Signaling mechanisms of parathyroid hormone regulation of insulin-like growth factor binding protein-5 in osteoblasts.
Published: 2006
Language: eng
Number of pages: 177
ISBN-13: 9780494219966
Description: Parathyroid hormone (PTH) is an essential regulator of extracellular calcium homeostasis. PTH is secreted from the parathyroid glands in response to low plasma calcium concentrations and restores levels to normal by stimulating calcium reabsorption in the kidney and increasing bone resorption to release mineralized calcium. In addition to its catabolic effects in bone, PTH is capable of stimulating bone deposition when administered intermittently. This thesis examined the signal transduction systems involved in PTH-regulation of an osteoblast marker of bone anabolic activity, insulin-like growth factor binding protein-5 (IGFBP-5).I determined that the transcription factor Sp1 bound in a PTH-dependent manner to the IGFBP-5-GC-element and a Sp-1 consensus oligonucleotide. Additionally, overexpression of Sp1 increased PTH-stimulated IGFBP-5 transcript levels. Sp1 was dephosphorylated following PTH treatment and the protein phosphatase inhibitor okadaic acid blocked the dephosphorylation of Sp1 by PTH and partially abrogated the PTH-induction of IGFBP-5 transcript levels.IGFBP-5 potentiates the anabolic actions of insulin-like growth factor-I in bone and PTH increases the expression of IGFBP-5 in osteoblasts. Using an osteoblastic cell-line, UMR106-01, I identified the signal transduction intermediates that mediate PTH induction of IGFBP-5 transcript levels. I confirmed an essential role of protein kinase A (PKA) and also demonstrated that the PKC pathway was required by using a non-selective PKC inhibitor. Using subcellular fractionation I identified PTH stimulation of PKC-delta nuclear translocation and a dominant negative mutant and selective inhibitor of PKC-delta, both inhibited the PTH-induction of IGFBP-5.In summary, my studies demonstrated that PTH increased IGFBP-5 gene transcription in osteoblasts by activating PKA and PKCdelta. These pathways in turn increased the activity of a protein phosphatase to dephosphorylate and thus activate Sp1 binding to a GC-rich element in the IGFBP-5 promoter. These studies have identified novel cellular targets of PTH signal transduction and may facilitate the development of improved therapeutic strategies for the treatment for osteoporosis.PTH stimulation had no effect on the half-life of the IGFBP-5 transcript based on mRNA stability experiments. A reporter gene assay using approximately 1000 base pairs of the IGFBP-5 5'flanking region indicated that PTH induced IGFBP-5 gene transcription. Deletion and mutation analysis identified a GC-rich element in the proximal promoter region of the IGFBP-5 gene that mediated approximately 50% of the PTH-induced increase in transcription.
Notes: Source: Dissertation Abstracts International, Volume: 68-01, Section: B, page: 0087.

Thesis (Ph.D.)--University of Toronto, 2006.

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