Susanne Klein‐Scory

Smad4/DPC4 (deleted in pancreatic carcinoma, locus 4) is a tumor suppressor gene lost at high frequency in cancers of the pancreas and other gastrointestinal organs. Smad4 encodes a key intracellular messenger in the transforming growth factor beta (TGF-beta) signaling cascade. TGF-beta is a potent inhibitor of the growth of epithelial cells; thus, it has been assumed that loss of Smad4 during tumor progression relieves this inhibition. Herein, we show that restoration of Smad4 to human pancreatic carcinoma cells suppressed tumor formation in vivo, yet it did not restore sensitivity to TGF-beta. Rather, Smad4 restoration influenced angiogenesis, decreasing expression of vascular endothelial growth factor and increasing expression of thrombospondin-1. In contrast to the parental cell line and to control transfectants that produced rapidly growing tumors in vivo, Smad4 revertants induced small nonprogressive tumors with reduced vascular density. These data define the control of an angiogenic switch as an alternative, previously unknown mechanism of tumor suppression for Smad4 and identify the angiogenic mediators vascular endothelial growth factor and thrombospondin-1 as key target genes.

MicroRNAs (miRNAs) are small non-coding RNAs that are involved in different biological processes by suppressing target gene expression. Altered expression of miR-30a-5p has been reported in colon carcinoma. To elucidate its potential biological role in colon cancer, miR-30a-5p was overexpressed via a lentiviral vector system in two different colon cancer cell lines. This induced in both lines miR-30a-5p-mediated growth inhibition, attributable to a cell cycle arrest at the G(1) phase and an induction of apoptosis. Combining global gene expression analyses of miR-30a-5p transgenic line HCT116 with in silico miRNA target prediction, we identified the denticleless protein homolog (DTL) as a potential miRNA-30a-5p target. Subsequent reporter gene assays confirmed the predicted miR-30a-5p binding site in the 3'untranslated region of DTL. Importantly, overexpression of DTL in HCT116 cells partially rescued these cells from miR-30a-5p-mediated growth suppression. In addition, TP53 and CDKN1A expression were increased in miR-30a-5p-overexpressing HCT116 cells, suggesting that miR-30a-5p is able to modulate the cell cycle via a DTL-TP53-CDKN1A regulatory circuit. Finally, 379 colorectal cancer tissues were screened for DTL expression and DTL was found to be overexpressed in 95.8% of human colorectal cancers compared with normal colon mucosa. In conclusion, our data identified miR-30a-5p as a tumor-suppressing miRNA in colon cancer cells exerting its function via modulation of DTL expression, which is frequently overexpressed in colorectal cancer. Thus, our data suggest that restoring miR-30a-5p function may prove useful as therapeutic strategy for tumors with reduced miR-30a-5p expression.

Improved non-invasive strategies for early cancer detection are urgently needed to reduce morbidity and mortality. Non-coding RNAs, such as microRNAs and small nucleolar RNAs, have been proposed as biomarkers for non-invasive cancer diagnosis. Analyzing serum derived from nude mice implanted with primary human pancreatic ductal adenocarcinoma (PDAC), we identified 15 diagnostic microRNA candidates. Of those miR-1246 was selected based on its high abundance in serum of tumor carrying mice. Subsequently, we noted a cross reactivity of the established miR-1246 assays with RNA fragments derived from U2 small nuclear RNA (RNU2-1). Importantly, we found that the assay signal discriminating tumor from controls was derived from U2 small nuclear RNA (snRNA) fragments (RNU2-1f) and not from miR-1246. In addition, we observed a remarkable stability of RNU2-1f in serum and provide experimental evidence that hsa-miR-1246 is likely a pseudo microRNA. In a next step, RNU2-1f was measured by qRT-PCR and normalized to cel-54 in 191 serum/plasma samples from PDAC and colorectal carcinoma (CRC) patients. In comparison to 129 controls, we were able to classify samples as cancerous with a sensitivity and specificity of 97.7% [95% CI = (87.7, 99.9)] and 90.6% [95% CI = (80.7, 96.5)], respectively [area under the ROC curve 0.972]. Of note, patients with CRC were detected with our assay as early as UICC Stage II with a sensitivity of 81%. In conclusion, this is the first report showing that fragments of U2 snRNA are highly stable in serum and plasma and may serve as novel diagnostic biomarker for PDAC and CRC for future prospective screening studies.

Smad4 is a tumor suppressor gene primarily involved in carcinogenesis of the pancreas and colon. The functional inactivation of Smad4 is a late step genetically. In pancreatic carcinogenesis, loss of Smad4 marks the transition to invasive growth. In colorectal cancers, the frequency of Smad4 inactivation is markedly increased in metastatic cancers. We have established cell biological models, re-expressing Smad4 in deficient human cancer cells, in which we could show that Smad4 is adequate to suppress tumor growth through suppression of angiogenic and invasive properties. Thus, pairs of Smad4-re-expressing and Smad4-deficient cells are prone to model the progression from premalignant stages to carcinomas in the carcinogenic process and may provide access to Smad4 targets of high clinical relevance. We present here a "differential secretome analysis", comparing all the proteins released in vitro from the Smad4-deficient and Smad4-re-expressing SW480 human colon carcinoma cells. The differential secretome catalog comprises more than 25 proteins including proteases and protease inhibitors, as well as established tumor biomarkers. In conclusion, this approach proved to be a sensitive tool to specifically detect Smad4 targets relevant for tumor-stroma interactions. It is also able to reflect complex alterations of cellular physiology. Moreover, the results support our hypothesis that human tumor markers detectable in serum may be identified through differential secretome analyses.