No abstract

Transforming growth factor-beta (TGF-beta) and TGF-beta-related factors induce apoptosis in a variety of tissues; however, the mechanism underlying this induction is largely unknown. Here, we demonstrate that TGF-beta induces the expression of the death-associated protein kinase (DAP-kinase) as an immediate early response in cells that undergo apoptosis in response to TGF-beta. DAP-kinase is a positive mediator of apoptosis induced by certain cytokines and oncogenes. We show that the DAP-kinase promoter is activated by TGF-beta through the action of Smad2, Smad3 and Smad4. Overexpression of DAP-kinase triggers apoptosis in the absence of TGF-beta, whereas inhibition of DAP-kinase activity protects cells from TGF-beta-induced apoptosis, blocks TGF-beta-induced release of cytochrome c from mitochondria and prevents TGF-beta-induced dissipation of the mitochondrial membrane potential. Our findings indicate that DAP-kinase mediates TGF-beta-dependent apoptosis by linking Smads to mitochondrial-based pro-apoptotic events.

Death-associated protein kinase (DAPK) is a death domain-containing serine/threonine kinase, and participates in various apoptotic paradigms. Here, we identify the extracellular signal-regulated kinase (ERK) as a DAPKinteracting protein. DAPK interacts with ERK through a docking sequence within its death domain and is a substrate of ERK. Phosphorylation of DAPK at Ser 735 by ERK increases the catalytic activity of DAPK both in vitro and in vivo. Conversely, DAPK promotes the cytoplasmic retention of ERK, thereby inhibiting ERK signaling in the nucleus. This reciprocal regulation between DAPK and ERK constitutes a positive feedback loop that ultimately promotes the apoptotic activity of DAPK. In a physiological apoptosis system where ERK-DAPK interplay is reinforced, downregulation of either ERK or DAPK suppresses such apoptosis. These results indicate that bidirectional signalings between DAPK and ERK may contribute to the apoptosis-promoting function of the death domain of DAPK.

Transforming growth factor-beta (TGF-beta) is a multifunctional protein that regulates cell proliferation and differentiation and extracellular matrix production. Although two receptor types, the type I and type II receptors, have been implicated in TGF-beta-induced signaling, it is unclear how the many activities of TGF-beta are mediated through these receptors. With the use of cells overexpressing truncated type II receptors as dominant negative mutants to selectively block type II receptor signaling, the existence of two receptor pathways was shown. The type II receptors, possibly in conjunction with type I receptors, mediate the induction of growth inhibition and hypophosphorylation of the retinoblastoma gene product pRB. The type I receptors are responsible for effects on extracellular matrix, such as the induction of fibronectin and plasminogen activator inhibitor I, and for increased JunB expression. Selective inactivation of the type II receptors alters the TGF-beta response in a similar manner to the functional inactivation of pRB, suggesting a role for pRB in the type II, but not the type I, receptor pathway.

Metastasis is the major cause of poor prognosis in colorectal cancer (CRC), and increasing evidence supports the contribution of miRNAs to cancer progression. Here, we found that high expression of miR-103 and miR-107 (miR-103/107) was associated with metastasis potential of CRC cell lines and poor prognosis in patients with CRC. We showed that miR-103/107 targeted the known metastasis suppressors death-associated protein kinase (DAPK) and Kr€ uppel-like factor 4 (KLF4) in CRC cells, resulting in increased cell motility and cell-matrix adhesion and decreased cell-cell adhesion and epithelial marker expression. miR-103/107 expression was increased in the presence of hypoxia, thereby potentiating DAPK and KLF4 downregulation and hypoxiainduced motility and invasiveness. In mouse models of CRC, miR-103/107 overexpression potentiated local invasion and liver metastasis effects, which were suppressed by reexpression of DAPK or KLF4. miR-103/107-mediated downregulation of DAPK and KLF4 also enabled the colonization of CRC cells at a metastatic site. Clinically, the signature of a miR-103/107 high, DAPK low, and KLF4 low expression profile correlated with the extent of lymph node and distant metastasis in patients with CRC and served as a prognostic marker for metastasis recurrence and poor survival. Our findings therefore indicate that miR-103/107-mediated repression of DAPK and KLF4 promotes metastasis in CRC, and this regulatory circuit may contribute in part to hypoxia-stimulated tumor metastasis. Strategies that disrupt this regulation might be developed to block CRC metastasis. Cancer Res; 72(14); 3631-41. Ó2012 AACR.

SUMMARY Pin1 is a phospho-specific prolyl isomerase that regulates numerous key signaling molecules and whose deregulation contributes to disease notably cancer. However, since prolyl isomerases are often believed to be constitutively active, little is known whether and how Pin1 catalytic activity is regulated. Here we identify death associated protein kinase 1 (DAPK1), a known tumor suppressor, as a kinase responsible for phosphorylation of Pin1 on Ser71 in the catalytic active site. Such phosphorylation fully inactivates Pin1 catalytic activity and inhibits its nuclear location. Moreover, DAPK1 inhibits the ability of Pin1 to induce centrosome amplification and cell transformation. Finally, Pin1 pSer71 levels are positively correlated with DAPK1 levels and negatively with centrosome amplification in human breast cancer. Thus, phosphorylation of Pin1 Ser71 by DAPK1 inhibits its catalytic activity and cellular function, providing strong evidence for an essential role of the Pin1 enzymatic activity for its cellular function.

Brk (for breast tumor kinase) is a nonreceptor tyrosine kinase containing SH3, SH2, and tyrosine kinase catalytic domains. Brk was originally identified from a human metastatic breast tumor, and its overexpression is frequently observed in breast cancer and several other cancer types. However, the molecular mechanism by which this kinase participates in tumorigenesis remains poorly characterized. In the present study, we not only identified paxillin as the binding partner and substrate of Brk but also discovered a novel signaling pathway by which Brk mediates epidermal growth factor (EGF)-induced paxillin phosphorylation. We show that EGF stimulation activates the catalytic activity of Brk, which in turn phosphorylates paxillin at Y31 and Y118. These phosphorylation events promote the activation of small GTPase Rac1 via the function of CrkII. Through this pathway, Brk is capable of promoting cell motility and invasion and functions as a mediator of EGF-induced migration and invasion. In accordance with these functional roles, Brk translocates to membrane ruffles, where it colocalizes with paxillin during cell migration. Together, our findings identify novel signaling and biological roles of Brk and indicate the first potential link between Brk and metastatic malignancy.Unraveling the signaling pathways responsible for the establishment of a metastatic phenotype in carcinoma cells is of crucial importance for the understanding of the pathology of cancer. The process of metastasis includes several components, such as the ability to invade through acquisition of cell motility, degradation of extracellular matrix and basement membrane, cell proliferation, and survival signaling. Aberrant tyrosine kinase signaling via stimulation of growth factor receptors or intracellular tyrosine kinases has been shown to contribute to various steps of tumor development and progression, including metastasis (6). Brk is an intracellular tyrosine kinase that was identified in a study for screening kinases expressed in human metastatic breast tumors (36). In addition to a typical tyrosine kinase domain, Brk possesses both SH3 and SH2 domains and thus is related to Src family kinases (36). However, unlike Src family kinases, Brk lacks an N-terminal consensus sequence for myristoylation and membrane association (36). Its genomic structure is also distinct from that of Src family kinases, suggesting that Brk has diverged significantly from Src kinases in evolution (37). The expression pattern and subcellular localization of Brk have suggested its role in tumorigenesis. In normal tissues, the expression of Brk or its mouse ortholog Sik is restricted to differentiating epithelial cells of the skin and gastrointestinal tract (27). However, it is highly expressed in many breast carcinoma cell lines and a significant portion of breast tumor tissues but not in human mammary epithelial cells (3, 34, 36) and mouse mammary glands at various developmental stages (27). Elevated expression of Brk has also been detected in metastatic melanoma cell li...

Activation of tumor suppressors for the treatment of human cancer has been a long sought, yet elusive, strategy. PTEN is a critical tumor suppressive phosphatase that is active in its dimer configuration at the plasma membrane. Polyubiquitination by the ubiquitin E3 ligase WWP1 (WW domain–containing ubiquitin E3 ligase 1) suppressed the dimerization, membrane recruitment, and function of PTEN. Either genetic ablation or pharmacological inhibition of WWP1 triggered PTEN reactivation and unleashed tumor suppressive activity. WWP1 appears to be a direct MYC (MYC proto-oncogene) target gene and was critical for MYC-driven tumorigenesis. We identified indole-3-carbinol, a compound found in cruciferous vegetables, as a natural and potent WWP1 inhibitor. Thus, our findings unravel a potential therapeutic strategy for cancer prevention and treatment through PTEN reactivation.