Protein kinase C (PKC), protein kinase A (PKA), prostaglandin synthesis, and various mitogen-activated protein kinases (MAPKs) have been reported to be activated in bone cells by mechanical loading. We studied the involvement of these signal transduction pathways in the downregulation of HB-GAM expression in osteoblastic cells after cyclic stretching. Specific antagonists and agonists of these signal transduction pathways were added to cells before loading and to non-loaded control cells. Quantitative RT-PCR was used to evaluate gene expression. The data demonstrated that the extracellular signal-regulated kinase (ERK) 1/2 pathway, PKC, PKA, p38, and C-Jun N-terminal kinase MAPK participated in the mechanical downregulation of HB-GAM expression, whereas prostaglandin synthesis did not seem to be involved. (c) 2006 Elsevier Inc. All rights reserved.