OSTEOBLAST NUCLEAR MATRIX REGULATION OF COLLAGEN

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Intermittent doses of parathyroid hormone (PTH) increase bone mass by selectively stimulating bone formation. The molecular mechanisms mediating this anabolic effect are unknown. Demonstrating this phenomenon is problematic using in vitro models because PTH attenuates collagen synthesis in osteoblast culture. Cell structure influences collagen synthesis and likely contributes to the opposing responses of bone and cultured osteoblasts to PTH. We propose that collagen expression is coupled to cell structure via the tissue matrix, the interlinking proteins of the extracellular matrix, cytoskeleton, and nuclear matrix. Furthermore, we propose that this structural pathway culminates with nuclear matrix architectural transcription factors, proteins that alter gene activity by bending promoter DNA in response to changes in tissue matrix organization. Therefore, PTH-induced changes in tissue matrix proteins may alter COL1A1 expression by "tugging" at the gene and altering promoter geometry. We have identified a novel family of nuclear matrix architectural transcription factors, NP/NMP4, that bind with sequence specificity to the promoter of the rat alpha1 (I) polypeptide chain (COL1A1) of type I collagen. These unique zinc finger proteins bind to the COL1A1 promoter, bend it, and contain basal promoter activity. PTH regulates NP/NMP4-COL1A1 binding activity and NP/NMP4 mRNA expression. Hormone-induced alterations in NP/NMP4-COL1A1 binding differ between bone and culture. Our goal is to investigate the functional role of NP/NMP4 in mediating basal and PTH-regulated COL1A1 transcription. The first study will test the hypothesis that the COL1A1 promoter NP/NMP4 binding elements contribute to osteoblast basal and PTH-modulated transcription in vitro and in vivo. The second study will determine the distinct functions of NP and NMP4 in mediating basal and PTH-regulated COL1A1 transcription. The third study will determine the functional domains of the NP/NMP4 proteins. This information will define a structural pathway for PTH action in osteoblasts and further clarify the cellular basis for the anabolic action of this hormone on bone.