related protein (PTHrP) plays an integral role in the syndrome, tumors also produce other hypercalcemic factors, such as transforming growth factor-α (TGF-α), which may modulate the effects of PTHrP. In order to determine if the effects of PTHrP on calcium homeostasis can be modulated by TGF-α, we have used a human squamous cell carcinoma cell line (RWGT2) which produces PTHrP alone and Chinese hamster ovarian (CHO) cells expressing only transfected human TGF-α complementary DNA (CHO/TGF-α). We studied the effects of these tumors on calcium homeostasis in nude mice bearing both tumors or each tumor alone. Whole blood ionized calcium concentrations (mean ± SEM in mmol/L) were significantly higher in mice bearing both RWGT2 and CHO/TGF-α tumors (3.11 ± 0.06, P < 0.05) when compared with mice bearing either RWGT2 alone (2.02 ± 0.06), CHO/TGF-α alone (1.42 ± 0.01), or RWGT2 and nontransfected CHO tumors (1.86 ± 0.01). This enhanced effect was also observed using continuous PTHrP-(1-34) infusion (2 micrograms/day) in mice bearing CHO/TGF-α tumors. In addition, tumor cell conditioned media was tested for bone resorbing activity in organ cultures of fetal rat long bones previously incorporated with ⁴⁵cal (⁴5Ca⁺⁺). Conditioned medium at 0.1% (vol/vol) from either RWGT2 or CHO/TGF-α had no bone resorbing activity over control (%45Ca++ release, mean ± SEM; control 23 ± RWGT2 19 ± CHO/TGF-α 23 ± 1). However, the combination of 0.1% conditioned medium from RWGT2 and CHO/TGF-α significantly increased bone resorption (53 ± 2, P < 0.05). These data demonstrate that the hypercalcemic effects of tumor-produced PTHrP are enhanced by TGF-α and that this effect may be due to increased bone resorption.