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Regulation of BAD phosphorylation at serine 112 by the Ras-mitogen-activated protein kinase pathway
1Department of Molecular Oncology, University of Texas MD Anderson Cancer Center, Houston 77030, USA.
Abstract:
The function of the pro-apoptotic molecule BAD is regulated by phosphorylation of two sites, serine-112 (Ser-112) and serine-136 (Ser-136). Phosphorylation at either site results in loss of the ability of BAD to heterodimerize with the survival proteins BCL-XL or BCL-2. Phosphorylated BAD binds to 14-3-3 and is sequestered in the cytoplasm. It has been shown that phosphorylation of BAD at Ser-136 is mediated by the serine/threonine protein kinase Akt-1/PKB which is downstream of phosphatidylinositol 3-kinase (PI3K). The signaling process leading to phophorylation of BAD at Ser-112 has not been identified. In this study, we show that phosphorylation of the two serine residues of BAD is differentially regulated. While Ser-136 phosphorylation is concordant with activation of Akt, Ser-112 phosphorylation does not correlate with Akt activation. Instead, we demonstrate that activated Ras and Raf, which are upstream of mitogen-activated protein kinases (MAPK), stimulate selective phosphorylation of BAD at Ser-112. Furthermore, phosphorylation of Ser-112, but not Ser-136 requires activation of the MAPK pathway as the MEK inhibitor, PD 98059, blocks EGF-, as well as activated Ras- or Raf-mediated phosphorylation of BAD at Ser-112. Therefore, the PI3K-Akt and Ras-MAPK pathways converge at BAD by mediating phosphorylation of distinct serine residues.
Insights
The pro-apoptotic molecule BAD
Area of Science:
- Cell Biology
- Molecular Biology
- Signal Transduction
Background:
- The pro-apoptotic protein BAD's function is regulated by phosphorylation at Ser-112 and Ser-136.
- Phosphorylation inactivates BAD, preventing interaction with survival proteins BCL-XL/BCL-2 and causing cytoplasmic sequestration.
- Ser-136 phosphorylation is mediated by Akt-1/PKB (PI3K pathway), but the mechanism for Ser-112 phosphorylation is unknown.
Purpose of the Study:
- To elucidate the distinct signaling pathways regulating BAD phosphorylation at Ser-112 and Ser-136.
- To investigate the role of Ras-MAPK pathway in BAD phosphorylation.
- To understand how PI3K-Akt and Ras-MAPK pathways converge on BAD.
Main Methods:
- Investigated differential regulation of BAD phosphorylation at Ser-112 and Ser-136.
- Assessed correlation between Ser-136 phosphorylation and Akt activation.
- Examined the role of Ras, Raf, and MAPK pathway in Ser-112 phosphorylation using MEK inhibitor PD 98059.
Main Results:
- Ser-136 phosphorylation correlates with Akt activation, while Ser-112 phosphorylation does not.
- Activated Ras and Raf, upstream of MAPK, induce selective phosphorylation of BAD at Ser-112.
- Ser-112 phosphorylation requires MAPK pathway activation; PD 98059 inhibits EGF-, Ras-, and Raf-mediated phosphorylation.
Conclusions:
- BAD phosphorylation at Ser-112 and Ser-136 are differentially regulated by distinct signaling pathways.
- The PI3K-Akt pathway phosphorylates BAD at Ser-136.
- The Ras-MAPK pathway phosphorylates BAD at Ser-112, demonstrating pathway convergence on BAD.