Ceramide regulates cellular homeostasis via diverse stress signaling pathways

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Abstract
The sphingolipid ceramide is an important second signal molecule that regulates diverse signaling pathways involving apoptosis, cell senescence, the cell cycle, and differentiation. For the most part, ceramide's effects are antagonistic to growth and survival. Interestingly, ceramide and the pro-growth agonist, diacylglycerol (DAG) appear to be regulated simultaneously but in opposite directions in the sphingomyelin cycle. While ceramide stimulates signal transduction pathways that are associated with cell death or at least are inhibitory to cell growth (eg stress-activated protein kinase, SAPK, pathways), DAG activates the classical and novel isoforms of the protein kinase C (PKC) family. These PKC isoforms are associated with cell growth and cell survival. Furthermore, DAG activation of PKC stimulates other signal transduction pathways that support cell proliferation (eg mitogen-activated protein kinase, MAPK, pathways). Thus, ceramide and DAG generation may serve to monitor cellular homeostasis by inducing pro-death or pro-growth pathways, respectively. The production of ceramide is emerging as a fixture of programmed cell death. Ceramide levels are elevated in response to diverse stress challenges including chemotherapeutic drug treatment, irradiation, or treatment with pro-death ligands such as tumor necrosis factor α, TNF α. Consistent with this notion, ceramide itself is a potent apoptogenic agent. Ceramide activates stress-activated protein kinases like c-jun N-terminal kinase (JNK) and thus affects transcription pathways involving c-jun. Ceramide activates protein phosphatases such as protein phosphatase 1 (PP1) and protein phosphatase 2 (PP2A). Ceramide activation of protein phosphatases has been shown to promote inactivation of a number of pro-growth cellular regulators including the kinases PKC α and Akt, Bcl2 and the retinoblastoma protein. A new role has recently emerged for ceramide in the regulation of protein synthesis. Ceramide-induced activation of double-stranded RNA-dependent protein kinase (PKR), a protein kinase important in anti-viral host defense mechanisms and recently implicated in cellular stress pathways, results in the inhibition of protein synthesis as a prelude to cell death. Taken together, these properties of ceramide suggest that this important second-signal molecule may have useful properties as an anti-neoplastic agent. Thus, strategies to promote ceramide metabolism or use of ceramide analogs directly may one day become useful in the treatment of diseases like leukemia.