Transforming Growth Factor-β1 (TGF-β1) Induces Human Osteoclast Apoptosis by Up-regulating Bim

Transforming growth factor-β1 (TGF-β1) is the most abundant TGF-β isoform detected in bone and is an important functional modulator of osteoclasts. TGF-β1 can induce osteoclast apoptosis; however, the apoptotic pathways involved in this process are not known. We show here that human osteoclasts express both type-I and type-II TGF-β receptors. In the absence of survival factors, TGF-β1 (1 ng/ml) induced osteoclast apoptosis. The expression of activated caspase-9, but not that of caspase-8, was increased by TGF-β1 stimulation, and the rate of TGF-β1-induced apoptosis was significantly lower in the presence of a caspase-9 inhibitor. To study further the mechanisms involved in TGF-β1-induced osteoclast apoptosis, we investigated TGF-β1 signaling, which primarily involves the Smad pathway, but also other pathways that may interfere with intracellular modulators of apoptosis, such as mitogen-activated protein (MAP) kinases and Bcl2 family members. We show here that early events consisted of a trend toward increased expression of extracellular signal-regulated kinase (ERK), and then TGF-β1 significantly induced the activation of p38 and Smad2 in a time-dependent manner. These signaling cascades may activate the intrinsic apoptosis pathway, which involves Bim, the expression of which was increased in the presence of TGF-β1. Furthermore, the rate of TGF-β1-induced osteoclast apoptosis was lower when Bim expression was suppressed, and inhibiting the Smad pathway abolished Bim up-regulation following TGF-β stimulation. This could correspond to a regulatory mechanism involved in the inhibition of osteoclast activity by TGF-β1.