Parathyroid Hormone-Related Protein: From Hypercalcemia of Malignancy to Developmental Regulatory Molecule

PTH-Related Protein Assays in Advanced Kidney Disease: Implications for Evaluation of Hypercalcemia

Hypercalcemia is a common and potentially serious electrolyte abnormality that is often observed in patients with chronic kidney disease (CKD). When malignancy is considered, parathyroid hormone-related protein (PTHrP) levels are often measured. PTHrP is produced by cancer cells and mimics the effects of parathyroid hormone (PTH) to elevate serum calcium concentrations. The amino and carboxy termini of PTHrP are of functional relevance. C-terminal PTHrP levels accumulate with CKD and can be elevated in normocalcemic CKD patients who lack malignancy. The existence of amino (N)-terminal and carboxy (C)-terminal PTHrP assays and how their concentrations are impacted by CKD are reviewed herein. The case of a patient on maintenance hemodialysis who developed prolonged hypercalcemia with elevated PTHrP concentrations is presented. The workup revealed suppressed intact PTH, low 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D levels. The initial PTHrP assay returned elevated. However, it was unappreciated that it was the C-terminal assay and the patient underwent an unnecessary search for malignancy. A subsequent N-terminal PTHrP assay returned within the normal range. Many commercial labs run the C-terminal PTHrP assay as their first-line test. This can lead to inaccurate differential diagnoses in hypercalcemic patients with CKD. We emphasize the need to specifically request N-terminal PTHrP assays in patients with advanced kidney disease when humoral hypercalcemia of malignancy is suspected.

Large-scale genotyping identifies 41 new loci associated with breast cancer risk

Breast cancer is the most common cancer among women. Common variants at 27 loci have been identified as associated with susceptibility to breast cancer, and these account for ∼9% of the familial risk of the disease. We report here a meta-analysis of 9 genome-wide association studies, including 10,052 breast cancer cases and 12,575 controls of European ancestry, from which we selected 29,807 SNPs for further genotyping. These SNPs were genotyped in 45,290 cases and 41,880 controls of European ancestry from 41 studies in the Breast Cancer Association Consortium (BCAC). The SNPs were genotyped as part of a collaborative genotyping experiment involving four consortia (Collaborative Oncological Gene-environment Study, COGS) and used a custom Illumina iSelect genotyping array, iCOGS, comprising more than 200,000 SNPs. We identified SNPs at 41 new breast cancer susceptibility loci at genome-wide significance (P < 5 × 10(-8)). Further analyses suggest that more than 1,000 additional loci are involved in breast cancer susceptibility.

Genome-wide association analysis identifies three new breast cancer susceptibility loci

Breast cancer is the most common cancer among women. To date, 22 common breast cancer susceptibility loci have been identified accounting for ∼8% of the heritability of the disease. We attempted to replicate 72 promising associations from two independent genome-wide association studies (GWAS) in ∼70,000 cases and ∼68,000 controls from 41 case-control studies and 9 breast cancer GWAS. We identified three new breast cancer risk loci at 12p11 (rs10771399; P = 2.7 × 10(-35)), 12q24 (rs1292011; P = 4.3 × 10(-19)) and 21q21 (rs2823093; P = 1.1 × 10(-12)). rs10771399 was associated with similar relative risks for both estrogen receptor (ER)-negative and ER-positive breast cancer, whereas the other two loci were associated only with ER-positive disease. Two of the loci lie in regions that contain strong plausible candidate genes: PTHLH (12p11) has a crucial role in mammary gland development and the establishment of bone metastasis in breast cancer, and NRIP1 (21q21) encodes an ER cofactor and has a role in the regulation of breast cancer cell growth.

Solution structure of parathyroid hormone related protein (residues 1-34) containing an Ala substituted for an Ile in position 15 (PTHrP[Ala15]-(1-34))

The structure of human parathyroid hormone (PTH) related protein (residues 1-34) containing an Ala substituted for an Ile in position 15 was studied by two-dimensional proton nuclear magnetic resonance spectroscopy. This mutant retains quite high levels of adenylate cyclase activity based on slightly reduced PTH receptor binding capacity. Three segments of helix were revealed extending from His5 to Lys11, Lys13 to Arg19, and from Phe22 to Thr33/Ala34, with a decided kink between the first two helices around Gly12. N- and C-terminal helices were stabilized by charged and hydrophobic side chain interactions between His5 and Glu30, Asp17 and both His9 and His25, and between Leu8 and Ala29, resulting in a globular molecule occupying a single conformation. While the structure of the entire mid-molecule region differed greatly from the structure of the native peptide, the structure of both N- and C-terminal regions remains essentially unaltered. The residues responsible for initiating signal transduction in the mutant are located in the vicinity of the residues responsible for receptor binding. The C-terminal amphipathic helix forming the receptor binding site exhibits reduced binding as a result of the closely applied N-terminal signal transduction-activating region. Although not contributing directly to receptor binding, the N-terminal region can sterically affect hormone binding through modifications to certain N-terminal side chains.

Parathyroid hormone-related protein enhances insulin-like growth factor-I expression by fetal rat dermal fibroblasts

Interactions between cells of differing embryonic origins comprise a common theme during tissue development and repair. Often, communication between them can be mediated by soluble growth mediators and in some cases is restricted in focus. That is, some cells respond to, but do not produce, mediators expressed by other cells within the tissue. Because keratinocytes respond to but do not express insulin-like growth factor I (IGF-I), another skin cell population, the dermal fibroblast, may supply this factor. However, keratinocytes express, but do not respond to parathyroid hormone related protein (PTHrp), which increases cAMP production by dermal fibroblasts. Based on earlier results where inducers of cAMP increase local IGF-I expression in skeletal tissue, we postulated that PTHrp might induce local IGF-I by dermal fibroblasts and provide a source of this factor for keratinocyte activity. Our studies reveal that IGF-I mRNA and protein levels increase in response to PTHrp in vitro, and that this effect is replicated by inducers of cAMP, but not by activators of protein kinase C. Consequently, these factors appear to comprise a paracrine loop within the skin, permitting focused but restricted IGF-I expression to support skin growth, remodeling, or repair.