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

Abstract

Different C-terminal fragments of parathyroid hormone (PTH)-(1-84) in blood participate in the regulation of calcium homeostasis by PTH-(1-84), and an antagonizing effect for the large carboxyl-terminal parathyroid hormone (C-PTH)-fragment (7-84) on calcium release has been described in vivo and in vitro. In this study the smaller C-PTH-fragment (53-84) and mid-regional PTH fragment (28-48), which represent discrete areas of activity in the PTH-(7-84) molecule, were assayed for their effects on calcium release and alkaline phosphatase (ALP) activity in a chick bone organ culture system. Neither PTH-(28-48) nor PTH-(53-84) had any effect on calcium release into the medium and both fragments stimulated ALP activity in the bone tissue, suggesting that the cAMP/ PKA signalling pathway was not affected by these fragments. However they suppressed the calcium release induced by PTH-(1-34) and attenuated the down regulation of ALP activity caused by PTH-(1-34), suggesting that the effect on the cAMP/PKA signalling pathway may be indirectly. In conclusion, the study shows that the PTH-fragments (53-84) and (28-48) antagonize the PTH-(1-34) induced effects on calcium release and inhibition of ALP activity in a chick bone organ culture system.

Intermittent administration of PTH(1-34) regulates the osteoblastic differentiation of human periodontal ligament cells via protein kinase C- and protein kinase A-dependent pathways in vitro

“…In less‐mature preconfluent cultures, only the N‐terminally intact PTH fragments containing the PKA‐activating domain enhanced ALP activity and osteocalcin production and those effects were antagonized by the PKA inhibitor, H8, but not by the PKC inhibitor, RO‐32‐0432, allowing for the conclusion that PTH(1–34) mediates its effects via a cAMP‐dependent mechanism at this maturation stage. These results strengthen the findings of previous reports on the predominant role of the cAMP/PKA‐dependent pathway in PTH signaling for the regulation of osteoblastic differentiation parameters, including ALP specific activity and bone sialoprotein (23–25). Conflicting evidence regarding the role of PKA as an activator or inhibitor of PTH‐induced target gene transcription (10,24,26,27) exists with the majority of the data obtained in osteoblast studies, supporting the concept that multiple mechanisms are involved in the PTH response by demonstrating the involvement of both cAMP and PKC in PTH signal transduction with a predominance of PKA as an activator of ALP activity (3,10,24).…”

Intermittent PTH(1–34) signals through protein kinase A to regulate osteoprotegerin production in human periodontal ligament cells in vitro

“…The cellular response to the signal-specific fragment truncated at the PKC-activating terminus, PTH(1-31), which was comparable to PTH , and the inability of the PKC inhibitor RO-32-0432 to antagonize the action of the stimulus further substantiate this interpretation. These data reinforce previous observations in other cell culture systems that PTH can signal through both PKA and PKC with a predominance of the cAMP/PKA-dependent pathway [55,[61][62][63]. Conflicting data exist regarding the role of PKA as an activator or inhibitor of PTH-induced target gene regulation [64,65].…”

“…In doing so, signal-specific PTH fragments and specific inhibitors to the protein kinases A and C were employed without measuring kinase activity directly. This approach is well established [40,49,54,55] and based on the confirmation that PTH (1-31) activates only PKA with no demonstrable effects on PKC or phospholipase C [56,57] as opposed to the analogs PTH and PTH(7-34) which both activate PKC, but not adenylyl cyclase [58][59][60]. The lack of an effect of the N-terminally truncated fragments PTH and PTH together with the ability of the PKA inhibitor H8 to inhibit the PTH(1-34)-induced changes in target protein expression leads to the conclusion that the cAMP/PKA-dependent pathway, but not PKC, is of predominant importance in PTH signal transduction with respect to osteoprotegerin expression.…”

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

“…These findings have been established convincingly through pharmacological experiments in sheep (289) and genetic studies in mice (175). Further evidence was provided (47) that the mid-molecule region producing the placental effect is within PTHrP (67)(68)(69)(70)(71)(72)(73)(74)(75)(76)(77)(78)(79)(80)(81)(82)(83)(84)(85)(86). This was ascribed to PTHrP (38 -94)amide, a product generated by secretory processing in the neuroendocrine RIN cells (382).…”