Frequency and partial characterization of adenylate cyclase-stimulating activity in tumors associated with humoral hypercalcemia of malignancy

Could use of Selective Serotonin Reuptake Inhibitors During Lactation Cause Persistent Effects on Maternal Bone?

The lactating mammary gland elegantly coordinates maternal homeostasis to provide calcium for milk. During lactation, the monoamine serotonin regulates the synthesis and release of various mammary gland-derived factors, such as parathyroid hormone-related protein (PTHrP), to stimulate bone resorption. Recent evidence suggests that bone mineral lost during prolonged lactation is not fully recovered following weaning, possibly putting women at increased risk of fracture or osteoporosis. Selective Serotonin Reuptake Inhibitor (SSRI) antidepressants have also been associated with reduced bone mineral density and increased fracture risk. Therefore, SSRI exposure while breastfeeding may exacerbate lactational bone loss, compromising long-term bone health. Through an examination of serotonin and calcium homeostasis during lactation, lactational bone turnover and post-weaning recovery of bone mineral, and the effect of peripartum depression and SSRI on the mammary gland and bone, this review will discuss the hypothesis that peripartum SSRI exposure causes persistent reductions in bone mineral density through mammary-derived PTHrP signaling with bone.

Colony stimulating factor-1 plays a role in osteoclast formation and function in bone resorption induced by parathyroid hormone and parathyroid hormone-related protein

Although colony stimulating factor-1 (CSF-1) plays a key role in osteoclast recruitment, studies examining the effect of CSF-1 on mature osteoclasts indicate that it may directly inhibit bone resorption by isolated rat osteoclasts. To define further CSF-1's role in bone remodeling, we examined the effect of neutralizing antisera to CSF-1 on basal and parathyroid hormone (PTH)-induced bone resorption using two organ culture assays designed to examine the recruitment of osteoclast precursors and the activation of mature osteoclasts, respectively. We first assessed whether PTH increases CSF-1 production from bone in organ culture by examining conditioned medium from 19-day-old fetal rat long bones in a mitogenesis assay employing a CSF-1-responsive cell line, CRX-1. Conditioned medium from untreated bones induced a titratable increase in CRX-1 cell proliferation, and treatment of bones with PTH for 72 h caused a significant increase in mitogenic activity. CSF-1 antiserum caused a significant decrease in mitogenic activity in conditioned medium, indicating that bone in organ culture produces CSF-1 constitutively and in response to PTH. To examine bone-derived CSF-1's role in bone resorption, we examined the effect of neutralizing antisera to CSF-1 on basal and PTH-induced bone resorption in the fetal rat long bone assay, which reflects activation of mature osteoclasts. Anti-CSF-1 caused a significant increase in unstimulated and PTH-induced bone resorption compared with control. By contrast, in the fetal mouse metacarpal assay, which examines proliferation and recruitment of osteoclast progenitors and precursors, anti-CSF-1 caused significant inhibition of PTH related protein (PTHrP)-induced bone resorption after 3 and 6 days of incubation. Consistent with these findings, histological examination of cultured 17-day-old fetal metacarpals demonstrated that anti-CSF-1 inhibits the formation of tartrate-resistant acid phosphatase-positive osteoclasts in PTHrP-treated explants, whereas it has no effect on unstimulated bones. We conclude that bone-derived CSF-1 may have a dual role in PTH/PTHrP-induced bone resorption by enhancing the appearance of osteoclast precursors while restraining the resorptive function of mature osteoclasts.

Overexpression of parathyroid hormone-related protein in the pancreatic islets of transgenic mice causes islet hyperplasia, hyperinsulinemia, and hypoglycemia

Parathyroid hormone-related protein (PTHrP) is produced by the pancreatic islet. It also has receptors on islet cells, suggesting that it may serve a paracrine or autocrine role within the islet. We have developed transgenic mice, which overexpress PTHrP in the islet through the use of the rat insulin II promoter (RIP). Glucose homeostasis in these mice is markedly abnormal; RIP-PTHrP mice are hypoglycemic in the postprandial and fasting states and display inappropriate hyperinsulinemia. At the end of a 24-hour fast, blood glucose values are 49 mg/dl in RIP-PTHrP mice, as compared to 77 mg/dl in normal littermates; insulin concentrations at this time are 6.3 and 3.9 ng/ml, respectively. Islet perifusion studies failed to demonstrate abnormalities in insulin secretion. In contrast, quantitative islet histomorphometry demonstrates that the total islet number and total islet mass are 2-fold higher in RIP-PTHrP mice than in their normal littermates. PTHrP very likely plays a normal physiologic role within the pancreatic islet. This role is most likely paracrine or autocrine. PTHrP appears to regulate insulin secretion either directly or indirectly, through developmental or growth effects on islet mass. PTHrP may have a role as an agent that enhances islet mass and/or enhances insulin secretion.

Rhabdoid tumor of the kidney with humoral hypercalcemia and parathyroid hormone-related protein production

This is a report of a 7-month-old infant with malignant rhabdoid tumor of the kidney (RTK). The patient (pt) demonstrated clinical and biochemical evidence of humoral hypercalcemia of malignancy (HHM). The hypercalcemia responded promptly to calcitonin treatment and tumor removal. Despite aggressive surgery and chemotherapy, the patient expired four months after diagnosis. The primary tumor displayed adenylate cyclase-stimulating activity (ACSA) indicating the production of parathyroid hormone-related protein (PTHrP) by the primary tumor. This is the first report of ACSA documented in a pt with RTK.

Impaired parathyroid hormone release in human immunodeficiency virus infection

Patients with human immunodeficiency virus type 1 (HIV-1) seropositivity exhibited significantly lower intact serum parathyroid hormone (PTH) values (mean, 13.6 ng/liter; n = 44) than healthy controls (mean, 38.1 ng/liter; p < 0.001; n = 50). The reduction was greater among patients with no or mild immunodeficiency (> 400 x 10(6) CD4+ lymphocytes/ml blood; n = 22) than in those with severe immunodeficiency (< 200 x 10(6) CD4+ lymphocytes/ml blood; n 22; p = 0.03), although total serum calcium was normal in all groups. Patients with severe immunodeficiency demonstrated an inverse correlation between total serum calcium and serum PTH (r2 = 0.367; p < 0.01), which was also present in healthy controls (r2 = 0.482; p < 0.001), but not among the seropositive patients with no or mild immunodeficiency (r2 = 0.017; p = 0.58). Parathyroid cells express a protein recognized by antibodies directed against CD4, the HIV-1 receptor. This implies that these cells may be directly infected with HIV-1 and also interact with circulating autoantibodies against CD4, thus resulting in impaired PTH release.

Endotoxin increases parathyroid hormone-related protein mRNA levels in mouse spleen. Mediation by tumor necrosis factor

Parathyroid hormone-related protein (PTHrP) causes hypercalcemia in malignancy. However, the role and regulation of PTHrP in normal physiology is just beginning to be explored. PTHrP is found in the spleen and has several other features common to cytokines. Since endotoxin (LPS) causes many of its effects indirectly by inducing cytokines, studies were undertaken to determine whether LPS might also induce splenic PTHrP expression. LPS (100 ng/mouse) increased splenic PTHrP mRNA levels 3.6-fold in C3H/OuJ mice. This effect was maximal at 2 h and returned to baseline by 4 h. PTHrP peptide levels also increased 3.3-fold in splenic extracts in response to LPS (1 microgram/mouse). Murine TNF-alpha and human IL-1 beta, cytokines that mediate many of the effects of LPS, also increased splenic PTHrP mRNA levels. LPS-resistant C3H/HeJ mice, which produce minimal amounts of TNF and IL-1 in response to LPS, were resistant to LPS induction of splenic PTHrP mRNA, while TNF-alpha and IL-1 beta readily increased PTHrP mRNA levels in C3H/HeJ mice. Anti-TNF antibody blocked LPS induction of splenic PTHrP mRNA in C3H/OuJ mice by 68%, indicating that TNF is a mediator of the LPS induction of PTHrP levels. In contrast, an IL-1 receptor antagonist (IL-1ra) was ineffective. The increase in PTHrP in the spleen during the immune response suggests that PTHrP may play an important role in immune modulation, perhaps by mediating changes in lymphocyte proliferation and/or function.

Production of parathyroid hormone and parathyroid-hormone-related protein by breast cancer cells in culture

Parathyroid-hormone-related protein (PTHrP) has been implicated in the origin of malignant hypercalcaemia. However, PTHrP production is not restricted to neoplastic cells, it is widespread among a variety of normal cell types and tissues. A physiological role for PTHrP has not been well defined. We describe a case of breast cancer with bone metastases and humoral hypercalcaemia of malignancy, with high levels of plasma C-terminal parathyroid hormone (PTH), mid-molecule PTH and PTHrP. Cells from breast cancer biopsies were cultured and medium samples assayed for the C-terminal and mid-molecule fragments, intact PTH and PTHrP. The data indicate a progressive increase in both PTH fragments and PTHrP levels, over a period of 30 days. No temporal parallelism exists between PTH fragments and PTHrP concentrations, the former being maximum at the 14th day, and the latter at the 30th day from the beginning of the culture. Our results indicate a coproduction of PTH and PTHrP by the breast cancer cells both in vivo and in vitro.

Co-purification of calcium transport-stimulating and DNA synthesis-stimulating agents with parathormone-like activity isolated from the hypercalcaemic strain of the Walker 256 tumour

One of the strains of the Walker 256 carcinosarcoma induces in the rat a humoral hypercalcaemia of malignancy (HHM) syndrome which is similar to that reported in human patients. We have isolated from this tumour a chromatographic fraction which displays an adenylate cyclase stimulating activity in dog kidney cortical membranes, similar to that of a parathormone (PTH) related protein isolated from various HHM related tumours. In addition, this fraction stimulated initial calcium (Ca) uptake in confluent Madin-Darby canine kidney (MDCK) cell monolayers in a dose-dependent manner. Maximal stimulation of Ca uptake was associated with an enhanced Ca efflux from MDCK cells preloaded with the cation, and with an increased DNA synthesis in these cells. These activities might be involved in development of increased tubular calcium reabsorption in Walker 256 tumour-bearing rats.

Parathyroid hormone-related protein and hypercalcemia in pancreatic neuro-endocrine tumors

We investigated the possible involvement of parathyroid hormone-related protein (PTHrP) in 2 cases of metastatic pancreatic neuro-endocrine tumors associated with severe hypercalcemia. Both patients displayed biochemical alterations in renal tubular reabsorption of calcium and phosphate, as well as in urinary cAMP excretion, similar to those encountered in primary hyperparathyroidism, although plasma levels of parathyroid hormone were within the normal range. Tumor protein extracts stimulated cAMP production, which was inhibited by the PTH-antagonist (8,18 Nle, 34 Tyr)bPTH-(3-34)amide, in the PTH-responsive osteoblastic cell line UMR-106. Northern blot analysis of tumor extracts revealed the presence of PTHrP mRNA transcripts, while PTH mRNA was undetectable. In contrast, neither PTHrP mRNA(s) nor cAMP-stimulating activity was detectable in other neuroendocrine tumors not accompanied by hypercalcemia. These results demonstrate that certain pancreatic neuroendocrine tumors associated with hypercalcemia can synthesize and release PTHrP.

Immunochemical characterization of circulating parathyroid hormone-related protein in patients with humoral hypercalcemia of cancer

Tumors from patients with humoral hypercalcemia of cancer produce a parathyroid hormone-related protein (PTHRP). We have developed two region-specific immunoassays capable of measuring PTHRP in plasma: an immunoradiometric assay directed toward PTHRP amino acid sequence 1 to 74 and a radioimmunoassay directed toward PTHRP amino acid sequence 109 to 138. Sixty normal subjects had low or undetectable plasma PTHRP (1 to 74) concentrations (mean, 1.9 pmol per liter) and undetectable PTHRP (109 to 138) concentrations (less than 2.0 pmol per liter). Patients with humoral hypercalcemia of cancer (n = 30) had elevated levels of both PTHRP (1 to 74) (mean, 20.9 pmol per liter) and PTHRP (109 to 138) (mean, 23.9 pmol per liter). The plasma concentrations of immunoreactive PTHRP correlated with the levels of urinary cyclic AMP excreted; in some patients, the concentrations decreased after the tumors were resected. Patients with chronic renal failure (n = 15) had plasma PTHRP (1 to 74) concentrations similar to those in the normal subjects, but their plasma PTHRP (109 to 138) concentrations were elevated (mean, 29.6 pmol per liter). The levels of both peptides were normal in patients with hyperparathyroidism and those with hypercalcemia due to various other causes. Breast milk contained high concentrations of PTHRP. An anti-PTHRP (1 to 36) immunoaffinity column failed to extract PTHRP (109 to 138) immunoactivity from plasma, suggesting that the C-terminal region circulates as a separate peptide. We conclude that plasma PTHRP concentrations are high in the majority of patients with cancer-associated hypercalcemia and that the circulating forms of PTHRP in such patients include both a large N-terminal (1 to 74) peptide and a C-terminal (109 to 138) peptide. Measuring the concentrations of PTHRPs may be useful in the differential diagnosis of hypercalcemia.

Renal vasodilatation and microvessel adenylate cyclase stimulation by synthetic parathyroid hormone-like protein fragments

The hypercalcemia caused by malignancy factor, also called parathyroid hormone-related protein (PTHrP), exhibits most of the biological activities of parathyroid hormone (PTH) in kidney and bone. On the basis of the well-documented vascular action of PTH, we characterized the vasodilator action of human (h) PTHrP-(1-34) on a preparation of the isolated rat kidney, and its activity to stimulate adenylate cyclase in microvessels isolated from rabbit kidney cortex. Injection of sequential cumulative doses of hPTHrP-(1-34) into the isolated kidney preparation produced increasing vasodilatation up to 10(-8) M (EC50 of 3 x 10(-9) M) and decreasing responses thereafter. The maximal effect represented 26% of the reference relaxation induced by papaverine. Single injections of hPTHrP-(1-34) resulted in a greater (over 60%) vasodilatation. These results were reminiscent of the tachyphylaxis that occurs after repeated exposure to the peptide. The (3-34) PTH antagonist inhibited the hPTHrP-induced vasodilatation. Human PTHrP-(1-34) was equipotent with hPTH-(1-34) (EC50 values of 3 x 10(-9) M) but 5-fold less potent than rat (r) PTH-(1-34) in stimulating microvessel adenylate cyclase. GTP enhanced the enzyme responses to the peptides but reduced their potency. Both (3-34) and (7-34) PTH antagonists were inhibitors of hPTHrP- or PTH-stimulated microvascular adenylate cyclase. Synthetic hPTHrP-(1-16) had neither vasodilator nor adenylate cyclase-stimulating activity. This hPTHrP fragment exhibited some inhibitory effect on the hPTHrP-(1-34)-induced stimulation of microvessel adenylate cyclase. These results indicate that hPTHrP possesses PTH-like activity to cause vasorelaxation and to stimulate microvascular adenylate cyclase in the kidney.

Hypercalcaemia of malignancy

Hypercalcaemia in malignancy is a major clinical problem. It contributes significantly to morbidity and mortality and can present difficult diagnostic and management dilemmas. Direct bony invasion by tumour cells rather than humorally mediated hypercalcaemia is probably the most common cause of malignant hypercalcaemia. Yet even in this situation the mechanism of bone resorption or the reason that the normal homeostatic mechanisms cannot cope with the calcium load are poorly understood. It is likely that the humoral and paracrine factors produced by tumours which result in hypercalcaemia or in osteosclerotic bone metastases, are interposing themselves into the normal regulatory processes and deranging them. Humoral hypercalcaemia of malignancy is an important model for studying these questions, and it also provides some insight into the normal regulation of bone turnover. This review will examine the animal models and human syndromes of malignant hypercalcaemia and show how animal models, although helpful, fail to delineate the relative importance of the various potential humoral factors. A most interesting recent development in this area is the description of a new hormone, the parathyroid hormone-related peptide, which may explain many of the cases of humoral hypercalcaemia of malignancy. It is also a useful model with multiple sites of action within the bone and calcium homeostatic process. The active hormonal form of vitamin D3, 1,25-dihydroxyvitamin D3, may also be involved in a small proportion of cases, but again it is a useful model of some of the factors that may operate. Of considerable interest are the tumour derived factors, such as the transforming growth factors, and the cytokines, such as tumour necrosis factors, interleukins, and haemopoietic colony stimulating factors. Prostanoids are seldom of major importance, but may be important in certain tumour types. Osteosclerotic metastases, although seldom associated with hypercalcaemia, may provide insight into osteoblast regulating factors. Treatment of hypercalcaemia is discussed to show ways in which response to treatment may shed light on underlying pathophysiological mechanisms. Most effective treatments have many potential modes of action, and further study of the interactions of these agents and tumour types may help to unravel some of the enigmas in this human syndrome. The major advances in this complex problem involve the realisation of the necessity of multiple sites of action, including renal calcium handling as well as relative increases in bone resorption and/or intestinal calcium absorption.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenylate cyclase-stimulating, bone-resorbing and B TGF-like activities in canine apocrine cell adenocarcinoma of the anal sac

Canine apocrine cell adenocarcinoma of the anal sac (APO-AS) is a spontaneously occurring tumor that causes humorally mediated hypercalcemia in 90% of cases. To further define the nature of the responsible mediator in APO-AS, we examined tumor extracts from five APO-AS and four control tumors for adenylate cyclase-stimulating activity (ACSA). All extracts from APO-AS contained potent ACSA, whereas the four control tumors did not. The ACSA extracted from one tumor demonstrated a dose response curve parallel to that of synthetic bovinePTH-(1-34) and was 80% inhibited by Nle8,18,Tyr34 bPTH-(3-34)amide at a concentration of 10(-5) M. Extracts from three APO-AS and three control tumors were also examined for in vitro bone-resorbing activity (BRA). All APO-AS contained significant BRA, stimulating resorption 1.47 to 2.13-fold over basal, whereas none of the control tumors stimulated resorption. Purification of one extract using C18 reverse-phase high pressure liquid chromatography (RP-HPLC) resulted in a single sharp peak of ACSA which was 400-fold purified compared with the initial extract. This pool also contained significant bone-resorbing activity, whereas none of the adjacent pools did. Purification of a second extract using sequential CN and C18 RP-HPLC followed by size exclusion HPLC resulted in material that was at least 10,000-fold purified, and showed co-purification of ACSA and B TGF-like activity.

A hypercalcemic nude rat model that completely mimics human syndrome of humoral hypercalcemia of malignancy

Tumors causing humoral hypercalcemia of malignancy (HHM) were implanted to athymic nude rats. In one of these rat models transplanted with uterine cancer (UCC), a complete reproduction of human HHM syndrome was achieved: hypercalcemia, hypophosphatemia with increased urinary phosphate and cyclic AMP excretion, and suppressed serum 1,25-dihydroxy-vitamin D (1,25(OH)2D) level. In another hypercalcemic nude rat model implanted with oral cavity cancer (OCC), all the features were similar except for markedly elevated serum 1,25(OH)2D. Hypercalcemia disappeared by surgical removal of the tumors in both models, confirming the humoral mechanisms for causing these features. Furthermore, in UCC tumor-bearing rats, hypophosphatemia, increased renal phosphate excretion, and reduced serum 1,25(OH)2D concentration were already present when these rats were only marginally hypercalcemic. These results raise the possibility that the changes in renal tubular phosphate handling and vitamin D metabolism in HHM are not secondary to hypercalcemia but are due to direct effects of the humoral factor(s) that cause this syndrome. Extracts of both tumors exhibited stimulation of cyclic AMP production in osteoblastlike cells, UMR 106, which could be almost completely inhibited by parathyroid hormone (PTH) antagonist, human PTH(3-34). By comparing the nature and characteristics of humoral factor(s) from UCC and OCC models, mechanisms responsible for causing these abnormalities can be explored. Thus, these nude rat models can be useful for elucidating the underlying mechanism of the development of HHM.

Identification of transcripts encoding a parathyroid hormone-like peptide in messenger RNAs from a variety of human and animal tumors associated with humoral hypercalcemia of malignancy

The syndrome of humoral hypercalcemia of malignancy (HHM) appears to be mediated in many instances by a parathyroid hormone-like peptide, which has recently been purified, sequenced, and cloned. Using a probe representing the coding region of the human PTH-like peptide, we examined by Northern analysis poly (A)+ RNA from a variety of human and animal tumors associated with HHM. Hybridizing transcripts were identified in mRNA from each of 12 human and each of four animal HHM-associated tumors, with a complex hybridization pattern observed in the human mRNAs and a relatively simple pattern observed in the animal mRNAs. Poly (A)+ RNA prepared from tumors of similar histological types unassociated with HHM failed to hybridize with the probe. Messenger RNA-dependent biological activity from the animal tumors was entirely eliminated in a hybridization-arrest experiment using a complementary oligonucleotide spanning the region of homology between human PTH and the PTH-like peptide. These findings indicate that the PTH-like peptide is associated with the syndrome of HHM in a wide spectrum of tumor types from a variety of mammalian species and that the PTH-like sequence in the proximal amino terminus of the peptide is highly conserved.

In vitro adenylate cyclase-stimulating activity predicts the occurrence of humoral hypercalcemia of malignancy in nude mice

A number of factors have been proposed as potential mediators of the syndrome of humoral hypercalcemia of malignancy (HHM), but to date no firm cause-and-effect relationship has been established. We attempted to establish such a relationship by determining whether the presence or absence of adenylate cyclase-stimulating activity (ACSA) in the media of cultured tumor cells predicted the occurrence of the syndrome of HHM when these cell lines were grown in nude mice in vivo. Conditioned media from 35 human renal carcinoma cell lines were surveyed for ACSA in the PTH-sensitive rat osteosarcoma 17/2.8 cell assay. 12 lines were positive (mean, 13.7-fold stimulation, range, 3.0 to 44.0), and 23 lines were negative (mean, 1.2-fold stimulation, range, 0.9 to 1.5). We were successful in establishing five of the positive and six of the negative lines in three to five nude mice per line. Mice implanted with the positive lines uniformly became hypercalcemic (mean serum calcium, 15.8 mg/dl), whereas mice implanted with the negative lines uniformly remained normocalcemic (mean serum calcium, 9.5 mg/dl), in spite of comparable mean tumor size. Acid-urea tumor extracts from each of four hypercalcemic animals contained potent in vitro ACSA (mean, 15.9-fold stimulation), while 5/5 extracts from normocalcemic animals did not (mean, 1.4-fold stimulation). Our study demonstrates that in this model system in vitro ACSA is a reliable predictive marker for HHM in vivo. Whether the protein responsible for this activity is also the mediator of the bone resorption seen in HHM remains to be demonstrated.

Pathogenesis of humoral hypercalcemia of malignancy

The three biologic activities most commonly associated with tumors that produce Humoral Hypercalcemia of Malignancy (HHM) include; 1) adenylate cyclase stimulating activity (PTH-like activity), 2) in vitro bone resorbing activity, and 3) transforming growth factor activity. The canine adenocarcinoma (CAC-8) model of HHM contains all three activities and the first two are inhibited by a PTH receptor antagonist. These data in light of the recent purification of PTH-related peptides from human tumors suggest that CAC-8 produces a PTH-related protein that is important in the pathogenesis of hypercalcemia. The CAC-8 tumor is a well characterized example of HHM and offers several advantages for further investigations on the pathogenesis of HHM: 1) transplantable tumor line from a spontaneous neoplasm in the dog, 2) tumor extracts contain the three biologic activities associated with HHM, 3) slow progressive growth rate in nude mice permits investigations on treatment of HHM, 4) increased bone resorption and formation in nude mice mimics the effects of PTH on bone, and 5) the only model of HHM that has been demonstrated to contain bone resorbing activity that can be inhibited by a PTH receptor antagonist.

Two forms of parathyroid hormone-like adenylate cyclase-stimulating protein derived from tumors associated with humoral hypercalcemia of malignancy

Tumors associated with humoral hypercalcemia of malignancy (HHM) contain parathyroid hormone-like adenylate cyclase-stimulating proteins (hACSPs). We previously purified a 17,000 MW hACSP from an HHM-associated breast carcinoma. This report describes the characterization of hACSPs from three additional HHM-associated tumors: two typical HHM-associated tumors (squamous carcinomas) and a third unusual tumor type (pheochromocytoma). Each tumor was extracted in acid-urea/ethanol-sodium chloride, and adenylate cyclase-stimulating activity (ACSA) was examined following reverse-phase and size-exclusion HPLC and isoelectric focusing. Each tumor contained a high molecular weight form of ACSA which co-eluted with the 17,000 molecular weight breast carcinoma hACSP in each of the three separation procedures. Each also contained a lower molecular weight form(s) of ACSA (6,000-9,000 molecular weight). Both forms were inhibited by Nle8,18Tyr34bPTH(3-34) amide. The high molecular weight form was not changed to the lower molecular weight form by a reducing agent. Some HHM-associated tumors contain two forms of hACSP, one with a molecular weight of 17,000 and another with a molecular weight of 6,000-9,000, which appears to be an amino-terminal cleavage product of the larger protein.

Parathyroid hormonelike protein from human renal carcinoma cells. Structural and functional homology with parathyroid hormone

A variety of solid tumors secrete proteins that are immunochemically distinct from parathyroid hormone (PTH) but activate PTH-responsive adenylate cyclase. Such PTH-like proteins have been proposed as mediators of the hypercalcemia and hypophosphatemia frequently associated with malignancies. We purified to apparent homogeneity a PTH-like protein with a molecular weight of 6,000, that is produced by human renal carcinoma cells. The amino-terminal sequence of the PTH-like protein and that of human PTH were found to display at least five identities in the first 13 positions. The purified protein bound to PTH receptors, activated adenylate cyclase in renal plasma membranes, and stimulated cAMP formation in rat osteosarcoma cells. The PTH-like protein reproduced two additional effects of PTH, stimulation of bone resorption in fetal rat limb bone cultures and inhibition of phosphate uptake in cultured opossum kidney cells. These properties are consistent with a role for PTH-like proteins as mediators of the syndrome of malignancy-associated hypercalcemia.

The effect of low calcium diet, mithramycin, and dichlorodimethylene bisphosphonate on humoral hypercalcemia of malignancy in nude mice transplanted with the canine adenocarcinoma tumor line (CAC-8)

The effect of a low calcium diet, mithramycin, or dichlorodimethylene bisphosphonate were evaluated in nude mice with humoral hypercalcemia of malignancy associated with the transplanted canine adenocarcinoma (CAC-8). Low calcium (0.01%) diet significantly reduced serum calcium levels in hypercalcemic nude mice and reduced urine calcium excretion to control levels. Mithramycin (8 mg/kg) decreased serum calcium concentration and urine calcium excretion to the range of control non-tumor-bearing nude mice at day 5 after a single injection, but there was no change in the number of tartrate-resistant acid phosphatase-positive osteoclasts in lumbar vertebrae. Osteoclasts from CAC 8-bearing nude mice after mithramycin administration were decreased in size, had small ruffled borders, and increased relative size of clear zones. Dichlorodimethylene bisphosphonate (Cl2MDP) (45 mg/kg) partially reduced serum calcium concentration of hypercalcemic tumor-bearing nude mice, decreased urine calcium excretion to control levels, and markedly reduced the numbers of tartrate-resistant acid phosphatase-positive osteoclasts in lumbar vertebrae. Osteoclasts from Cl2MDP-treated nude mice were smaller and had a reduced frequency of ruffled borders than saline-treated hypercalcemic nude mice. In vitro bone resorption induced by CAC-8 extract was significantly reduced by Cl2MDP and mithramycin. The results of these investigations suggest that the hypercalcemia and hypercalciuria associated with HHM in nude mice with CAC-8 are the combined result of altered calcium homeostasis in the bone, kidney, and intestine. Chemotherapeutic agents that specifically affect only bone or feeding a low calcium diet alone may not completely ameliorate the hypercalcemia of HHM.

Hypercalcemia in breast cancer. Reassessment of the mechanism

Hypercalcemia in patients with breast cancer is usually attributed to osteolytic bone metastases. Seventeen patients with biopsy-proved breast cancer and hypercalcemia were identified in a prospective, unselected manner. Biochemical and clinical evaluation included measurements of parathyroid hormone, nephrogenous cAMP, vitamin D metabolites, fasting calcium excretion, and maximal tubular phosphate reabsorption, and bone radionuclide scanning. Tumor histologic findings were also reviewed. Four of the 17 patients (23.5 percent) had no evidence of bone involvement by bone scanning or radiography. Two additional patients (a total of 35 percent) appeared to have a humoral component to their hypercalcemia as determined by the presence of elevated nephrogenous cAMP excretion. These observations suggest that humoral, tumor-derived products may play a more important role in the hypercalcemia of breast cancer than has been previously recognized.

The relative potency of a human tumor-derived PTH-like adenylate cyclase-stimulating preparation in three bioassays

Three bioassays are widely employed for the measurement of PTH-like adenylate cyclase-stimulating factors (ACSFs) derived from tumors associated with humoral hypercalcemia of malignancy. These include renal cortical adenylate cyclase (RAC) assays, rat osteosarcoma (ROS) adenylate cyclase assays, and fetal bone resorption (FBR) assays. A previous study has suggested that the potency of one human tumor-derived ACSF, expressed in PTH equivalents, was 30-fold higher in the ROS assay than in the RAC assay, but no study has directly compared all three bioassays using a single PTH standard and a single ACSF preparation. We compared one partially purified ACSF preparation to a single lot of bPTH 1-34 in all three bioassays. The results indicate that the relative potency of this ACSF as compared to the PTH standard varied with the assay employed, with the ROS assay yielding a specific activity estimate 47.5-fold higher than the RAC, and the FBR 6.7-fold higher than the RAC but 7.1-fold lower than the ROS. These findings support the possibility that distinct subpopulations of PTH receptors exist on different PTH target tissues. Further, they underscore the importance of bioassay choice when estimating the specific activity of tumor-derived ACSF preparations.