Bioactive Sphingolipids Mediate Apoptosis and Senescence in Human Pancreatic Cancer Cells

Ceramide is a second messenger involved in apoptosis, cell differentiation, and growth arrest. We hypothesize that in tumor cells, insufficient sphingomyelin (SM) limits the amount of ceramide which can be generated for the probagation of the apoptotic signal resulting from treatment with chemotherapy. As a result, we postulated that exogenous SM will facilitate apoptosis and synergize with chemotherapeutic agents. The current studies examined the response of the Panel human pancreatic cancer cell line to gemcitabine, a nucleoside analog, in the presence and absence of sub-toxic doses of SM. Cytotoxic dose response relationships demonstrated an increased chemosensitivity to gemcitabine with SM inclusion which was synergistic. Treatment with gemcitabine and/or SM led to an increased percentage of apoptotic cells with combination treatment as compared to the single agents. Gemcitabine treatment caused a decrease in the expression of anti-apoptotic bcl-2 family proteins in Panel cells, irrespective of the inclusion of SM. However, treatment of Panel cells with a combination of gemcitabine and exogenous SM resulted in a greater percentage of cells accumulating in the S phase when compared to single agent treatments. Changes in morphology and /3-galactosidase activity in cells treated with gemcitabine were consistent with senescence and gemcitabine-induced senescence was abrogated by concomitant SM treatment. Moreover, exposure of Panel cells to Cg-ceramide demonstrated ceramide induces both apoptosis and senescence in a concentration dependent manner with senescence occurring at lower concentrations and apoptosis at high concentrations. These findings support our proposed model which suggests a cell’s ability to progress through the cell cycle, undergo apoptosis, or enter into senescence is associated with intracellular ceramide levels. By supporting ceramide generation via exogenous SM addition, it is possible to increase apoptosis signaling in tumor cells and increase gemcitabine cytotoxicity. However, failure to generate sufficient signaling ceramide will redirect cells to senescence

Quantitative structure–mesothelioma potency model optimization for complex mixtures of elongated particles in rat pleura: A retrospective study

<p>Cancer potencies of mineral and synthetic elongated particle mixtures, including asbestos fibers, are influenced by changes in fiber dose composition, bioavailability, and biodurability in combination with relevant cytotoxic dose-response relationships. An extensive rat intrapleural dose characterization data set with a wide variety of elongated particles physicochemical properties facilitated statistical analyses of pleural mesothelioma response data combined from several studies for evaluation of alternative dose-response models. Utilizing logistic regression of individual elongated particle dimensional variations within each test sample, four major findings emerged: (1) Mild acid leaching provides superior prediction of tumor incidence compared to samples that were not leached; (2) sum of the elongated particle surface areas from mildly acid-leached samples provides the optimum holistic dose-response model; (3) progressive removal of dose associated with very short and/or thin elongated particles significantly degrades the resultant particle count and surface area dose-based predictive model fits; and (4) alternative biologically plausible model adjustments provide evidence for reduced potency of elongated particles with aspect ratios less than 8 and lengths greater than 80 µm. Regardless of these adjustments, the optimum predictive models strongly incorporate potency attributable to abundant short elongated particles in proportion to their surface area. Transmission electron microscopy analyses of low-temperature-ashed pleural membrane and lung tissues 5.5 mo post intrapleural exposures do not support hypotheses that short elongated particles that reach the pleural space are rapidly eliminated. Low-aspect-ratio elongated particles were still abundant in pleural membrane tissues but may have reduced potencies due to aggregation tendencies and therefore lower potential for intracellular presence.</p