Modeling pancreatic cancer in vivo: from xenograft and carcinogen-induced systems to genetically engineered mice.

In the last 10 years, there has been a relative explosion of new rodent systems that recapitulate both genetic and cellular lesions that lead to the development of pancreatic cancer. These models now need to be considered when selecting an appropriate in vivo system to study disease etiology, cell signaling, and drug development. The majority of these evaluations have used transplantation of cancer cells and the use of carcinogens, which still maintain their value when investigating human cancer and epigenetic contributors. Xenograft models utilize cultured or primary pancreatic cancer cells that are placed under the skin or implanted within the pancreas of immunocompromised mice. Carcinogen-induced systems rely on administration of certain chemicals to generate cellular changes that rapidly lead to pancreatic cancer. Genetically modified mice are more advanced in their design in that relevant genetic mutations can be inserted into mouse genomic DNA in both a conditional and inducible manner. Generation of mice that develop spontaneous pancreatic cancer from a targeted genetic mutation is a valuable research tool, considering the broad spectrum of genes and cell targets that can be used, producing a variety of neoplastic lesions and cancer that can reflect many aspects of human pancreatic ductal adenocarcinoma.

Increased expression of the Akt/PKB inhibitor TRB3 in osteoarthritic chondrocytes inhibits insulin-like growth factor 1-mediated cell survival and proteoglycan synthesis.

OBJECTIVE: The chondrocyte response to insulin-like growth factor 1 (IGF-1) is reduced with aging and in osteoarthritis (OA). IGF-1 signals through the phosphatidylinositol 3-kinase/Akt pathway. TRB3, a tribbles homolog, has been shown to inhibit IGF-1-mediated activation of Akt in HEK 293 cells. This study was undertaken to determine if TRB3 is expressed in chondrocytes, and whether the chondrocyte response to IGF-1 is reduced by TRB3. METHODS: Human articular cartilage was obtained from normal tissue donors and from patients with OA at the time of knee replacement surgery. TRB3 was assessed in the tissue samples by reverse transcription-polymerase chain reaction, immunoblotting, and immunohistochemistry. Overexpression of TRB3 was induced by transient transfection to determine the effects of TRB3 on cell survival and proteoglycan synthesis. RESULTS: TRB3 messenger RNA was detected in normal human chondrocytes. TRB3 protein levels were low in cells from normal cartilage but significantly increased in cells from OA cartilage. Incubation with 2 agents that induce endoplasmic reticulum stress, tunicamycin and thapsigargin, increased TRB3 levels in normal cells. Overexpression of TRB3 inhibited Akt phosphorylation and reduced chondrocyte survival and proteoglycan synthesis. CONCLUSION: These results are the first to demonstrate that TRB3 is present in human chondrocytes, and that the level of TRB3 is increased in OA cartilage and in isolated OA chondrocytes. Because it is an inhibitor of Akt activation, elevated TRB3 production could play a role in the increased cell death and reduced response to IGF-1 observed in OA cartilage.

IGF-I stimulation of proteoglycan synthesis by chondrocytes requires activation of the PI 3-kinase pathway but not ERK MAPK.

The IGF-I (insulin-like growth factor-I) signalling pathway responsible for regulation of proteoglycan synthesis in chondrocytes has not been defined and is the focus of the present study. Chondrocytes isolated from normal human articular cartilage were stimulated with IGF-I in monolayer culture or in suspension in alginate. IGF-I activated members of both the PI3K (phosphoinositide 3-kinase) pathway and the ERK (extracellular-signal-regulated kinase)/MAPK (mitogen-activated protein kinase) pathway. The PI3K inhibitors LY294002 and wortmannin blocked IGF-I-stimulated Akt phosphorylation without blocking ERK phosphorylation and this was associated with complete inhibition of proteoglycan synthesis. A decrease in IGF-I-stimulated proteoglycan synthesis was also observed upon inhibition of mTOR (mammalian target of rapamycin) and p70S6 kinase, both of which are downstream of Akt. The MEK (MAPK/ERK kinase) inhibitors PD98059 and U0126 blocked IGF-I-stimulated ERK phosphorylation but did not block the phosphorylation of Akt and did not decrease proteoglycan synthesis. Instead, in alginate- cultured chondrocytes, the MEK inhibitors increased IGF-I-stimulated proteoglycan synthesis when compared with cells treated with IGF-I alone. This is the first study to demonstrate that IGF-I stimulation of the PI3K signalling pathway is responsible for the ability of IGF-I to increase proteoglycan synthesis. Although IGF-I also activates the ERK/MAPK pathway, ERK activity is not required for proteoglycan synthesis and may serve as a negative regulator.