The transcript (mRNA), protein levels, enzyme activity, and cellular localization of four protein kinase C (PKC) isozymes identified in rat osteogenic sarcoma cells (UMR-108) were studied at confluent density and during mechanical stress (cyclic stretch). Western blot analysis indicated that growth to confluent density significantly increased the protein levels of cPKC-α (11.6-fold), nPKC-δ (5.3-fold), and nPKC-ϵ (22.0-fold) but not aPKC-ζ. Northern blot analysis indicated a significant (2.3-fold) increase in the 10 kb transcript of cPKC-α, a slight (1.3-fold) increase in that of nPKC-ϵ but no detectable change in that of the remaining isozymes. Enzyme activity assays of the individually immunoprecipitated isozymes yielded detectable kinase activity only for PKC-α, PKC-δ, and PKC-ϵ and only in confluent cells, corroborating the selective increase of these isozymes at confluent density. The UMR-108 cells showed a dramatic orientation response to mechanical stress with cell reshaping and alignment of the cell long axis perpendicular to the axis of force, remodeling of the actin cytoskeleton, and the appearance of multiple peripheral sites which stained for actin, vinculin, and PKC in separate experiments. Longer term mechanical stress beyond 24 h, however, resulted in no significant change in the mRNA level, protein level, or enzyme activity of any of the four PKC isozymes investigated. The results indicate that there are isozyme-selective increases in the protein levels of PKC isozymes of osteoblastic UMR-108 cells upon growth to confluence which may be regulated at the transcriptional or the post-transcriptional level. The results from UMR-108 cells support the earlier proposal (Carvalho RS, Scott JE, Suga DM, Yen EH. 1994. J Bone Miner Res 9(7):999–1011) that PKC could be involved in the early phase of mechanotransduction in osteoblasts through the activation of focal adhesion assembly/disassembly and the remodeling of the actin cytoskeleton. © 2001 Wiley-Liss, Inc.