Incidence and Outcomes of Pituitary Microadenomas in Children with Short Stature/Growth Hormone Deficiency

BACKGROUND/AIMS

Patients with short stature (SS)/growth hormone deficiency (GHD) and precocious puberty (PP) undergo brain MRI to evaluate for structural brain abnormalities or pituitary lesions, and pituitary microadenomas are a common finding. Theoretically, a mass effect from these lesions could cause GHD and growth hormone treatment could cause them to enlarge, but they should not cause PP, at least in females.

METHODS

We investigated if pituitary microadenomas cause GHD by comparing their incidence in patients with SS/GHD to that in females with PP. We performed a retrospective chart review of patients with these disorders who had a brain MRI between 2000 and 2013.

RESULTS

The incidence of microadenoma was high in both groups, 18.5% for SS (n = 346) and 21.1% for PP females (n = 194), but did not differ between groups (p = 0.46). In patients with microadenomas, repeat imaging showed resolution in 58% (SS, n = 33) and 67% (PP females, n = 21). Importantly, none of the lesions grew, even in patients treated with growth hormone.

CONCLUSIONS

Pituitary microadenomas are common in children with GHD/SS and PP, but it does not appear that they are a cause of GHD. They appear to be of limited clinical significance and should not be considered a contraindication to growth hormone therapy.

Abstract 4184: Novel parathyroid hormone antagonist prevents breast cancer bone metastasis and cortical bone destruction

The majority of patients (~70%) with advanced breast cancer will develop bone metastases and suffer from severe pain, bone fracture and eventually death. Current palliative treatments have only limited efficacy highlighting an urgent need for development of targeted agents. Bone metastatic breast cancer cells secrete parathyroid hormone related peptide (PTHrP) that promotes autocrine tumor growth, induces bone turnover that releases additional tumor stimulatory factors, and creates cavities in the bone that permit tumor outgrowth. Previous PTH/PTHrP antagonists failed clinically due to short half-life and inadequate targeting to bone. We created a novel bone-targeted PTH antagonist drug, PTH(7-33)-CBD, by fusing an N-terminally truncated analog of PTH (aa 7-33) with the collagen binding domain (CBD) from ColH collagenase (Clostridium histolyticum). Our previous preliminary study demonstrated in vivo efficacy of PTH(7-33)-CBD against tumor burden and cortical bone destruction using an established model of breast cancer bone metastasis by injecting a bone-trophic variant of estrogen receptor-negative MDA-MB-231 breast cancer cells expressing luciferase (MDA-MB-231-BM/luc+) into the tibia marrow of nude mice. PTH(7-33)-CBD treatment did not result in hypocalcemia or reduce animal body weight. We extended this previous study by using increased animal numbers, administration of a control drug, and assessment of apoptosis of breast cancer cell in vitro by PTH(7-33)-CBD. PTH(7-33)-CBD reduced PTH agonist-stimulated cAMP accumulation in SaOS-2 osteosarcoma cells by 71% confirming the PTH antagonist activity of the compound. PTH(7-33)-CBD exhibited a direct cytotoxic effect towards MDA-MB-231 breast cancer cells in vitro by increasing apoptosis as assessed by the Caspase Glow Assay. To assess in vivo efficacy, on Day 0 nude mice were treated IP with either vehicle, 1000 µg/kg control drug PTH-7-34 (without CBD domain), or 1000 µg/kg PTH(7-33)-CBD (N=8/treatment group). On Day 1 mice were injected with 2x106 MDA-MB-231-BM/luc+ cells into the tibia marrow and tumor burden and bone density were assessed weekly by bioluminescence imaging and X-ray imaging, respectively. PTH(7-33)-CBD significantly reduced tumor burden in bone from weeks 4-8 post-tumor cell inoculation compared to vehicle (P<0.05), and from weeks 6-7 compared to PTH-7-34 control drug (P<0.05). PTH(7-33)-CBD reduced cortical bone destruction from weeks 3-7 weeks compared to both vehicle and PTH-7-34 control drug (P<0.05). Taken together, these data demonstrate that PTH(7-33)-CBD reduces both bone metastatic breast tumor burden and osteolytic lesions in nude mice, and induces apoptosis of MDA-MB-231 breast cancer cells in vitro.

Support: RCG and BGR are co-principal investigators for Breast Cancer Research Program (BCRP) Breakthrough Award, BC150685.

Citation Format: Murali Anbalagan, Tulasi Ponnapakkam, Yibin Kang, Robert C. Gensure, Brian G. Rowan. Novel parathyroid hormone antagonist prevents breast cancer bone metastasis and cortical bone destruction [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4184.

Therapy for Alopecia Areata in Mice by Stimulating the Hair Cycle with Parathyroid Hormone Agonists Linked to a Collagen-Binding Domain

Alopecia areata is a common disorder in which autoimmune destruction of hair follicles results in patchy hair loss. Currently there is no adequate therapy, although immune modulator therapies are currently in development. Parathyroid hormone (PTH) is a hair cycle stimulator which shows promise in treating various forms of alopecia, although its short half-life limits its clinical use. PTH-CBD is a PTH analog which binds collagen, prolonging retention in skin. We tested effects of PTH-CBD in C3H/HeJ-engrafted mice, the animal model for alopecia areata, on hair growth and found that a significant proportion of animals had reduced hair loss (PTH-CBD: 13/21, 62% vs.

CONTROL

3/10, 30%; P<0.01). Histological analysis showed no change in immune response, but there was increased number of anagen hair follicles and increased production of beta-catenin, a factor which initiates the anagen phase of the hair cycle. PTH-CBD thus shows promise as a therapy for alopecia areata, either alone or in conjunction with immune modulation therapy.

Clinical response to combined therapy of cyclosporine and prednisone

Reported is a patient with severe alopecia areata, multiple autoimmune diseases (chronic lymphocytic thyroidis, primary ovarian failure), and Down syndrome. She had a poor response to topical treatment with glucocorticoids and minoxidil, but showed some improvement with glucocorticoid injections. At the time of evaluation, she had hair loss on 85-90% of her scalp. She was treated initially with oral prednisone 50 mg per day for 2 weeks, followed by a 3-month course of prednisone 10 mg per day and cyclosporine 125 mg (4 mg kg(-1)) two times per day. She responded well with excellent regrowth of hair on the scalp, and prednisone was tapered and ultimately discontinued. Importantly, her parents noted marked improvement in sense of well-being. Several months after discontinuing treatment, she developed hyperpigmentation on the trunk consistent with confluent and reticulated papillomatosis; she has several known risk factors for this disorder, but it is not clear if this is related to her previous treatment.

Treating osteoporosis by targeting parathyroid hormone to bone

Osteoporosis is a major public health problem despite widespread use of bisphosphonate therapy. PTH(1-34) is a more effective treatment; but its use has been limited by side effects (hypercalcemia, tumor risk) and inconvenient dosing (daily injection). Long-acting forms of PTH are also effective but cause severe hypercalcemia, presumably from effects in kidney. We hypothesized that targeted delivery of PTH to bone using a collagen binding domain (PTH-CBD) could reduce hypercalcemia. PTH-CBD is cleared from serum within 12hours after subcutaneous administration. In ovariectomized rats, monthly administration of PTH-CBD increased spinal BMD by 14.2% with no associated hypercalcemia. Such bone-targeted anabolic agents may ultimately allow the superior efficacy of anabolic therapy to be obtained with the dosing convenience of bisphosphonates.

A single injection of the anabolic bone agent, parathyroid hormone-collagen binding domain (PTH-CBD), results in sustained increases in bone mineral density for up to 12 months in normal female mice

Parathyroid hormone (PTH) is the most effective osteoporosis treatment, but it is only effective if administered by daily injections. We fused PTH(1-33) to a collagen binding domain (PTH-CBD) to extend its activity, and have shown an anabolic bone effect with monthly dosing. We tested the duration of action of this compound with different routes of administration. Normal young C57BL/6J mice received a single intraperitoneal injection of PTH-CBD (320 μg/kg). PTH-CBD treated mice showed a 22.2 % increase in bone mineral density (BMD) at 6 months and 12.8 % increase at 12 months. When administered by subcutaneous injection, PTH-CBD again caused increases in BMD, 15.2 % at 6 months and 14.3 % at 12 months. Radiolabeled PTH-CBD was concentrated in bone and skin after either route of administration. We further investigated skin effects of PTH-CBD, and histological analysis revealed an apparent increase in anagen VI hair follicles. A single dose of PTH-CBD caused sustained increases in BMD by >10 % for 1 year in normal mice, regardless of the route of administration, thus showing promise as a potential osteoporosis therapy.

Treatment for chemotherapy-induced alopecia in mice using parathyroid hormone agonists and antagonists linked to a collagen binding domain

Parathyroid hormone (PTH) agonists and antagonists have been shown to improve hair growth after chemotherapy; however, rapid clearance and systemic side-effects complicate their usage. To facilitate delivery and retention to skin, we fused PTH agonists and antagonists to the collagen binding domain (CBD) of Clostridium histolyticum collagenase. in-vitro studies showed that the agonist fusion protein, PTH-CBD, bound collagen and activated the PTH/parathyroid hormone-related peptide receptor in SaOS-2 cells. The antagonist fusion proteins, PTH(7-33)-CBD and PTH([-1]-33)-CBD, also bound collagen and antagonized PTH(1-34) effect in SaOS-2 cells; however, PTH(7-33)-CBD had lower intrinsic activity. Distribution studies confirmed uptake of PTH-CBD to the skin at 1 and 12 hr after subcutaneous injection. We assessed in vivo efficacy of PTH-CBD and PTH(7-33)-CBD in C57BL/6J mice. Animals were depilated to synchronize the hair follicles; treated on Day 7 with agonist, antagonist, or vehicle; treated on Day 9 with cyclophosphamide (150 mg/kg i.p.) or vehicle; and sacrificed on Day 39. Normal mice (no chemo and no treatment) showed rapid regrowth of hair and normal histology. Chemo+Vehicle mice showed reduced hair regrowth and decreased pigmentation; histology revealed reduced number and dystrophic anagen/catagen follicles. Chemo+Antagonist mice were grossly and histologically indistinguishable from Chemo+Vehicle mice. Chemo+Agonist mice showed more rapid regrowth and repigmentation of hair; histologically, there was a normal number of hair follicles, most of which were in the anagen phase. Overall, the agonist PTH-CBD had prominent effects in reducing chemotherapy-induced damage of hair follicles and may show promise as a therapy for chemotherapy-induced alopecia.

Prevention of chemotherapy-induced osteoporosis by cyclophosphamide with a long-acting form of parathyroid hormone

BACKGROUND

Most chemotherapeutics reduce bone mineral density (BMD) and increase risk for fractures by causing gonadal suppression, which in turn increases bone removal. Cyclophosphamide (CYP) also has a direct effect of inhibiting bone formation and removal, making the resulting bone loss particularly difficult to treat with antiresorptive therapy.

AIM

We tested whether a single dose of the anabolic agent PTH linked to a collagen binding domain (PTHCBD) could prevent the effects of CYP-induced bone loss.

METHODS

Mice received either buffer alone, CYP, or CYP+ PTH-CBD. BMD and alkaline phosphatase were measured every 2 weeks for a total of 8 weeks.

RESULTS

After 6 weeks, mice treated with CYP showed expected reductions in BMD (increase from baseline: 7.4 ± 6.9 vs 24.35 ± 4.86% in mice without chemotherapy, p<0.05) and decrease in alkaline phosphatase levels (42.78 ± 6.06 vs 60.62 ± 6.23 IU/l in mice without chemotherapy, p<0.05), consistent with osteoporosis from impaired bone formation. Administration of a single dose of PTH-CBD (320 μg/kg ip) prior to CYP treatment improved BMD (change from baseline: 23.4 ± 5.4 vs 7.4 ± 6.9%, CYP treatment alone, p<0.05) and increased alkaline phosphatase levels (50.14 ± 4.86 vs 42.78 ± 6.06 IU/l in CYP treatment alone, p<0.05). BMD values and alkaline phosphatase levels were restored to those seen in mice not receiving chemotherapy.

CONCLUSIONS

A single dose of PTHCBD prior to chemotherapy reversed CYP-induced suppression of bone formation and prevented CYP-induced bone loss in mice.

Monthly administration of a novel PTH-collagen binding domain fusion protein is anabolic in mice

We synthesized fusion proteins of parathyroid hormone (PTH) (1-33) and the collagen binding domain of ColH (CBD) and tested them for anabolic bone activity in mice. Two fusion proteins were synthesized, linking the carboxy terminus of PTH(1-33) either directly to the amino terminal of the CBD or to the CBD through an adjacent ColH domain (PTH-PKD-CBD). Both PTH-CBD and PTH-PKD-CBD increased cAMP accumulation in cells stably transfected with the PTH/PTHrP receptor, and both peptides bound to type 1 collagen in flow-through assays. Distribution studies indicated that the PTH-CBD was concentrated in the bone and skin, tissues with abundant collagen and blood flow. Administration of 320 μg/kg PTH-CBD either weekly (for 8 weeks) or monthly (for 6 months) to 7-week-old C57BL/6J mice resulted in a sustained increase in bone mineral density (BMD) (15% for weekly studies, 13% for monthly studies; P < 0.05). PTH-PKD-CBD showed only 5% increases in BMD after weekly administration, and, as expected, neither weekly nor monthly PTH(1-34) affected BMD. PTH-CBD increased serum alkaline phosphatase levels. Importantly, there were no significant increases in serum calcium observed. Collectively, the data suggest that PTH-CBD has a sustained anabolic effect in bone with either weekly or monthly administration. This approach of targeted delivery of PTH to bone may show promise for the treatment of disorders of low bone mass, such as postmenopausal osteoporosis.

A new technique for precisely and accurately measuring lumbar spine bone mineral density in mice using clinical dual energy X-ray absorptiometry (DXA)

Dual Energy X-ray Absorptiometry (DXA) is effective in measuring bone mineral density (BMD) in mice for early detection of osteoporosis. However, scanners designed for use with small animals (i.e. PIXImus) are very expensive. Used human DXA machines are cheaper to obtain, but analysis of scans from these instruments is operator-dependent. Obtaining reliable data depends on having a single operator analyze the scans in a blinded fashion. Scan quality is improved by excising the bone prior to scanning, which does not allow serial measurements. This study describes a novel method of analyzing lumbar spine BMD in mice using whole body DXA. This non-invasive technique has a high degree of precision and reproducibility, with good correlation between multiple observers. Inter-observer variability (0.063 ± 0.00317 g/cm² [mean ± SD], 5.05 [% coefficient of variation (CV)], repeat scan variability (0.063 ± 0.00364 g/cm² [mean ± SD], 5.94 [%CV]) were very low compared to variability between different animals (0.063 ± 0.00588 g/cm² [mean ± SD], 9.64 [%CV]) and variability seen in same animal over time (0.011 ± 0.00885 g/cm² [mean ± SD], 80.68 [%CV]). The measurement error is thus smaller than the biological variation. Accuracy was determined by comparing average peak BMD from two scans per mouse in-vivo (0.066 g/cm²) versus excised spine (0.065 g/cm²). Furthermore, correlation between bone ash weights and whole body lumbar spine BMD measurements (p < 0.0001) was highly significant. This technique thus shows a high degree of precision and accuracy, even with multiple observers, for measuring BMD in mice using a DXA machine designed for clinical use.

Haplotype frequencies and linkage disequilibrium analysis of four frequent polymorphisms at the PTH/PTH-related peptide receptor gene locus

The parathyroid hormone (PTH)/PTH-related peptide receptor is a critical component in the control of mineral ion metabolism and in bone development. This receptor is encoded by a single gene (PTHR1) on chromosome 3p21.1-p24.2, and mutations in this gene have been found in several clinical disorders of bone and mineral metabolism. To facilitate future genetic studies of this important gene, we determined haplotype frequencies and performed linkage disequilibrium (LD) analysis of four different polymorphisms at the PTHR1 locus. Combined analysis of Caucasian, African-American and Asian individuals indicated that LD exists between all but one pair of the four polymorphisms. However, the pattern of LD differed substantially among the three subpopulations; for example, LD between two closely spaced (154-bp apart) single nucleotide polymorphisms appeared to be present only in Asians. Depending on the population under study, genetic association studies may need to test even more closely spaced polymorphic markers when screening the PTHR1 locus. These findings may thus affect the design and interpretation of future genetic studies involving PTHR1.

A novel COL1A1 mutation in infantile cortical hyperostosis (Caffey disease) expands the spectrum of collagen-related disorders

Infantile cortical hyperostosis (Caffey disease) is characterized by spontaneous episodes of subperiosteal new bone formation along 1 or more bones commencing within the first 5 months of life. A genome-wide screen for genetic linkage in a large family with an autosomal dominant form of Caffey disease (ADC) revealed a locus on chromosome 17q21 (LOD score, 6.78). Affected individuals and obligate carriers were heterozygous for a missense mutation (3040Ctwo head right arrowT) in exon 41 of the gene encoding the alpha1(I) chain of type I collagen (COL1A1), altering residue 836 (R836C) in the triple-helical domain of this chain. The same mutation was identified in affected members of 2 unrelated, smaller families with ADC, but not in 2 prenatal cases and not in more than 300 chromosomes from healthy individuals. Fibroblast cultures from an affected individual produced abnormal disulfide-bonded dimeric alpha1(I) chains. Dermal collagen fibrils of the same individual were larger, more variable in shape and size, and less densely packed than those in control samples. Individuals bearing the mutation, whether they had experienced an episode of cortical hyperostosis or not, had joint hyperlaxity, hyperextensible skin, and inguinal hernias resembling symptoms of a mild form of Ehlers-Danlos syndrome type III. These findings extend the spectrum of COL1A1-related diseases to include a hyperostotic disorder.

A novel STX16 deletion in autosomal dominant pseudohypoparathyroidism type Ib redefines the boundaries of a cis-acting imprinting control element of GNAS

A unique heterozygous 3-kb microdeletion within STX16, a closely linked gene centromeric of GNAS, was previously identified in multiple unrelated kindreds as a cause of autosomal dominant pseudohypoparathyroidism type Ib (AD-PHP-Ib). We now report a novel heterozygous 4.4-kb microdeletion in a large kindred with AD-PHP-Ib. Affected individuals from this kindred share an epigenetic defect that is indistinguishable from that observed in patients with AD-PHP-Ib who carry the 3-kb microdeletion in the STX16 region (i.e., an isolated loss of methylation at GNAS exon A/B). The novel 4.4-kb microdeletion overlaps with the previously identified deletion by 1,286 bp and, similar to the latter deletion, removes several exons of STX16 (encoding syntaxin-16). Because these microdeletions lead to AD-PHP-Ib only after maternal transmission, we analyzed expression of this gene in lymphoblastoid cells of affected individuals with the 3-kb or the 4.4-kb microdeletion, an individual with a NESP55 deletion, and a healthy control. We found that STX16 mRNA was expressed in all cases from both parental alleles. Thus, STX16 is apparently not imprinted, and a loss-of-function mutation in one allele is therefore unlikely to be responsible for this disorder. Instead, the region of overlap between the two microdeletions likely harbors a cis-acting imprinting control element that is necessary for establishing and/or maintaining methylation at GNAS exon A/B, thus allowing normal G alpha(s) expression in the proximal renal tubules. In the presence of either of the two microdeletions, parathyroid hormone resistance appears to develop over time, as documented in an affected individual who was diagnosed at birth with the 4.4-kb deletion of STX16 and who had normal serum parathyroid hormone levels until the age of 21 mo.

Parathyroid hormone and parathyroid hormone-related peptide, and their receptors

Parathyroid hormone (PTH) has a central role in the regulation of serum calcium and phosphate, while parathyroid hormone-related peptide (PTHrP) has important developmental roles. Both peptides signal through the same receptor, the PTH/PTHrP receptor (a class B G-protein-coupled receptor). The different biological effects of these ligands result from their modes of regulation and secretion, endocrine vs. paracrine/autocrine. The importance of PTH and PTHrP is evident by the variety of clinical syndromes caused by deficiency or excess production of either peptide, and the demonstration that intermittent injection of PTH increases bone mass, and thus provides a means to treat osteoporosis. This, in turn, has triggered increased interest in understanding the mechanisms of PTH/PTHrP receptor action and the search for smaller peptide or non-peptide agonists that have efficacy at this receptor when administered non-parenterally.

Identification and characterization of the zebrafish and fugu genes encoding tuberoinfundibular peptide 39

Although the PTH type 2 receptor (PTH2R) has been isolated from mammals and zebrafish, only its mammalian agonist, tuberoinfundibular peptide 39 (TIP39), has been characterized thus far. To determine whether zebrafish TIP39 (zTIP39) functions similarly with the zebrafish PTHR (zPTH2R) and human PTH2Rs and to determine its tissue-specific expression, fugu (Takifugu rubripes) and zebrafish (Danio rerio) genomic databases were screened with human TIP39 (hTIP39) sequences. A single TIP39 gene was identified for each fish species, which showed significant homology to mammalian TIP39. Using standard molecular techniques, we isolated cDNA sequences encoding zTIP39. The fugu TIP39 precursor was encoded by a gene comprising at least three exons. It contained a hydrophobic signal sequence and a predicted prosequence with a dibasic cleavage site, similar to that found in mammalian TIP39 ligands. Phylogenetic analyses suggested that TIP39 forms the basal group from which PTH and PTHrP have been derived. Functionally, subtle differences in potency could be discerned between hTIP39 and zTIP39. The human PTH2R and zPTH2R were stimulated slightly better by both hTIP39 and zTIP39, whereas zTIP39 had a higher potency at a previously isolated zPTH2R splice variant. Whole-mount in situ hybridization of zebrafish revealed strong zTIP39 expression in the region of the hypothalamus and in the heart of 24- and 48-h-old embryos. Similarly, zPTH2R expression was highly expressed throughout the brain of 48- and 72-h-old embryos. Because the mammalian PTH2R was also most abundantly expressed in these tissues, the TIP39-PTH2R system may serve conserved physiological roles in mammals and fishes.

Identification and characterization of two parathyroid hormone-like molecules in zebrafish

Zebrafish (Danio rerio) have receptors homologous to the human PTH (hPTH)/PTHrP receptor (PTH1R) and PTH-2 receptor (PTH2R) and an additional receptor (PTH3R) with high homology to the PTH1R. To find natural ligands for zPTH1R and zPTH3R, we searched the zebrafish genomic database and discovered two distinct regions that, when translated (zPTH1 and zPTH2), showed high homology to hPTH. Isolation of cDNAs and determination of the intron/exon boundaries revealed genomic structures which were similar to known PTHs. Peptides consisting of the first 34 amino acids after the pre- and prosequences of the zebrafish PTHs (zPTHs) were synthesized and were shown to be fully active at the hPTH1R. zPTH2(1-34) was, however, approximately 30-fold less potent at the zPTH1R than hPTH(1-34), hPTHrP(1-36), and zPTH1(1-34). When tested with zPTH3R, zPTH1(1-34) and hPTHrP(1-36) showed similar potencies, whereas the potency of zPTH2(1-34) was moderately (3-fold) reduced. To determine whether other fishes have multiple PTHs, we searched the genomic database of the Japanese pufferfish (Takifugu rubripes) and identified zPTH1 and zPTH2 homologs. Phylogenetic analysis showed that PTHs from zebrafish and pufferfish are more closely related to each other than to known mammalian PTH homologs or to PTHrP and tuberoinfundibular peptide of 39 residues. This is consistent with evolution of two teleost PTH-like peptides occurring after the evolutionary divergence between fishes and mammals. Overall, the PTH system appears more complex in fishes than in mammals, providing evidence of continued evolution in nontetrapod species. The availability of multiple forms of fish PTH and their receptors provide additional tools for PTH ligand/receptor structure-function studies.

Identification of a contact site for residue 19 of parathyroid hormone (PTH) and PTH-related protein analogs in transmembrane domain two of the type 1 PTH receptor

Recent functional studies have suggested that position 19 in PTH interacts with the portion of the PTH-1 receptor (P1R) that contains the extracellular loops and seven transmembrance helices (TMs) (the J domain). We tested this hypothesis using the photoaffinity cross-linking approach. A PTHrP(1-36) analog and a conformationally constrained PTH(1-21) analog, each containing para-benzoyl-l-phenylalanine (Bpa) at position 19, each cross-linked efficiently to the P1R expressed in COS-7 cells, and digestive mapping analysis localized the cross-linked site to the interval (Leu232-Lys240) at the extracellular end of TM2. Point mutation analysis identified Ala234, Val235, and Lys240 as determinants of cross-linking efficiency, and the Lys240-->Ala mutation selectively impaired the binding of PTH(1-21) and PTH(1-19) analogs, relative to that of PTH(1-15) analogs. The findings support the hypothesis that residue 19 of the receptor-bound ligand contacts, or is close to, the P1R J domain-specifically, Lys240 at the extracellular end of TM2. The findings also support a molecular model in which the 1-21 region of PTH binds to the extracellular face of the P1R J domain as an alpha-helix.

A mutant PTH/PTHrP type I receptor in enchondromatosis

Enchondromas are common benign cartilage tumors of bone. They can occur as solitary lesions or as multiple lesions in enchondromatosis (Ollier and Maffucci diseases). Clinical problems caused by enchondromas include skeletal deformity and the potential for malignant change to chondrosarcoma. The extent of skeletal involvement is variable in enchondromatosis and may include dysplasia that is not directly attributable to enchondromas. Enchondromatosis is rare, obvious inheritance of the condition is unusual and no candidate loci have been identified. Enchondromas are usually in close proximity to, or in continuity with, growth-plate cartilage. Consequently, they may result from abnormal regulation of proliferation and terminal differentiation of chondrocytes in the adjoining growth plate. In normal growth plates, differentiation of proliferative chondrocytes to post-mitotic hypertrophic chondrocytes is regulated in part by a tightly coupled signaling relay involving parathyroid hormone related protein (PTHrP) and Indian hedgehog (IHH). PTHrP delays the hypertrophic differentiation of proliferating chondrocytes, whereas IHH promotes chondrocyte proliferation. We identified a mutant PTH/PTHrP type I receptor (PTHR1) in human enchondromatosis that signals abnormally in vitro and causes enchondroma-like lesions in transgenic mice. The mutant receptor constitutively activates Hedgehog signaling, and excessive Hedgehog signaling is sufficient to cause formation of enchondroma-like lesions.

Identification of determinants of inverse agonism in a constitutively active parathyroid hormone/parathyroid hormone-related peptide receptor by photoaffinity cross-linking and mutational analysis

We have investigated receptor structural components responsible for ligand-dependent inverse agonism in a constitutively active mutant of the human parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor type 1 (hP1R). This mutant receptor, hP1R-H223R (hP1R(CAM-HR)), was originally identified in Jansen's chondrodysplasia and is altered in transmembrane domain (TM) 2. We utilized the PTHrP analog, [Bpa(2),Ile(5),Trp(23),Tyr(36)]PTHrP-(1-36)-amide (Bpa(2)-PTHrP-(1-36)), which has valine 2 replaced by p-benzoyl-l-phenylalanine (Bpa); this substitution renders the peptide a photoreactive inverse agonist at hP1R(CAM-HR). This analog cross-linked to hP1R(CAM-HR) at two contiguous receptor regions as follows: the principal cross-link site (site A) was between receptor residues Pro(415)-Met(441), spanning the TM6/extracellular loop three boundary; the second cross-link site (site B) was within the TM4/TM5 region. Within the site A interval, substitution of Met(425) to Leu converted Bpa(2)-PTHrP-(1-36) from an inverse agonist to a weak partial agonist; this conversion was accompanied by a relative shift of cross-linking from site A to site B. The functional effect of the M425L mutation was specific for Bpa(2)-containing analogs, as inverse agonism of Bpa(2)-PTH-(1-34) was similarly eliminated, whereas inverse agonism of [Leu(11),d-Trp(12)]PTHrP-(5-36) was not affected. Overall, our data indicate that interactions between residue 2 of the ligand and the extracellular end of TM6 of the hP1R play an important role in modulating the conversion between active and inactive receptor states.

Multiple sites of contact between the carboxyl-terminal binding domain of PTHrP-(1--36) analogs and the amino-terminal extracellular domain of the PTH/PTHrP receptor identified by photoaffinity cross-linking

The carboxyl-terminal portions of parathyroid hormone (PTH)-(1--34) and PTH-related peptide (PTHrP)-(1-36) are critical for high affinity binding to the PTH/PTHrP receptor (P1R), but the mechanism of receptor interaction for this domain is largely unknown. To identify interaction sites between the carboxyl-terminal region of PTHrP-(1--36) and the P1R, we prepared analogs of [I(5),W(23),Y(36)]PTHrP-(1--36)-amide with individual p-benzoyl-l-phenylalanine (Bpa) substitutions at positions 22--35. When tested with LLC-PK(1) cells stably transfected with human P1R (hP1R), the apparent binding affinity and the EC(50) of agonist-stimulated cAMP accumulation for each analog was, with the exception of the Bpa(24)-substituted analog, similar to that of the parent compound. The radiolabeled Bpa(23)-, Bpa(27)-, Bpa(28)-, and Bpa(33)-substituted compounds affinity-labeled the hP1R sufficiently well to permit subsequent mapping of the cross-linked receptor region. Each of these peptides cross-linked to the amino-terminal extracellular domain of the P1R: [I(5),Bpa(23),Y(36)]PTHrP-(1-36)-amide cross-linked to the extreme end of this domain (residues 33-63); [I(5),W(23),Bpa(27),Y(36)]PTHrP-(1--36)-amide cross-linked to residues 96--102; [I(5),W(23),Bpa(28),Y(36)]PTHrP-(1--36)- amide cross-linked to residues 64--95; and [I(5),W(23), Bpa(33),Y(36)]PTHrP-(1--36)-amide cross-linked to residues 151-172. These data thus predict that residues 23, 27, 28, and 33 of native PTHrP are each near to different regions of the amino-terminal extracellular receptor domain of the P1R. This information helps define sites of proximity between several ligand residues and this large receptor domain, which so far has been largely excluded from models of the hormone-receptor complex.

Selective and nonselective inverse agonists for constitutively active type-1 parathyroid hormone receptors: evidence for altered receptor conformations

The spontaneous signaling activity of some G protein-coupled receptors and the capacity of certain ligands (inverse agonists) to inhibit such constitutive activity are poorly understood phenomena. We investigated these processes for several analogs of PTH-related peptide (PTHrP) and the constitutively active human PTH/PTHrP receptors (hP1Rcs) hP1Rc-H223R and hP1Rc-T410P. The N-terminally truncated antagonist PTHrP(5-36) functioned as a weak partial/neutral agonist with both mutant receptors but was converted to an inverse agonist for both receptors by the combined substitution of Leu(11) and D-Trp(12). The N-terminally intact analog [Bpa(2)]PTHrP(1-36)-a partial agonist with the wild-type hP1Rc-was a selective inverse agonist, in that it depressed basal cAMP signaling by hP1Rc-H223R but enhanced signaling by hP1Rc-T410P. The ability of [Bpa(2)]PTHrP(1-36) to discriminate between the two receptor mutants suggested that H223R and T410P confer constitutive receptor activity by inducing distinct conformational changes. This hypothesis was confirmed by the observations that: 1) the double mutant receptor hP1Rc-H223R/T410P exhibited basal cAMP levels that were 2-fold higher than those of either single mutant; and 2) hP1Rc-H223R and hP1Rc-T410P internalized (125)I-PTHrP(5-36) to markedly different extents. The overall results thus reveal that two different types of inverse agonists are possible for PTHrP ligands (nonselective and selective) and that constitutively active PTH-1 receptors can access different conformational states.

Tuberoinfundibular peptide 39 binds to the parathyroid hormone (PTH)/PTH-related peptide receptor, but functions as an antagonist

The tuberoinfundibular peptide TIP39 [TIP-(1-39)], which exhibits only limited amino acid sequence homology with PTH and PTH-related peptide (PTHrP), stimulates cAMP accumulation in cells expressing the PTH2 receptor (PTH2R), but it is inactive at the PTH/PTHrP receptor (PTH1R). However, when using either (125)I-labeled rat [Nle(8,21),Tyr(34)]PTH-(1-34)amide (rPTH) or (125)I-labeled human [Tyr(36)]PTHrP-(1-36)amide [PTHrP-(1-36)] for radioreceptor studies, TIP-(1-39) bound to LLCPK(1) cells stably expressing the PTH1R (HKrk-B7 cells), albeit with weak apparent affinity (243 +/- 52 and 210 +/- 64 nM, respectively). In comparison to the parent peptide, the apparent binding affinity of TIP-(3-39) was about 3-fold higher, and that of TIP-(9-39) was about 5.5-fold higher. However, despite their improved IC(50) values at the PTH1R, both truncated peptides failed to stimulate cAMP accumulation in HKrk-B7 cells. In contrast, the chimeric peptide PTHrP-(1-20)/TIP-(23-39) bound to HKrk-B7 cells with affinities of 31 +/- 8.2 and 11 +/- 4.0 nM when using radiolabeled rPTH and PTHrP-(1-36), respectively, and it stimulated cAMP accumulation in HKrk-B7 and SaOS-2 cells with potencies (EC(50), 1.40 +/- 0.3 and 0.38 +/- 0.12 nM, respectively) and efficacies (maximum levels, 39 +/- 8 and 31 +/- 3 pmol/well, respectively) similar to those of PTH-(1-34) and PTHrP-(1-36). In both cell lines, TIP(9-39) and, to a lesser extent, TIP-(1-39) inhibited the actions of the three agonists with efficiencies similar to those of [Leu(11),D-Trp(12),Trp(23),Tyr(36)]PTHrP-(7-36)amide, an established PTH1R antagonist. Taken together, the currently available data suggest that the carboxyl-terminal portion of TIP-(1-39) interacts efficiently with the PTH1R, at sites identical to or closely overlapping those used by PTH-(1-34) and PTHrP-(1-36). The amino-terminal residues of TIP-(1-39), however, are unable to interact productively with the PTH1R, thus enabling TIP-(1-39) and some of its truncated analogs to function as an antagonist at this receptor.

Homologous up-regulation of vitamin D receptors is tissue specific in the rat

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) receptors (VDR) are expressed in multiple tissues within the body. VDR levels are increased by 1,25(OH)2D3 in intestine and kidney and in numerous cell models. The ability of 1,25(OH)2D3 to affect VDR levels in other target tissues in vivo was studied by assessing VDR levels by the 3H-1,25(OH)2D3 binding assay under varied physiological conditions in the rat. When compared with vitamin D-deficient (-D) controls, rats raised on a normal vitamin D-sufficient (+D) diet showed elevated VDR levels in kidney (391 +/- 53 vs. 913 +/- 76 fmol/g of tissue;p < 0.05), but not in testis, heart, or lung. Up-regulation of the VDR also occurred in kidney of +D rats 1 day after a single 100-ng dose of 1,25(OH)2D3 (454 +/- 43 vs. 746 +/- 113 fmol/mg of DNA; p < 0.05), but no changes were seen in intestine, testis, or lung. Because 1,25(OH)2D3-induced hypercalcemia may independently affect VDR regulation, 1,25(OH)2D3 was infused into -D rats, and normocalcemia was maintained by reduced dietary calcium intake. In this model, the renal VDR was again up-regulated (446 +/- 115 vs. 778 +/- 58 fmol/mg of DNA; p < 0.05), but VDR levels in testis and lung were unaffected. Scatchard analysis and tests of 1,25(OH)2D3 dose (1-100 ng/day for 7 days) and temporal (100 ng/day for 1-7 days) responsiveness further supported the tissue-specific nature of the homologous VDR regulation. Assay of VDR levels by L-1-tosylamido-2-phenylethyl chloromethyl ketone-3H-1,25(OH)2D3 exchange assay ruled out differences in endogenous 1,25(OH)2D3 occupancy as the basis for the observed differences in VDR regulation. Finally, coidentity of the VDR-like sites in kidney versus testis was confirmed by competitive binding analysis comparing their relative affinities for 25(OH)D3 versus 1,25(OH)2D3 (30.5 +/- 6.4 vs. 35.6 +/- 3.6 in kidney and testis, respectively) and by immunoblot analysis using a highly specific monoclonal anti-rat VDR antibody. Thus, under a wide variety of experimental conditions, homologous up-regulation of the VDR occurs in the rat kidney in vivo, but not in several other target tissues which do not regulate plasma calcium homeostasis. Moreover, this differential VDR regulation did not result from secondary changes in plasma calcium, from differential 1,25(OH)2D3 responsiveness in the various tissues, nor from differences in endogenous 1,25(OH)2D3 occupancy of the VDR. These studies thus establish that, in contrast to observations in vitro, the widely described phenomenon of homologous VDR up-regulation in kidney and intestine is not a universal property of 1,25(OH)2D3 target tissues in vivo in the rat.

Dexamethasone downregulates vitamin D receptors in rat kidney, unmasking a high affinity binding site

Dexamethasone exerted tissue-specific effects on rat vitamin D receptor levels: upregulation of receptors in intestine, downregulation in kidney, and no effect on receptors in testis, heart, and lung. Scatchard analysis showed selective downregulation of the low affinity receptor site (Kd = 0.9 nM) in kidney, thus unmasking a high affinity binding site (Kd = 0.05 nM). The single low affinity site in intestine (Kd = 0.3-0.7 nM) was upregulated, while the single high affinity site in testis (Kd = 0.07-0.09 nM) was not changed. These results demonstrate the existence of two types of nuclear binding sites in rat kidney and establish that there are similarities between the high affinity binding site in the rat kidney and the single high affinity receptor site in the testis.

Evidence for two classes of 1,25-dihydroxyvitamin D3 binding sites in classical vs. nonclassical target tissues

Possible differences in 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] binding sites in classical and nonclassical target tissues were tested by Scatchard analysis of [3H]1,25(OH)2D3 binding in parallel chromatin preparations of rat kidney vs. testis. Two distinct binding components were resolved in kidney (p less than 0.005). Moreover, the single binding site in testis exhibited a 10-fold lower Kd (p less than 0.05) than did the principal binding site in kidney (50 +/- 4 vs. 405 +/- 142 pM). Secondly, regulation of [3H]1,25(OH)2D3 binding sites also differed. 1,25(OH)2D3 injection resulted in increased 1,25(OH)2D3 binding (p less than 0.05) in kidney (92%) and intestine (415%), but not in testis, lung or heart. These results suggest that the principal 1,25(OH)2D3 binding sites in classical targets kidney and intestine may be intrinsically different from those in at least some nonclassical targets.

Arterial pressure effects on preglomerular microvasculature of juxtamedullary nephrons

Videomicroscopic and micropuncture techniques were utilized to determine segmental microvascular responses of in vitro blood-perfused juxtamedullary nephrons to step changes in renal arterial perfusion pressure (PP). At a PP of 104 +/- 2 mmHg, inside diameters of arcuate arteries (ARC), interlobular arteries (ILA), and afferent arterioles (AFF) averaged 68.6 +/- 6.4, 35.7 +/- 1.5, and 20.4 +/- 2.3 microns, respectively. Variations in PP within the range of 70-180 mmHg elicited alterations in microvessel diameters with the following slopes: ARC, -0.15 micron/mmHg; ILA, -0.13 micron/mmHg; and AFF, -0.14 micron/mmHg. In other experiments, intravascular pressures were measured during changes in PP. Glomerular capillary pressure was well regulated (slope = 0.19 +/- 0.03 mmHg/mmHg), and mid-AFF pressure was partially regulated (slope = 0.60 +/- 0.17 mmHg/mmHg); however, pressure measured at the ILA-AFF branch point responded passively to changes in PP (slope = 0.95 +/- 0.06 mmHg/mmHg). These observations reveal that, although the entire preglomerular vasculature of juxtamedullary nephrons is capable of actively responding to changes in PP, afferent arterioles are responsible for the predominant resistance adjustment throughout the normal autoregulatory range.