Association of a new c-Cbl related protein with the very first stages of apoptosis induction

Cbl negatively regulates JNK activation and cell death

Here, we explore the role of Cbl proteins in regulation of neuronal apoptosis. In two paradigms of neuron apoptosis - nerve growth factor (NGF) deprivation and DNA damage - cellular levels of c-Cbl and Cbl-b fell well before the onset of cell death. NGF deprivation also induced rapid loss of tyrosine phosphorylation (and most likely, activation) of c-Cbl. Targeting c-Cbl and Cbl-b with siRNAs to mimic their loss/inactivation sensitized neuronal cells to death promoted by NGF deprivation or DNA damage. One potential mechanism by which Cbl proteins might affect neuronal death is by regulation of apoptotic c-Jun N-terminal kinase (JNK) signaling. We demonstrate that Cbl proteins interact with the JNK pathway components mixed lineage kinase (MLK) 3 and POSH and that knockdown of Cbl proteins is sufficient to increase JNK pathway activity. Furthermore, expression of c-Cbl blocks the ability of MLKs to signal to downstream components of the kinase cascade leading to JNK activation and protects neuronal cells from death induced by MLKs, but not from downstream JNK activators. On the basis of these findings, we propose that Cbls suppress cell death in healthy neurons at least in part by inhibiting the ability of MLKs to activate JNK signaling. Apoptotic stimuli lead to loss of Cbl protein/activity, thereby removing a critical brake on JNK activation and on cell death.

Androgen-dependent apoptosis in male germ cells is regulated through the proto-oncoprotein Cbl

The proto-oncoprotein Cbl is known to control several signaling processes. It is highly expressed in the testis, and because spermatogenesis is androgen dependent, we investigated the androgen dependency expression of Cbl through its testicular sublocalization and its expression levels in rats that were exposed to the antiandrogen flutamide or were hypophysectomized. We report the androgen dependency of Cbl as it localizes in pachytene spermatocytes during androgen-dependent stages, is down-regulated upon flutamide exposure, and is up-regulated with testosterone in hypophysectomized rats. Coculture experiments showed the key control exerted by the Sertoli cell on Cbl activity. As flutamide induces germ cell apoptosis, we investigate members of the Bcl-2 family upon flutamide exposure. We show that the proapoptotic Bcl-2 family member Bim mirrored Cbl expression through a posttranscriptional process. We also show that in Cbl knockout mouse testes, the imbalance between the high expression of Bim and Smac/Diablo and antiapoptotic factors such as cellular inhibitor of apoptosis 2 favors a survival process, which makes these mice unresponsive to androgen withdrawal and could explain their hypofertility.