Cbl-b: its role of expression and regulation in T-lymphocyte activation and ageing
The aging process is strongly associated with decreased activity in the immune system. Dysregulation of T-lymphocyte function, such as reduced proliferation, is one problem faced by most elder people, which prevents them from successfully dealing with exogenous pathogens. Effective regulation of T-lymphocyte activity depends on the proper and prompt transduction of both positive and negative signals within Tlymphocytes and reflects the balance between positive and negative effects. Decline of positive signaling in aging has been studied and reported, while mechanisms concerning up-regulation of negative signaling with age and its role in immune senescence are still unclear. Cbl-b, an E3 ubiquitin ligase, was studied by our lab since it regulates the ubiquitin process, a protein modification process that has suppressive effects on signaling pathways. We first determined the reaction of Cbl-b to different stimuli in young rat splenic T-lymphocytes, and showed that there is a decrease in Cbl-b protein expression upon CD28 stimulation and such protein degradation is proteasome-dependent only. We also showed the mechanism of Cbl-b expression regulation involves the intracellular movement of Nedd4 toward Cbl-b and an up-regulation of Nedd4 expression. Then we proved that in old splenic T-lymphocytes, decreased proteasome activity was unable to down-regulate the Cbl-b protein. High levels of Cbl-b in old T-lymphocytes are functional in preventing PI3K activity and are associated with reduced T-lymphocyte proliferation upon regular stimulation. T-lymphocytes from old Cbl-b knock-out mice show similar proliferative reaction to CD3 stimulation as T-lymphocytes from young wild-type, which establishes the causeeffect relationship between sustained Cbl-b expression and decreased T-lymphocyte proliferation. In summary, these data suggest a unique role of Cbl-b in regulating Tlymphocyte signal transduction and provide critical preliminary data for extending Cbl-b studies into other fields, such as carcinogenesis.