Ceramide Differentially Regulates Protein Kinase C/ Mitogen Activated Protein Kinase Pathways: Implications for Growth Arrest

Open Access
Author:
Bourbon, Nicole A.
Graduate Program:
Pharmacology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 19, 2001
Committee Members:
  • Steven S Levison, Committee Member
  • Mark Kester, Committee Chair
  • Elliot Saul Vesell, Committee Member
  • Melvin Lee Billingsley, Committee Member
  • Robert G Levenson, Committee Member
Keywords:
  • PKC
  • MAPK
  • Ceramide
  • SAPK
  • ERK
  • Signaling Complexes
  • Sphingolipids
Abstract:
Ceramide, a sphingomyelin-derived second messenger, has been shown to stimulate signaling pathways that lead to growth arrest, differentiation or apoptosis. However, the mechanisms by which ceramide induces these cellular phenotypes are unclear at this time. Our studies establish a new paradigm for actions of lipid-derived second messengers: coordinating assembly of multi-factoral signaling complexes. The Stress-Activated Protein Kinase (SAPK) pathway has been implicated in cell growth arrest and/or apoptosis. In addition, studies have linked ceramide with activation of the SAPK cascade. However, the mechanism by which ceramide leads to activation of this cascade is unclear at this time. Our studies demonstrate that ceramide activates the SAPK cascade via direct activation of Protein Kinase C zeta (PKCz) in human embryonic kidney cells (HEK 293). Upon activation by ceramide, PKCz forms a signaling complex with upstream components of the SAPK cascade, including MEKK1 and SEK1. These studies demonstrate a novel mechanism by which ceramide activates the SAPK pathway to induce cell cycle arrest. A parallel pathway to the SAPK cascade is the Extracellular signal-Regulated Kinase (ERK) cascade. We and others have shown that the inhibitory action of ceramide on cell growth involves inhibition of the ERK pathway. Therefore, we investigated the mechanism by which ceramide inhibits this pathway. Our studies demonstrate that ceramide inhibits PKC epsilon activity and subsequent interaction with upstream components of the ERK cascade. These studies characterize a mechanism by which ceramide induces growth arrest through inhibition of the ERK cascade. Another signaling pathway critical in cell survival and proliferation is the PI3K/Akt1 cascade. Recent studies have demonstrated that ceramide inhibits Akt1. However, the mechanism of Akt1 inhibition by ceramide is unclear. Therefore, we investigated the role of PKCz in ceramide-mediated inhibition of the Akt1 cascade. Our studies revealed that inhibition of Akt1 by ceramide is PKCz-dependent. Collectively, our studies demonstrate several complimentary mechanisms by which ceramide induces cell growth arrest. The ability of ceramide to induce growth arrest, without inducing significant apoptosis or necrosis, may be of therapeutic value in the prevention or control of cell proliferation during inflammatory renal and vascular diseases.