Akt phosphorylation of deleted in liver cancer 1 abrogates its suppression of liver cancer tumorigenesis and metastasis.
OBJECTIVE
Deleted in liver cancer 1 (DLC1), which encodes a Rho GTPase activating protein, is a bona fide tumor suppressor in hepatocellular carcinoma. Underexpression of DLC1 in cancer has been attributed to genomic deletion and epigenetic silencing. However, the regulatory mechanism of the tumor suppressive activity of DLC1 remains elusive. In this study, we elucidated a novel post-translational modification by which the activity of DLC1 is functionally regulated.


METHODS
Molecular and biochemical approaches were employed to study Akt phosphorylation of DLC1. In vitro and in vivo functional assays were performed to elucidate the functional significance of Akt phosphorylation of DLC1.


RESULTS
Phosphorylation of ectopically expressed and endogenous DLC1 was enhanced upon insulin induction or with Akt expression in liver cancer cell lines. Conversely, addition of a phosphatidylinositol 3-kinase/Akt pathway inhibitor or silencing of Akt attenuated the phosphorylation level of DLC1. Site-directed mutagenesis was employed to replace the serine residue of the consensus Akt substrate motifs of DLC1 with alanine. S567 of DLC1 was identified as the only target of Akt phosphorylation. S567 is well conserved in all DLC family members. DLC2 was phosphorylated by Akt at the corresponding residue. Functional assays demonstrated that the S567D phosphomimetic DLC1 mutant lost its inhibitory activities in tumorigenesis and metastasis of oncogenically transformed hepatoblasts in a mouse model.


CONCLUSIONS
This study has revealed a novel post-translational modification that functionally deregulates the biologic activities of DLC1. Phosphorylation of DLC1 and DLC2 by Akt at the conserved residue points to a common regulatory mechanism of the DLC tumor suppressor family.
Ko
Frankie Chi Fat
FC
Department of Pathology, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong, China.
Chan
Lo-Kong
LK
Tung
Edmund Kwok-Kwan
EK
Lowe
Scott W
SW
Ng
Irene Oi-Lin
IO
Yam
Judy Wai Ping
JW
D000818:Animals; D002460:Cell Line; D002471:Cell Transformation, Neoplastic / Q000473:pathology*; D020690:GTPase-Activating Proteins / Q000502:physiology*; D006801:Humans; D008113:Liver Neoplasms / Q000473:pathology / Q000517:prevention & control*; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D009362:Neoplasm Metastasis / Q000517:prevention & control; D010766:Phosphorylation; D051057:Proto-Oncogene Proteins c-akt / Q000502:physiology*; D025521:Tumor Suppressor Proteins / Q000502:physiology*
C112352:DLC1 protein, human; D020690:GTPase-Activating Proteins; C473607:STARD13 protein, human; D025521:Tumor Suppressor Proteins; D051057:Proto-Oncogene Proteins c-akt