PTHRP

Structure

Human PTHrP encodes a signal peptide of 36 aa and a 141-aa mature protein. In teleosts, another PTHrPB gene that shares 67% homology with PTHrPA in pufferfish, Takifugu rubripes, has been found. The mature PTHrPAs of the pufferfish, sea bream, and European flounder comprise 126, 125, and 129 aa, respectively, and share approximately 90% sequence similarity. Although the total amino acid identity among the teleost and mammalian PTHrPs is low (~36%), it is higher (~60%) in the N-terminal region. Eight of the first 13 residues of the human PTHrP are identical to those in PTH . In PTHrP as well as PTH, the first 34 aa have the structural requirements for full PTHrP biological activity. A comparison of the C-terminal region of PTHrPs reveals it to be much shorter in fish than in tetrapods.

Properties

Mammalian PTHrPs: Mr. ~17 kDa. Soluble in water, and stable in solution at pH 4.8. To prevent enzymatic degradation, plasma samples should be collected in EDTA-containing tubes. Precooling the sampling tubes and the addition of protease inhibitors further improve the stability.

Gene and mRNA

The human PTHrP gene has been located on chromosome 12 (12p11.22). The human PTHrP gene has eight exons and seven introns, and produces several splice variants encoding mature proteins with 139, 141, and 173 aa4. In the chicken, two different mRNA isoforms encoding mature proteins with 139 and 141 aa were found while the mouse and rat genes encode only a single isoform. On the other hand, there is a high similarity of the organization of the pufferfish pthrp gene to the human PTH, chicken pth, and pufferfish pth genes, having three exons spanning a 2.25-kb region.

Synthesis and release

Increasing Ca2+ concentrations stimulate PTHrP secretion via CaSR in a number of normal and cancerous cell types. CaSR is important in the modulation of PTHrP secretion for the maintenance of the placentalfetal Ca2+ transport. In addition, the PTHrP gene contains consensus regulatory motifs for cAMP, 1,25- dihydroxy vitamin D3 (1,25(OH)2D3), and glucocorticoids. Dexamethasone, vitamin D, cortisol, estrogens, and androgens have been shown to inhibit PTHrP production in vitro.

Receptors

PTH1R, but not PTH2R, is activated by PTHrP. The organization of the PTH1R gene is conserved in three mammalian species: the rat, human, and mouse. As described in the PTH section, PTH1R interacts with the first 34 aa of the N-termini of both PTH and PTHrP.1 Zebrafish and other teleosts possess a third receptor, PTH3R, which may be a result of the duplication of PTH1R. Kd in the human kidney plasma membrane, canine kidney plasma membrane, and chick bone cell membrane is 1–5 nM.

Agonists and Antagonists

PTHrP (1–36) as well as PTH (1–34) are thought to interact with the PTH1R through distinct binding and activating domains. PTH (1–14) and PTH (1–21) analogs show high-affinity binding and efficient activation of the PTH1R. [Leu11, D-Trp12] PTHrP (7–34) and [Ile5 , Trp23] PTHrP (5–36) bind with high affinity but are devoid of signaling activity. The substitution or modification of the second valine in PTH (1–34) or PTHrP (1–36) with a bulky amino acid or an amino acid derivative (Trp, Arg, or Bpa) also results in the generation of potent antagonists.

Biological functions

PTHrP is known to be a critical regulator of cellular and organ growth, development, migration, differentiation, survival, and epithelial calcium ion transport. In mammals, judging from the distribution and expression of mRNA, PTHrP has a greater importance in early development, which determines the rate of chondrogenesis and structure mineralization. In teleosts, on the other hand, the possible involvement of PTHrP in the process of expansion and relaxation of the organs was suggested. In terrestrial vertebrates, PTHrP acts as a paracrine or autocrine regulator. While high concentrations (0.4–1.2 nM) in the blood of fish predict a classical endocrine function, to date no PTHrP-producing gland has been clearly identified in fish.

Description

This was the first hormone isolated as a hypercalcemic factor involved in the humoral hypercalcemia of malignancy. PTHrP is a drug target for osteoporosis. The existence of a PTH-like factor in cancers has been reported. It was isolated in 1987 from a human lung cancer cell line. A second PTHrP gene, pPTHrPB, was first identified in the pufferfish genome. In teleosts, two PTHrP (PTHrPA and PTHrPB) appear to be derived by the teleost-specific third round genome duplication.

Clinical Use

Plasma PTHrP levels have been used for the differential diagnosis of humoral HHM based on a secreted humoral bone resorption factor and a judgment of its curative effect from various tumors. The plasma PTHrP concentration is routinely measured to diagnose malignant tumors (including lung, esophageal, oral, head and neck, uterine, breast, pancreatic, ovarian, hepatocellular, kidney, and bladder cancers). It has been noted that specimens for PTHrP testing are collected according to the requirements given by the referral laboratory.