Ceramide: a novel second messenger.
The data discussed in the preceding sections suggest that information may be transmitted both through synthesis and through degradation of sphingomyelin. Although the sphingomyelin pathway holds promise as a new signaling system coupling TNF receptor activation to cellular stimulation (Fig. 5), the work is still at a preliminary stage. A physical association of receptors with neutral sphingomyelinase has yet to be established and the ceramide-activated protein kinase has yet to be isolated. Endogenous substrates for the kinase have also to be identified. Furthermore, the exact role of this pathway has not been defined. It is unclear whether this pathway is specific to monocyte differentiation or cytokine action. It also seems unlikely that a single signal transduction mechanism can account for all the diverse effects of TNF-alpha in different systems (Vilcek and Lee, 1991). Interactions with other signaling systems are sure to complicate elucidation of the exact role(s) of this pathway. Nevertheless, availability of cell-permeable analogs of ceramide, localization of many components of the system at the cell surface, and recent development of anti-TNF receptor antibodies to receptor isotypes may allow for greater definition of the sphingomyelin pathway in the near future.