Abstract
Pancreatic cancer remains extremely difficult to treat, with the average lifespan following diagnosis being only 3-6 months, resulting in a death to incidence ratio of 0.94. A major reason for this high mortality rate is resistance to the main chemotherapeutic agent used to treat this disease, gemcitabine. Alterations in nucleoside and gemcitabine metabolism, specifically over-expression of ribonucleotide reductase, have been implicated as a major mechanism of resistance to this drug. Here, we show that inhibition of sphingosine kinase-2 by the specific inhibitor ABC294640 is synergistically cytotoxic with gemcitabine toward three human pancreatic cancer cell lines. Treatment with ABC294640 results in decreased expression of both RRM2 and MYC in all three cell lines. Additionally, expression of c-Myc protein and phosphorylation of Rb at S780 both decrease in a dose-dependent manner in response to ABC294640, while acetylation of H3-K9 and p21 levels increase. Pretreatment with the protein phosphatase 1 inhibitor okadaic acid or the ceramide synthase inhibitor fumonisin B1 fails to prevent the effects of ABC294640 on Rb phosphorylation. These data indicate a role for sphingosine kinase-2 in E2F and c-Myc mediated transcription through alteration of histone acetylation and p21 expression. These effects of ABC294640 suggest that it may be an effective agent for pancreatic cancer, particularly in combination with gemcitabine.
Highlights
For the last ten years the incidence rate of pancreatic cancer has been rising in the US while the five year survival rate remains near 6% [1]
Each cell line was treated with multiple concentrations of ABC294640 and gemcitabine and cell survival was quantified after 96 hours
Because elevation of p21 is associated with G1 arrest, we examined the effect of ABC294640 on the cell cycle distributions of the three pancreatic cancer cell lines
Summary
For the last ten years the incidence rate of pancreatic cancer has been rising in the US while the five year survival rate remains near 6% [1]. The two main reasons for this alarming statistic are the lack of early warning signs of the disease which leads to a delay in diagnosis, often after the cancer has already grown beyond the borders of the pancreas, and resistance to the mainline chemotherapeutic drug, gemcitabine, a difluorinated nucleoside analog of deoxycytidine. A gemcitabine resistant cell line generated by incremental increases in exposure to gemcitabine was found to have a 9and 2-fold increase in RRM2 mRNA and protein expression, respectively [5]. Expression levels of RRM2 in tumors were shown to be predictive of treatment responsiveness to gemcitabine [6]. The inhibition of RRM2 by RNAi technology [7] or exposure to flavopiridol [8], an upstream inhibitor of RRM2 transcription, has been shown to restore sensitivity to gemcitabine
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