Signal transduction pathways mediating parathyroid hormone stimulation of bone sialoprotein gene expression in osteoblasts.
Bone sialoprotein is a major noncollagenous protein of bone. Parathyroid hormone (PTH) was shown to cause a 2-4-fold increase in the steady-state levels of bone sialoprotein mRNAs within primary cultures of embryonic osteoblasts. The induction could be mimicked by both forskolin and phorbol 12-myristate 13-acetate and was not inhibited by cycloheximide. Transient expression of a approximately 1200-base pair avian 5' bsp promoter/reporter construct demonstrated similar inductions as mRNA levels. Co-transfection of an expression plasmid encoding heat-stable inhibitor of cAMP-dependent protein kinase, a peptide inhibitor of PKA, decreased both the basal and PTH-induced bsp transcription, while co-expression of the catalytic subunit of PKA-induced bsp expression 3-fold. Protein kinase C activation, on the other hand, did not appear to work through its activation of c-fos, since co-transfection of an expression clone for c-fos had no effect. Interestingly, heat-stable inhibitor of cAMP-dependent protein kinase also inhibited the phorbol 12-myristate 13-acetate induction, suggesting that the protein kinase C acts through some form of interaction with the cAMP/PKA pathway. A half-cAMP response element site in the bsp promoter was identified as the cis-acting element that mediated the PTH response by the transient transfections with reporter constructs containing nested deletions of the promoter or a heterologous promoter containing the cAMP response element. In conclusion, these data indicate that PTH stimulation of bsp gene expression is specific to osteoblasts and mediated by changing cellular cAMP/PKA levels. They further suggest that although protein kinase C is capable of stimulating the gene by itself, it plays a minimal role in mediating the PTH induction of bone sialoprotein.
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Department of Orthopedic Surgery, Children's Hospital and Harvard Medical School, Boston, Massachusetts 02115, USA. gerstenfeld@a1.tch.harvard.edu
Gerstenfeld
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D000818:Animals; D002478:Cells, Cultured; D002645:Chickens; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects*; D015759:Ionomycin / Q000494:pharmacology; D010006:Osteoblasts / Q000187:drug effects* / Q000378:metabolism; D010281:Parathyroid Hormone / Q000494:pharmacology*; D011401:Promoter Regions, Genetic; D011493:Protein Kinase C / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D015290:Second Messenger Systems; D012795:Sialoglycoproteins / Q000235:genetics*; D015398:Signal Transduction; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D010281:Parathyroid Hormone; D012333:RNA, Messenger; D012795:Sialoglycoproteins; D005576:Colforsin; D015759:Ionomycin; D000242:Cyclic AMP; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate