Smad4 Decreases the Population of Pancreatic Cancer-Initiating Cells through Transcriptional Repression of ALDH1A1.

Cancer progression involves a rare population of undifferentiated cancer-initiating cells that have stem cell-like properties for self-renewal capacity and high tumorigenicity. We investigated how maintenance of pancreatic cancer-initiating cells is influenced by Smad4, which is frequently deleted or mutated in pancreatic cancers cells. Smad4 silencing up-regulated the expression of aldehyde dehydrogenase 1A1 (ALDH1A1) mRNA, whereas forced expression of Smad4 in pancreatic cancer cells down-regulated it. Smad4 and ALDH1 expression inversely correlated in some human clinical pancreatic adenocarcinoma tissues, suggesting that ALDH1 in pancreatic cancer cells was associated with decreased Smad4 expression. We then examined whether ALDH1 served as a marker of pancreatic cancer-initiating cells. Pancreatic cancer cells contained ALDH1(hi) cells in 3% to 10% of total cells, with high tumorigenic potential. Because Smad4 is a major mediator of transforming growth factor (TGF)-ß family signaling, we investigated the regulatory mechanism of ALDH activity by TGF-ß and bone morphogenetic proteins. Treatment with TGF-ß attenuated ALDH1(hi) cells in several pancreatic cancer cells, whereas bone morphogenetic protein-4 was not as potent. Biochemical experiments revealed that TGF-ß regulated ALDH1A1 mRNA transcription through binding of Smad4 to its regulatory sequence. It appears that TGF-ß negatively regulates ALDH1 expression in pancreatic cancer cells in a Smad-dependent manner and in turn impairs the activity of pancreatic cancer-initiating cells.

Autocrine TGF-ß protects breast cancer cells from apoptosis through reduction of BH3-only protein, Bim.

Cancer cells undergo multi-step processes in obtaining the ability to metastasize, and are constantly exposed to signals that induce apoptosis. Acquisition of anti-apoptotic properties by cancer cells is important for metastasis, and recent studies suggest that transforming growth factor (TGF)-ß promotes the survival of certain types of cancer cells. Here, we found that in highly metastatic breast cancer cells, JygMC(A), JygMC(B) and 4T1, TGF-ß ligands were produced in autocrine fashion. Pharmacological inhibition of endogenous TGF-ß signalling by a TGF-ß type I receptor kinase inhibitor in serum-free conditions increased the expression of BH3-only protein, Bim (also known as Bcl2-like 11) in JygMC(A) and JygMC(B) cells, and caused apoptotic cell death. We also found that induction of Bim by TGF-ß was not observed in Foxc1 knocked-down cancer cells. These findings suggest that TGF-ß plays a crucial role in the regulation of survival of certain types of cancer cells through the TGF-ß-Foxc1-Bim pathway, and that specific inhibitors of TGF-ß signalling might be useful as apoptosis inducers in breast cancer cells.

A novel RNA-binding protein, Ossa/C9orf10, regulates activity of Src kinases to protect cells from oxidative stress-induced apoptosis.

During the process of tumor progression and clinical treatments, tumor cells are exposed to oxidative stress. Tumor cells are frequently resistant to such stress by producing antiapoptotic signaling, including activation of Src family kinases (SFKs), although the molecular mechanism is not clear. In an attempt to identify the SFK-binding proteins selectively phosphorylated in gastric scirrhous carcinoma, we identified an uncharacterized protein, C9orf10. Here we report that C9orf10 (designated Ossa for oxidative stress-associated Src activator) is a novel RNA-binding protein that guards cancer cells from oxidative stress-induced apoptosis by activation of SFKs. Exposure to oxidative stress such as UV irradiation induces the association of Ossa/C9orf10 with regulatory domains of SFKs, which activates these kinases and causes marked tyrosine phosphorylation of C9orf10 in turn. Tyrosine-phosphorylated Ossa recruits p85 subunits of phosphatidylinositol 3-kinase (PI3-kinase) and behaves as a scaffolding protein for PI3-kinase and SFKs, which activates the Akt-mediated antiapoptotic pathway. On the other hand, the carboxyl terminus of Ossa has a distinct function that directly binds RNAs such as insulin-like growth factor II (IGF-II) mRNA and promotes the extracellular secretion of IGF-II. Our findings indicate that Ossa is a dual-functional protein and might be a novel therapeutic target which modulates the sensitivity of tumors to oxidative stress.