Last update:
09-Jul-2004

OBJECTIVE A primary goal of bone research is to understand the mechanism(s) by which mechanical forces dictate the cellular and metabolic activities of osteoblasts, the bone-forming cells. Several studies indicate that osteoblastic cells respond to physical loading by transducing signals that alter gene expression patterns. Accumulated data have documented the fundamental role of the osteoblast-specific transcription factor Cbfa1 in osteoblast differentiation and function.

METHOD Primary cultures of human osteoblast-like cells were subjected to lowlevel continuous mechanical strain employing an established system for applying calibrated stretch. A combination of Northern/Western blotting, mobility-shift, protein kinase inhibition, in-gel kinase, and co-immunoprecipitation assays were used to address the role of Cbfa1 in mechanotransduction.

RESULTS It was demonstrated that physiological amounts of mechanical deformation (stretching) directly up-regulate the expression and DNAbinding activity of Cbfa1 in human osteoblastic cells. This effect seems to be “fine-tuned” by stretch-triggered induction of distinct mitogen-activated protein kinase (MAPK) cascades. Importantly, it is shown that activated extracellular signal-regulated kinase (ERK) MAPK physically interacts and phosphorylates endogenous Cbfa1 in vivo, ultimately potentiating this transcription factor.

CONCLUSIONS The findings of this study provide a molecular link between mechanostressing and stimulation of osteoblast differentiation. Elucidation of the specific modifiers and co-factors that operate in this mechanotranscription circuitry will contribute to a better understanding of mechanical load-induced bone formation which may set the basis for non-pharmacological intervention in bone loss pathologies.

Key words: Cbfa1, ERK, MAP kinase, Mechanical stimulation, Osteoblastic cell.