Effects of peptide fragments of protein kinase C on isolated rat osteoclasts

The intracellular mechanisms responsible for inhibition of osteoclast activ ity are of significant interest in the search for more effective ways of ma naging bone diseases associated with enhanced bone resorption. Previous stu dies have suggested that the protein kinase C (PKC) pathway is an important inhibitory second messenger in osteoclasts. We, therefore, investigated th e effects of the synthetic peptide fragments, PKC(530-558) and (19-36), whi ch correspond to parts of the catalytic and regulatory domains of PKC, on t he activity of isolated osteoclasts. These fragments have been shown to act ivate and inhibit PKC, respectively, in biochemical studies employing isola ted rat brain PKC, but have rarely been employed in studies of cellular act ivity. PKC(19-36), an enzyme inhibitor (PKC-I), had no effect by itself on osteoclastic bone resorption. However, PKC(530-558), a PKC activator (PKC-A ), caused a dose-responsive inhibition of bone resorption, which was accomp anied by a rapid and distinctive change in osteoclast morphology. This effe ct was reversible: (a) upon removal of PKC-A, (b) upon continuous exposure to this fragment for more than 36 h, or (c) in the presence of PKC-I. In co nclusion, a short synthetic peptide fragment of PKC (PKC-A) significantly i nhibits osteoclastic bone resorption; this, together with the fact that the inhibitory effect is abolished in the presence of PKC-I, provides further evidence for an important physiological role for the PKC pathway in the reg ulation of osteoclast activity. Selective activation of this pathway may ha ve important therapeutic implications for the management of bone diseases a ssociated with enhanced resorption.