Differential effects of parathyroid hormone fragments on collagen gene expression in chondrocytes.

Abstract

Abstract. The effect of parathyroid hormone (PTH) in vivo after secretion by the parathyroid gland is mediated by bioactive fragments of the molecule. To elucidate their possible role in the regulation of cartilage matrix metabolism, the influence of the amino-terminal (NH2-terminal), the central, and the carboxyl-terminal (COOH-terminal) portion of the PTH on collagen gene expression was studied in a serum free cell culture system of fetal bovine and human chondrocytes. Expression of al (I), oil (II), al (III), and otl (X) mRNA was investigated by in situ hybridization and quantified by Northern blot analysis. NH2-terminal and mid-regional fragments containing a core sequence between amino acid residues 28-34 of PTH induced a significant rise in etl (II) mRNA in proliferating chondrocytes. In addition, the COOH-terminal portion (aa 52-84) of the PTH molecule was shown to exert a stimulatory effect on etl(II) and etl (X) mRNA expression in chondrocytes from the hypertrophic zone of bovine epiphyseal cartilage. PTH peptides harboring either the functional domain in the central or COOH-terminal region of PTH can induce cAMP independent Ca 2+ signaling in different subsets of chondrocytes as assessed by microfluorometry of Fura-2/AM loaded cells. These results support the hypothesis that different hormonal effects of PTH on cartilage matrix metabolism are exerted by distinct effector domains and depend on the differentiation stage of the target cell.p ARATHYROID hormone plays a predominant role in the regulation of calcium homeostasis by acting mainly on its target tissues in the renal cortex and bone (15,42). Soon after secretion the parathyroid hormone (PTH) I molecule undergoes rapid proteolysis in the liver resulting in multiple fragments (7). Since most of the calcium regulatory functions could be mapped to the NH2-terminal portion (PTH 1-34) of PTH, it was thought that this fragment contains all structural requirements for biological activity of the entire molecule (43, 52). The other fragments were regarded as inactive metabolites whose functional importance was confined to processing and intracellular transport events during hormone secretion by the cells of the parathyroid gland (PTH 53-84, references 35, 46). However, there is now increasing evidence for a broader spectrum of target tissues, including cartilage (26,33,34), and of hormone action in growth (30,48) ferentiation processes (8, 13) which are mediated by additional functional domains on the mid-regional (23) and COOH-terminal portion (39, 40, 44) of PTH. For example, PTH (53-84) increases alkaline phosphatase activity in osteoblastic cell lines (39) and more recent studies showed that PTH (39-84) and PTH (53-84) dose dependently stimulate the differentiation of osteoclast precursors into osteoclast-like cells (25). Moreover, for two domains of PTH their functional role in the induction of second messenger pathways has been elucidated: the first two NH2-terminal amino acids of PTH are needed for adenylate cyclase stimulation via the "classical" ...

hPTH‐fragments (53–84) and (28–48) antagonize the stimulation of calcium release and repression of alkaline phosphatase activity by hPTH‐(1–34) in vitro

“…This contrasts with the more typical dose-dependence of the PTH-(53-84) effect and seems to imply that the two peptides have different mechanisms of action. Interestingly both fragments induced ALP activity in isolation, demonstrating that they are not simply inert peptides that prevent the binding of hPTH-(1-34), which is also supported by other studies both in vivo and in vitro [4][5][6][7][8][9][10]. In this report we show that the fragments induced ALP activity at low concentrations, whereas PTH-(1-34) significantly repressed ALP activity at concentrations of 0.1-10 nM.…”

“…The mid-regional fragment PTH-(28-48) augments cell proliferation in bone forming cells in vivo [4] and in vitro in chondrocytes [5] and osteoblasts [6,7]. It also induces alkaline phosphatase (ALP) activity and osteocalcin mRNA expression in osteoblast-like cells [8] and collagen II gene expression in chondrocytes [9]. The C-terminal fragment PTH-(53-84) induces also ALP activity in bone cells and regulates calcium influx in chondrocytes [10].…”

High-Level Production of Human Parathyroid Hormone (hPTH) by Induced Expression inSaccharomyces cerevisiae

“…Recent data indicate that the circulating bioactive Nterminal fragment is hPTH(1-37), not 1-34 as earlier thought (6). For the C-terminal region, osteoblast binding sites with specificity for aa 69 -84 have been characterized (7), and a new effector domain for chondrocytes that affects collagen gene expression has been localized to aa 52-84 (8).…”

Terminal Differentiation of Chick Embryo Chondrocytes Requires Shedding of a Cell Surface Protein That Binds 1,25-Dihydroxyvitamin D3

“…In a feedback loop of paracrine control, perichondrial cells, induced by chondrocytederived Indian Hedgehog, produce parathyroid hormone-related peptide that delays progression of late differentiation at late proliferative stages (10). However, parathyroid hormone can promote differentiation at other stages (11). Finally, extracellular signals also can stem from suprastructures of the extracellular matrix.…”