Parathyroid hormone (PTH) plays a central role in regulation of calcium metabolism but also has a role as an anabolic hormone for bone.  The hormone appears to act through a single receptor on the osteoblast to elicit all of its skeletal effects, including its anabolic effects.  We have shown that low concentrations of PTH (1012 and I 0" M) stimulate DNA synthesis and proliferation of osteoblastic cells in culture, and that this is due to selective activation of protein kinase C (PKC) resulting in activation of extra-cellular signal regulated kinases (ERKs).  The most likely reason is because these concentrations of PTH interact with their receptor in such a way as to selectively activate certain Ga proteins (such as Gaq or Ga 2/13) leading to activation of PKC.  The activation and role of this pathway has not been well documented in the osteoblast.  Thus, the overall goal of the present competing renewal is to delineate how low concentrations of PTH signal through PKC and what the eventual targets of ERK activation are which lead to proliferation of osteoblastic cells.  This will be done as a two-pronged approach: 1.  determining how low concentrations of PTH signal though PKC in osteoblastic cells by: a) ascertaining the type of PKC isozyme activated by low concentrations of the hormone, b) assessing the signal transduction pathway activating the PKC isozyme(s), c) determining the G protein activating the particular phospholipase(s).  2.  delineating the eventual targets of ERK activation which lead to proliferation of osteoblastic cells by: a) examining whether known ERK target genes are affected by treatment of osteoblastic cells with low concentrations of PTH, b) identifying other genes regulated by low concentrations of PTH, c) examining whether the changes in mRNA for the gene(s) are reflected in changes in protein levels, d) ablating the regulated gene(s) by either using dominant negative forms of the protein or inducible anti-sense RNA, and ascertaining if this prevents enhanced proliferation of osteoblastic cells by low doses of PTH.  The results of this work will provide insight into one of the key pathways of regulation of the osteoblast.  In so doing, the data may provide a foundation for development of small molecules that can be used in place of injected PTH for treatment of osteoporosis.