Dependence of humoral hypercalcemia of malignancy on parathyroid hormone-related protein expression in the canine anal sac apocrine gland adenocarcinoma (CAC-8) nude mouse model

Circulating parathyroid hormone-related protein (PTHrP) is the primary humo ral factor in dogs with spontaneous humoral hypercalcemia of malignancy (HH M) and adenocarcinomas derived from apocrine glands of the anal sac. A cani ne apocrine adenocarcinoma model of HHM in nude mice (CAC-8) was developed and characterized. After 32 passages in vivo, a spontaneous variant of the tumor (CAC-8 Lo Ca) that has altered cellular morphology and that fails to induce HHM in tumor-bearing nude mice has been discovered. The hypercalcemi c and nonhypercalcemic tumor lines were compared by tumor weight, effect on body weight, serum calcium concentration, plasma PTHrP concentration, hist opathology, expression of PTHrP protein by radioimmunoassay and immunohisto chemistry, and expression of PTHrP mRNA by in situ hybridization and northe rn blot analysis. Messenger RNA expression for other factors and cytokines known to alter PTHrP secretion or bone resorption in vivo, including tumor necrosis factor alpha (TNF alpha), interleukin (IL)-1, IL-6, and transformi ng growth factor beta (TGF beta), were also measured in the adenocarcinomas . There was no significant difference in weight of individual tumors. Nude mice bearing the CAC-8 (Lo Ca) tumor maintained normal body weight as compa red with non-tumor-bearing control mice. In contrast, mice with the CAC-8 ( Hi Ca) tumor had markedly decreased body weights. The CAC-8 (Hi Ca) tumor-b earing mice had severe hypercalcemia ((x) over bar = 13.4 mg/dl) and increa sed plasma concentrations of PTHrP (30.4 pM), whereas the CAC-8 (Lo Ca) tum or-bearing mice had a mean serum calcium concentration of 10.1 mg/dl and mi ldly increased PTHrP concentrations (5.7 pM) as compared with control mice (9.0 mg/dl and 1.0 pM, respectively). The original tumor (CAC-8 [Hi Ca]) is a well-differentiated adenocarcinoma, whereas the variant tumor (CAC-8 [Lo Ca]) is a solid carcinoma with both polygonal and spindle-shaped cells. Th e CAC-8 (Lo Ca) tumor had decreased PTHrP mRNA expression and protein synth esis. Messenger RNA expression of TGF beta, TNF alpha, IL-1, and IL-6 was s imilar in both tumors and was consistent with the central role of PTHrP in the induction of hypercalcemia in this animal model.