Silencing the constitutive active transcription factor CREB by the LKB1-SIK signaling cascade.
Cyclic AMP responsive element (CRE)-binding protein (CREB) is known to activate transcription when its Ser133 is phosphorylated. Two independent investigations have suggested the presence of Ser133-independent activation. One study identified a kinase, salt-inducible kinase (SIK), which repressed CREB; the other isolated a novel CREB-specific coactivator, transducer of regulated CREB activity (TORC), which upregulated CREB activity. These two opposing signals are connected by the fact that SIK phosphorylates TORC and induces its nuclear export. Because LKB1 has been reported to be an upstream kinase of SIK, we used LKB1-defective HeLa cells to further elucidate TORC-dependent CREB activation. In the absence of LKB1, SIK was unable to phosphorylate TORC, which led to constitutive activation of CRE activity. Overexpression of LKB1 in HeLa cells improved the CRE-dependent transcription in a regulated manner. The inactivation of kinase cascades by 10 nm staurosporine in LKB1-positive HEK293 cells also induced unregulated, constitutively activated, CRE activity. Treatment with staurosporine completely inhibited SIK kinase activity without any significant effect on the phosphorylation level at the LKB1-phosphorylatable site in SIK or the activity of AMPK, another target of LKB1. Constitutive activation of CREB in LKB1-defective cells or in staurosporine-treated cells was not accompanied by CREB phosphorylation at Ser133. The results suggest that LKB1 and its downstream SIK play an important role in silencing CREB activity via the phosphorylation of TORC, and such silencing may be indispensable for the regulated activation of CREB.
Katoh
Yoshiko
Y
Molecular Physiological Chemistry, Osaka University Medical School, Japan.
Takemori
Hiroshi
H
Lin
Xing-Zi
XZ
Tamura
Mitsuhiro
M
Muraoka
Masaaki
M
Satoh
Tomohiro
T
Tsuchiya
Yuko
Y
Min
Li
L
Doi
Junko
J
Miyauchi
Akira
A
Witters
Lee A
LA
Nakamura
Haruki
H
Okamoto
Mitsuhiro
M
D055372:AMP-Activated Protein Kinases; D021581:Active Transport, Cell Nucleus; D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D002467:Cell Nucleus / Q000378:metabolism; D002522:Chlorocebus aethiops; D017362:Cyclic AMP Response Element-Binding Protein / Q000235:genetics / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D020868:Gene Silencing; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000378:metabolism; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D012694:Serine / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics; D019311:Staurosporine / Q000494:pharmacology; D013912:Threonine / Q000378:metabolism; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
C504923:Crtc2 protein, mouse; D017362:Cyclic AMP Response Element-Binding Protein; D009097:Multienzyme Complexes; D015534:Trans-Activators; D014157:Transcription Factors; D013912:Threonine; D012694:Serine; D017346:Protein Serine-Threonine Kinases; C509080:SIK1 protein, human; C491654:Stk11 protein, mouse; D055372:AMP-Activated Protein Kinases; D019311:Staurosporine