Parathyroid hormone-related protein (PTHrP), calcium and human osteoblast-like cells

Abstract

Parathyroid hormone-related protein (PTHrP) may cause humoral hypercalcernia of malignancy (HHM) when overproduced by malignant tumors, but is also produced in lower amounts under normophysiological conditions in practically all cell types, including osteoblasts. One of the aims of the present study was to investigate interactions between PTHrP and osteoblasts, to elucidate bone-regulating functions of PTHrP. The well-characterized primary cultures of normal human osteoblast-like cells (hOB cells) that were used, reflect human bone physiology better than osteoblastic cell lines.

The N-terminal fragment of PTHrP increased intracellular cyclic adenosine monophospate (cAMP) in hOB cells, whereas there was no effect on free cytoplasmic calcium ([Ca2+]i), The content of insulin-like growth factor (IGF) I and II, IGF binding protein (IGFBP) 4 and 5 in conditioned medium from hOB cells was also stimulated by N-terminal PTHrP. IGF-I, IGF-II and IGFBP-5 are anabolic factors, whereas IGFBP-4 has catabolic effects. Therefore, the present findings give insight into the mechanisms behind the ambiguous effects of PTHrP, which may have both anabolic and catabolic effects on bone. There was also a negative correlation between IGFBP-4 and IGFBP-5, as well as a negative correlation between IGFBP-4 in conditioned medium and the age of the hOB cell donors. This implies that the catabolic effect of IGFBP-4 from osteoblasts may be amplified by a relatively low production of IGFBP-5, and vice versa, and that an intrinsic age-related increase in locally produced IGFBP-4 in osteoblasts may play a role in the development of age-related osteoporosis.

IL- 10 increased messenger RNA expression and secretion of PTHrP in hOB cells, whereas the production of TGF-ß was decreased. Thus, IL-1beta], which has well documented bone-resorbing effects, may exert indirect effects on osteoblasts by regulating the production of PTHrP, which increases bone resorption, and TGF-ß, which is an important anabolic factor for bone. TGF-ß is also considered to be a coupling factor that controls the interaction between osteoclasts and osteoblasts in the bone remodeling process. Patients with HHM may display increased levels of IL-1, in addition to elevated PTHrP. The fact that IL-1ß decreased TGF-ß in hOB cells therefore indicates a possible mechanism behind the uncoupled bone remodeling process in patients with HEM.

Oscillations in the [Ca2+]i level were found in hOB cells, but only in the presence of albumin with a high lipid content, and not when exposed to low lipid albumin. The oscillations were dependent on extracellular calcium ([Ca2+]O), as well as intracellular calcium stores, and intercellular gap junction communication. High lipid albumin also had a stimulatory effect on DNA synthesis compared to low lipid albumin. However, the precise relationship between [Ca2+]i oscillations and DNA synthesis has not been fully clarified. A relative decrease in pericellular osmolality induced [Ca2+]i transients in hOB cells. This indicates a potential signaling pathway for the anabolic effects of mechanical stimuli on bone, since a decrease in osmolality causes cellular swelling and stretching of the plasma membrane, and therefore mimics mechanical stimulation.

A high concentration of [Ca2+]O increased [Ca2+]i in hOB cells and stimulated DNA synthesis through a protein kinase C-dependent mechanism. Since osteoblasts can be exposed to high concentrations of [Ca2+]O released during osteoclastic bone resorption, this indicates that [Ca2+], may be a coupling factor in the bone remodeling process, which recruits and stimulates osteoblasts through these signaling pathways.