Signalling from parathyroid hormone

Sustained signalling by PTH modulates IP3 accumulation and IP3 receptors through cyclic AMP junctions

Parathyroid hormone (PTH) stimulates adenylyl cyclase through type 1 PTH receptors (PTH₁R) and potentiates the Ca²⁺ signals evoked by carbachol, which stimulates formation of inositol 1,4,5-trisphosphate (IP₃). We confirmed that in HEK cells expressing PTH₁R, acute stimulation with PTH(1-34) potentiated carbachol-evoked Ca²⁺ release. This was mediated by locally delivered cyclic AMP (cAMP), but unaffected by inhibition of protein kinase A (PKA), exchange proteins activated by cAMP, cAMP phosphodiesterases (PDEs) or substantial inhibition of adenylyl cyclase. Sustained stimulation with PTH(1-34) causes internalization of PTH₁R–adenylyl cyclase signalling complexes, but the consequences for delivery of cAMP to IP₃R within cAMP signalling junctions are unknown. Here, we show that sustained stimulation with PTH(1-34) or with PTH analogues that do not evoke receptor internalization reduced the potentiated Ca²⁺ signals and attenuated carbachol-evoked increases in cytosolic IP₃. Similar results were obtained after sustained stimulation with NKH477 to directly activate adenylyl cyclase, or with the membrane-permeant analogue of cAMP, 8-Br-cAMP. These responses were independent of PKA and unaffected by substantial inhibition of adenylyl cyclase. During prolonged stimulation with PTH(1-34), hyperactive cAMP signalling junctions, within which cAMP is delivered directly and at saturating concentrations to its targets, mediate sensitization of IP₃R and a more slowly developing inhibition of IP₃ accumulation.

Human parathyroid hormone increases the mRNA expression of the IGF system and hematopoietic growth factors in osteoblasts, but does not influence expression in mesenchymal stem cells

Osteoblasts, which are derived from pluripotent mesenchymal stem cells (MSCs), play an important role in hematopoiesis. Human parathyroid hormone (hPTH) induces osteoblasts to produce many factors that are essential to hematopoietic stem cells. However, little is known about the impact of hPTH on MSCs to enhance hematopoiesis. We determined the optimal dose of hPTH that was necessary in vitro for increased osteoblast function. In addition, we compared MSC and osteoblast function to explore the role of hPTH in hematopoiesis. The mRNA expression levels of granulocyte colony-stimulating factor (G-CSF), granulocyte macrophage colony-stimulating factor (GM-CSF), interleukin 6, stromal cell-derived factor 1, insulin-like growth factor 1 (IGF-1), IGF-2, insulin-like growth factor-binding protein 1 (IGFBP-1), IGFBP-2, and IGFBP-3 were comparable in osteoblasts and human cord blood-derived MSCs. However, G-CSF, GM-CSF, IGF-2, IGFBP-1, IGFBP-2, and IGFBP-3 expression levels in osteoblasts were markedly increased after treatment with 50 or 100 nM of hPTH. In conclusion, hPTH does not affect the ability of MSCs to differentiate into osteoblasts. In addition, hPTH may enhance hematopoiesis by activating the IGF system (IGF-2, IGFBP-1, IGFBP-2, and IGFBP-3) and hematopoietic growth factors (G-CSF and GM-CSF) in osteoblasts, but not in MSCs.

Parathyroid hormone suppresses insulin signaling in adipocytes

Previous reports suggest that parathyroid hormone (PTH) is associated with insulin resistance. This research investigated the effects of PTH on insulin signaling in differentiated 3T3-L1 adipocytes. PTH (10 nM, 24 h) treatment induced a reduction in insulin-stimulated glucose uptake, AKT activity (phosphorylated AKT/total AKT protein expression) and a decrease in GLUT4 and IRS-1 protein expression compared to vehicle treated controls in differentiated adipocytes. PTH treatment also induced increased phosphorylation of IRS-1 on serine 307, which suppresses insulin signaling. In addition, treatment of cells with adenyl cyclase inhibitor SQ52236 ameliorated the effects of PTH on insulin-stimulated glucose uptake, whereas inhibition of phospholipase C alpha (U73122) did not significantly alter the effects of PTH. Thus, PTH treatment of differentiated 3T3-L1 adipocytes suppresses insulin-stimulated glucose uptake and insulin signaling via cAMP pathway, potentially through the phosphorylation of IRS-1 at serine 307.

The role of parathyroid hormone and insulin-like growth factors in hematopoietic niches: physiology and pharmacology

Hematopoietic stem cells (HSC) capable of both self-renewal and differentiation into all blood lineages reside within the bone marrow in specialized microenvironmental niches. While the precise location and composition of these niches largely remains unknown, it is now believed that osteoblasts at the endosteal surface play critical roles. Among the molecules demonstrated to influence the function of these niches are parathyroid hormone (PTH) and the insulin-like growth factors (IGF). Administration of PTH to both mice and men expands the number of bone marrow HSC, and an increase in the number of those cells in peripheral blood following treatment with mobilizing agents. Several molecules downstream of PTH are capable of signaling to HSC, including IGF that appear to regulate both the survival and expansion of hematopoietic stem and progenitor cells. As our current understanding of the role for PTH and IGF in hematopoietic niches is limited, we believe it is important that both their physiological importance and pharmacological potential be more fully investigated.

NHE3 phosphorylation at serines 552 and 605 does not directly affect NHE3 activity

Direct phosphorylation of sodium hydrogen exchanger type 3 (NHE3) is a well-established physiological phenomenon; however, the exact role of NHE3 phosphorylation in its regulation remains unclear. The objective of this study was to evaluate whether NHE3 phosphorylation at serines 552 and 605 is physiologically regulated in vivo and, if so, whether changes in phosphorylation at these sites are tightly coupled to changes in transport activity. To this end, we directly compared PKA-induced NHE3 inhibition with site-specific changes in NHE3 phosphorylation in vivo and in vitro. In vivo, PKA was activated using an intravenous infusion of parathyroid hormone in Sprague-Dawley rats. In vitro, PKA was activated directly in opossum kidney (OKP) cells using forskolin and IBMX. NHE3 activity was assayed in microvillar membrane vesicles in the rat model and by 22Na uptake in the OKP cell model. In both cases, NHE3 phosphorylation at serines 552 and 605 was determined using previously characterized monoclonal phosphospecific antibodies directed to these sites. In vivo, we found dramatic changes in NHE3 phosphorylation at serines 552 and 605 with PKA activation but no corresponding alteration in NHE3 activity. This dissociation between NHE3 phosphorylation and activity was further verified in OKP cells in which phosphorylation clearly preceded transport inhibition. We conclude that although phosphorylation of NHE3 at serines 552 and 605 is regulated by PKA both in vivo and in vitro, phosphorylation of these sites does not directly alter Na+/H+ exchange activity.