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Vol. 16, Issue 9, 4437-4453, September 2005

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Calcium-activated RAF/MEK/ERK Signaling Pathway Mediates p53-dependent Apoptosis and Is Abrogated by B-Crystallin through Inhibition of RAS Activation

David Wan-Cheng Li ^*  , Jin-Ping Liu ^*, Ying-Wei Mao  , Hua Xiang  ^||, Juan Wang  ^¶, Wei-Ya Ma ^*, Zigang Dong ^*, Helen M. Pike ^*, Rhoderick E. Brown ^*, and John C. Reed ^#

^* Hormel Institute, University of Minnesota, Austin, MN 55912; Department of Molecular Biology, University of Medicine and Dentistry of New Jersey, Stratford, NJ 08084; College of Life Sciences, Hunan Normal University, Changsha, China 410081; and ^# The Burnham Institute, La Jolla, CA 92037

Submitted January 7, 2005; Revised May 16, 2005; Accepted June 27, 2005
Monitoring Editor: Carl-Henrik Heldin

The ocular lens is the only organ that does not develop spontaneous tumor. The molecular mechanism for this phenomenon remains unknown. Through examination of the signaling pathways mediating stress-induced apoptosis, here we presented evidence to show that different from most other tissues in which the extracellular signal-regulated kinases (ERKs) pathway is generally implicated in mediation of survival signals activated by different factors, the RAF/MEK/ERK signaling pathway alone plays a key role in stress-activated apoptosis of lens epithelial cells. Treatment of N/N1003A cells with calcimycin, a calcium mobilizer, activates the RAF/MEK/ERK pathway through RAS, which is indispensable for the induced apoptosis because inhibition of this pathway by either pharmacological drug or dominant negative mutants greatly attenuates the induced apoptosis. Calcimycin also activates p38 kinase and JNK2, which are not involved in calcium-induced apoptosis. Downstream of ERK activation, p53 is essential. Activation of RAF/MEK/ERK pathway by calcimycin leads to distinct up-regulation of p53. Moreover, overexpression of p53 enhances calcimycin-induced apoptosis, whereas inhibition of p53 expression attenuates calcimycin-induced apoptosis. Up-regulation of p53 directly promotes Bax expression, which changes the integrity of mitochondria, leading to release of cytochrome c, activation of caspase-3 and eventually execution of apoptosis. Overexpression of B-crystallin, a member of the small heat-shock protein family, blocks activation of RAS to inhibit ERK1/2 activation, and greatly attenuates calcimycin-induced apoptosis. Together, our results provide 1) a partial explanation for the lack of spontaneous tumor in the lens, 2) a novel signaling pathway for calcium-induced apoptosis, and 3) a novel antiapoptotic mechanism for B-crystallin.

Present address: Department of Pharmacology, University of Michigan, Ann Arbor, MI 48109

^|| Present address: State Key Laboratory of Microbial Resources and Center for Molecular Microbiology, Institute of Microbiology, Chinese Academy of Sciences, Beijing 100080, China

^¶ Present address: Division of Endocrinology, Diabetes, and Metabolism, Departments of Medicine and Genetics and The Penn Diabetes Center, University of Pennsylvania, Philadelphia, PA 19104

Address correspondence to: David Wan-Cheng Li (dwcli{at}hi.umn.edu).

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