Abstract

Apoptosis and proliferation are two dynamically and tightly regulated processes, which together maintain the homeostasis of renewable tissues. Anoikis is a subtype of apoptosis induced by detachment of adherent cells from the extracellular matrix. By using the defined mTeSR1 medium and collecting freshly detached cells, we found here that human pluripotent stem (PS) cells including embryonic stem (ES) cells and induced pluripotent stem (iPS) cells are subject to constant anoikis in culture, which is escalated in the absence of basic fibroblast growth factor (bFGF). Withdrawal of bFGF also promotes apoptosis and differentiation of the remaining adherent cells without affecting their cell cycle progression. Insulin-like growth factor 2 (IGF2) has previously been reported to act downstream of FGF signaling to support self-renewal of human ES cells. However, we found that IGF2 cannot substitute bFGF in the TeSR1-supported culture, although endogenous IGF signaling is required to sustain self-renewal of human ES cells. On the other hand, all the bFGF withdrawal effects observed here can be markedly prevented by the caspase inhibitor zVAD-FMK. We further demonstrated that the bFGF-repressed anoikis is dependent on activation of ERK and AKT, and associated with inhibition of BIM and the caspase-ROCK1-myosin signaling. Anoikis is independent of pre-detachment apoptosis and differentiation of the cells. Since previous studies of human PS cells have been focused on attached cells, our findings revealed a neglected role of bFGF in sustaining self-renewal of human PS cells - preventing them from anoikis via inhibition of caspase activation.