Overexpression of cAMP-dependent protein kinase (PKA) type I isozyme is associated with cell proliferation and neoplastic transformation. The presence of PKA on the external surface of LS-174T human colon carcinoma cells has been shown. Here, we show that cancer cells of various cell types excrete PKA into the conditioned medium. This extracellular PKA (ECPKA) is present in active, free catalytic subunit (C subunit) form, and its activity is specifically inhibited by PKA inhibitory protein, PKI. Overexpression of the C␣ or RI␣ subunit gene of PKA in an expression vector, which upregulates intracellular PKA type I, markedly up-regulates ECPKA expression. In contrast, overexpression of the RII␤ subunit, which eliminates PKA type I, up-regulates PKA type II, and reverts the transformed phenotype, down-regulates ECPKA. A mutation in the C␣ gene that prevents myristylation allows the intracellular PKA up-regulation but blocks the ECPKA increase, suggesting that the NH 2-terminal myristyl group of C␣ is required for the ECPKA expression. In serum of cancer patients, the ECPKA expression is up-regulated 10-fold as compared with normal serum. These results indicate that the ECPKA expression is an ordered cellular response of a living cell to actively exclude excess intracellular PKA molecules from the cell. This phenomenon is up-regulated in tumor cells and has an inverse relationship with the hormone dependency of breast cancer. Thus, the extracellular PKA may serve as a potential diagnostic and prognostic marker for cancer.

Background: Glutathione peroxidase 3 was identified as a new tumor suppressor. However, the mechanism is not clear. Results: Our analysis indicates that glutathione peroxidase 3 activates p53-induced gene 3 both in vivo and in vitro. Conclusion: p53-induced gene 3 is a major mediator of GPx3-induced cell death. Significance: Glutathione peroxidase 3-p53-induced gene 3 signaling represents a novel signaling pathway for cell death.

The ubiquitously expressed Src tyrosine kinases (c-Src, c-Yes, and c-Fyn) regulate intestinal cell growth and differentiation. Src activity is also elevated in the majority of malignant and premalignant tumors of the colon. The development of fibroblasts with the three ubiquitously expressed kinases deleted (SYF cells) has identified the role of Src proteins in the regulation of actin dynamics associated with increased cell migration and invasion. Despite this, unexpectedly nothing is known about the role of the individual Src kinases on intestinal cell cytoskeleton and/or cell migration. We have previously reported that villin, an epithelial cell-specific actin-modifying protein that regulates actin reorganization, cell morphology, cell migration, cell invasion, and apoptosis, is tyrosine-phosphorylated. In this report using the SYF cells reconstituted individually with c-Src, c-Yes, c-Fyn, and wild type or phosphorylation site mutants of villin, we demonstrate for the first time the absolute requirement for c-Src in villin-induced regulation of cell migration. The other major finding of our study is that contrary to previous reports, the nonreceptor tyrosine kinase, Jak3 (Janus kinase 3), does not regulate phosphorylation of villin or villin-induced cell migration and is, in fact, not expressed in intestinal epithelial cells. Further, we identify SHP-2 and PTP-PEST (protein-tyrosine phosphatase proline-, glutamate-, serine-, and threonine-rich sequence) as negative regulators of c-Src kinase and demonstrate a new function for these phosphatases in intestinal cell migration. Together, these data suggest that in colorectal carcinogenesis, elevation of c-Src or downregulation of SHP-2 and/or PTP-PEST may promote cancer metastases and invasion by regulating villin-induced cell migration and cell invasion.Src proteins play a critical role in the proliferation and differentiation of normal intestinal epithelial cells (1). In the normal intestinal mucosa, proliferative, undifferentiated cells at the base of the crypts express high c-Src activity; in contrast, differentiated cells at the tips of intestinal villi show decreased c-Src activity (1). Deregulation of Src leads to constitutive kinase activity, which also leads to cellular transformation (2, 3). Src activation is evident in 80% of human colon cancers relative to normal colonic epithelium (4) and is highly activated in colon cancers that metastasize to the liver (4, 5). The risk of colon cancer is particularly high in patients with inflammatory bowel disease, and elevated c-Src activity has also been noted in neoplastic ulcerative colitis epithelia (6). Although up-regulation of c-Yes is less frequent, it has been noted in about 50% of colorectal carcinomas (7-10). Despite the fact that there is considerable evidence associating aberrant Src activation in the development and progression of colorectal cancer, and although inhibitors of Src family kinases have entered phase I clinical trials as anticancer agents, surprisingly few investigations have analyzed th...

Integrin α7 (ITGA7) is a tumor-suppressor gene that is critical for suppressing the growth of malignant tumors; however, the mechanisms allowing ITGA7 to suppress the growth of cancer cells remain unclear. Herein, we show that ITGA7 binds to tissue inhibitor of metalloproteinase 3 (TIMP3) in prostate cancer cells. The ITGA7-TIMP3 binding led to a decreased protein level of tumor necrosis factor α, cytoplasmic translocation of NF-κB, and down-regulation of cyclin D1. These changes led to an accumulation of cells in G0/G1 and a dramatic suppression of cell growth. Knocking down TIMP3 or ITGA7/TIMP3 binding interference largely abrogated the signaling changes induced by ITGA7, whereas a mutant ITGA7 lacking TIMP3 binding activity had no tumor-suppressor activity. Interestingly, knocking down ITGA7 ligand laminin β1 enhanced ITGA7-TIMP3 signaling and the downstream tumor-suppressor activity, suggesting the existence of a counterbalancing role between extracellular matrix and integrin signaling. As a result, this report demonstrates a novel and critical signaling mechanism of ITGA7, through the TIMP3/NF-κB/cyclin D1 pathway.