Diet induced insulin resistance is due to induction of PTEN expression.

Type 2 Diabetes (T2D) is a condition that is often associated with obesity and defined by reduced sensitivity of PI3K signaling to insulin (insulin resistance), hyperinsulinemia and hyperglycemia. Molecular causes and early signaling events underlying insulin resistance are not well understood. Insulin activation of PI3K signaling causes mTOR dependent induction of PTEN translation, a negative regulator of PI3K signaling. We speculated that insulin resistance is due to insulin dependent induction of PTEN protein that prevent further increases in PI3K signaling. Here we show that in a diet induced model of obesity and insulin resistance, PTEN levels are increased in fat, muscle and liver tissues. Onset of hyperinsulinemia and PTEN induction in tissue is followed by hyperglycemia, hepatic steatosis and severe glucose intolerance. Treatment with a PTEN phosphatase inhibitor prevents and reverses these phenotypes, whereas an mTORC1 kinase inhibitor reverses all but the hepatic steatosis. These data suggest that induction of PTEN by increasing levels of insulin elevates feedback inhibition of the pathway to a point where downstream PI3K signaling is reduced and hyperglycemia ensues. PTEN induction is thus necessary for insulin resistance and the type 2 diabetes phenotype and a potential therapeutic target.

ARAF protein kinase activates RAS by antagonizing its binding to RASGAP NF1.

RAF protein kinases are effectors of the GTP-bound form of small guanosine triphosphatase RAS and function by phosphorylating MEK. We showed here that the expression of ARAF activated RAS in a kinase-independent manner. Binding of ARAF to RAS displaced the GTPase-activating protein NF1 and antagonized NF1-mediated inhibition of RAS. This reduced ERK-dependent inhibition of RAS and increased RAS-GTP. By this mechanism, ARAF regulated the duration and consequences of RTK-induced RAS activation and supported the RAS output of RTK-dependent tumor cells. In human lung cancers with EGFR mutation, amplification of ARAF was associated with acquired resistance to EGFR inhibitors, which was overcome by combining EGFR inhibitors with an inhibitor of the protein tyrosine phosphatase SHP2 to enhance inhibition of nucleotide exchange and RAS activation.

Regulation of PTEN translation by PI3K signaling maintains pathway homeostasis.

The PI3K pathway regulates cell metabolism, proliferation, and migration, and its dysregulation is common in cancer. We now show that both physiologic and oncogenic activation of PI3K signaling increase the expression of its negative regulator PTEN. This limits the duration of the signal and output of the pathway. Physiologic and pharmacologic inhibition of the pathway reduces PTEN and contributes to the rebound in pathway activity in tumors treated with PI3K inhibitors and limits their efficacy. Regulation of PTEN is due to mTOR/4E-BP1-dependent control of its translation and is lost when 4E-BP1 is deleted. Translational regulation of PTEN is therefore a major homeostatic regulator of physiologic PI3K signaling and plays a role in reducing the pathway activation by oncogenic PIK3CA mutants and the antitumor activity of PI3K pathway inhibitors. However, pathway output is hyperactivated in tumor cells with coexistent PI3K mutation and loss of PTEN function.

Mucin 16 (cancer antigen 125) expression in human tissues and cell lines and correlation with clinical outcome in adenocarcinomas of the pancreas, esophagus, stomach, and colon.

Mucin 16 (cancer antigen 125) is a cell surface glycoprotein that plays a role in promoting cancer cell growth in ovarian cancer. The aims of this study were to examine mucin 16 expression in a large number of digestive tract adenocarcinomas and precursors and to determine whether mucin 16 up-regulation is correlated with patient outcome. Tissue microarrays were constructed using surgical resection tissues and included pancreatic (115 normal, 29 precursors, 200 pancreatic ductal adenocarcinomas), esophageal (86 normal, 104 precursors, 95 esophageal adenocarcinomas, 35 lymph node metastases), gastric (211 normal, 8 precursors, 119 gastric adenocarcinomas, 62 lymph node metastases), and colorectal (34 normal, 17 precursors, 39 colorectal adenocarcinomas) tissues. Mucin 16 was detected in 81.5%, 69.9%, 41.2%, and 64.1% of the pancreatic ductal adenocarcinomas, esophageal adenocarcinomas, gastric adenocarcinomas, and colorectal adenocarcinomas, respectively. Mucin 16 was seen in a subset of the precursors. On multivariate analysis, moderate/diffuse mucin 16 in pancreatic ductal adenocarcinomas was strongly associated with poor survival (P < .001), independent of other prognosis predictors. A similar trend was observed for esophageal adenocarcinomas (P = .160) and gastric adenocarcinomas (P = .080). Focal mucin 16 in colorectal adenocarcinomas was significantly correlated (P = .044) with a better patient outcome, when compared with mucin 16-negative cases. Using Western blot analysis, we found mucin 16 expression in 3 of 6 pancreatic ductal adenocarcinoma and 1 of 2 esophageal adenocarcinoma cell lines. We conclude that most of the digestive tract adenocarcinomas and a subset of their precursors express mucin 16. Mucin 16 expression is an independent predictor of poor outcome in pancreatic ductal adenocarcinomas and potentially in esophageal adenocarcinomas and gastric adenocarcinomas. We propose that mucin 16 may function as a prognostic marker and therapeutic target in the future.

Molecular determinants of retinoic acid sensitivity in pancreatic cancer.

PURPOSE: To identify a predictive molecular "signature" for sensitivity to retinoic acid in pancreatic cancer. EXPERIMENTAL DESIGN: Fourteen patient-derived, low-passage pancreatic ductal adenocarcinoma (PDAC) lines with varied expression of fatty acid-binding protein 5 (FABP5) and cellular retinoic acid-binding protein 2 (CRABP2) were used to evaluate the response to all-trans retinoic acid (ATRA). Cell proliferation, apoptosis, and migration/invasion assays were used to measure the in vitro response. Tumor growth was monitored in subcutaneous xenografts in athymic nude mice for 4 weeks. RESULTS: Response to ATRA was observed to be dependent upon differential expression of FABP5 versus CRABP2. Thus, elevated FABP5 expression was associated with minimal cytotoxicity and tumor growth inhibition and a paradoxical increase in migration and invasion. Conversely, CRABP2 expression in the absence of FABP5 was associated with significant tumor growth inhibition with ATRA, even in gemcitabine-resistant tumors. The ATRA-resistant phenotype of FABP5(high)CRABP2(null) cells could be circumvented by ectopic expression of CRABP2. Alternatively, reexpression of endogenous CRABP2 could be enabled in FABP5(high)CRABP2(null) PDAC lines by exposure to decitabine and trichostatin A, thereby relieving epigenetic silencing of the CRABP2 gene promoter. Immunohistochemical staining for FABP5 in archival human tissue microarrays identifies a subset of cases (13 of 63, ~20%) which are negative for FABP5 expression and might be candidates for ATRA therapy. CONCLUSIONS: The widely used agent ATRA deserves a "second look" in PDAC, but needs to be targeted to patient subsets with biopsy-proven FABP5-negative tumors, or be combined with a chromatin-modifying agent to reexpress endogenous CRABP2.