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Definition
Parathyroid hormone–related peptide (PTHrP), a peptide hormone derived from normal and tumor cells that regulates bone metabolism and vascular tone, is a naturally occurring angiogenesis inhibitor.

Discovery
PTHrP was discovered in association with certain types of cancer that caused elevated blood Ca2+ levels (a syndrome called humoral hypercalcemia of malignancy, or HHM) in affected patients 1.

Structural Characteristics
The mature PTHrP of teleost fish is 161 amino acids long 2. There are regions of high conservation across all the vertebrates, including 22 of the first 34 amino acids, which suggest they interact with the common PTH/PTHrP receptors. Other regions of high conservation are 11 of the 18 amino acids of the Arg-rich RNA binding region 3 and 7 of the 15 amino acids of the nuclear localization sequence 4. Comparison of the COOH-terminal region of PTHrPs reveals it is much shorter in fish than in tetrapods 2.

Mode of Action
In a study, mice with a targeted deletion of parathyroid hormone (PTH)-related peptide (PTHrP) develop a form of dyschondroplasia resulting from diminished proliferation and premature maturation of chondrocytes. Therefore, PTHrP appears to modulate cell proliferation and differentiation in both the pre and post natal period. PTH/PTHrP receptor expression in the mouse is controlled by two promoters. Furthermore, it is found that, while the downstream promoter controls PTH/PTHrP receptor gene expression in bone and cartilage, it is differentially regulated in the two tissues. 1-alpha, 25-dihydroxyvitamin D3 downregulated the activity of the downstream promoter in osteoblasts, but not in chondrocytes, both in vivo and in vitro. Most of the biological activity of PTHrP is thought to be mediated by binding of its amino terminus to the PTH/PTHrP receptor. However, recent evidence suggests that amino acids 87-107, outside of the amino terminal binding domain, act as a nucleolar targeting signal. Chondrocytic cell line, CFK2, transfected with wild-type PTHrP cDNA showed PTHrP in the nucleoli as well as in the secretory pathway. Therefore, PTHrP appears to act as a bifunctional modulator of both chondrocyte proliferation and differentiation, through signal transduction linked to the PTH/PTHrP receptor and by its direct action in the nucleolus 5.

Functions
Effect of PTHrP on human platelet activation: A study conducted was to determine whether PTHrP might be involved in human platelet activation, by using a turbidimetric method to determine platelet aggregation. The expression of PTH1R (PTH type 1 receptor) in human platelets was analysed by western blot and flow cytometry analyses. PTHrP-(1–36) by itself failed to modify the activation of platelets. However, it significantly enhanced ADP-induced platelet activation, and also increased the ability of other agonists (thrombin, collagen and arachidonic acid) to induce platelet aggregation 6. H89 and Rp-cAMPS two protein kinase A inhibitors, and  bisindolylmaleimide I, a protein kinase C inhibitor, partially decreased the enhancing effect of PTHrP-(1–36) on ADP-induced platelet activation. Meanwhile, PTHrP-(7–34), a PTH1R antagonist, as well as PD098059, a MAPK (mitogen-activated protein kinase) inhibitor, or a farnesyltransferase inhibitor abolished this effect of PTHrP-(1–36). Moreover, PTHrP-(1–36) increased (2-fold over control) MAPK activation in human platelets. PTH1R was detected in platelets, and the number of platelets expressing it on their surface in patients during AMI (acute myocardial infarction) was not different from that in a group of patients with similar cardiovascular risk factors without AMI 6. Western blot analysis showed that total PTH1R protein levels were markedly higher in platelets from control than those from AMI patients. These results indicate that human platelets express the PTH1R. PTHrP can interact with this receptor to enhance human platelet activation induced by several agonists through a MAPK-dependent mechanism 6.

Localization of PTHrP in breast cancer metastases: PTHrP has recently been identified in 60% of a series of primary breast cancers. The detection of a bone-resorbing factor in tumors with a propensity to metastasize to bone prompted study of PTHrP in breast cancer metastasis. PTHrP was localized by immunohistology in 12 of 13 (92%) breast cancer metastases in bone and in 3 of 18 (17%) metastases in non-bone sites. The statistical difference was highly significant (P < 0.0001). Production of PTHrP as a bone-resorbing agent may contribute to the ability of breast cancers to grow as bone metastases.

References

 

1.     Moseley JM, Kubota M, Diefenbach-Jagger H, Wettenhall RE, Kemp BE, Suva LJ, Rodda CP, Ebeling PR, Hudson PJ, Zajac JD (1987). Parathyroid hormone-related protein purified from a human lung cancer cell line. PNAS., 84:5048–5052.


2.     Canario AVM, Rotllant J, Fuentes J, Guerreiro PM, Rita Teodosio H, Power DM, Clark MS (2006). Novel bioactive parathyroid hormone and related peptides in teleost fish. FEBS Lett., 580:291-299.


3.     Aarts MM, Levy D, He B, Stregger S, Chen T, Richard S, Henderson JE (1999). Parathyroid hormone-related protein interacts with RNA. J. Biol. Chem., 274:4832–4838.


4.     Henderson JE, Amizuka N, Warshawsky H, Biasotto D, Lanske BM, Goltzman D, Karaplis AC (1995). Nucleolar localization of parathyroid hormone- related peptide enhances survival of chondrocytes under conditions that promote apoptotic cell death. Mol. Cell. Biol., 15:4064–4075.

5.     Amizuka N, Henderson JE, White JH, Karaplis AC, Goltzman D, Sasaki T, Ozawa H (2000). Recent studies on the biological action of parathyroid hormone (PTH)-related peptide (PTHrP) and PTH/PTHrP receptor in cartilage and bone. Histol Histopathol., 15(3):957-970.


6.     Ortega A, Pérez de Prada MT, Mateos-Cáceres PJ, Ramos Mozo P, González-Armengol JJ, González Del Castillo JM, Martín Sánchez J, Villarroel P, Santiago JL, Bosch RJ, Macaya C, Esbrit P, López-Farré AJ (2007). Effect of parathyroid-hormone-related protein on human platelet activation. Clinical Science, 113(7):319–327.


7.     Powell GJ, Southby J, Danks JA, Stillwell RG, Hayman JA, Henderson MA, Bennett RC, Martin TJ (1991). Localization of Parathyroid Hormone-related Protein in Breast Cancer Metastases: Increased Incidence in Bone Compared with Other Sites. Cancer Research, 51:3059-3061.

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