Effects of D-mannoheptulose upon D-glucose metabolism in tumoral pancreatic islet cells

D-[H-3]mannoheptulose was recently reported to be poorly taken up by tumora l pancreatic islet cells of the RINm5F and INS-1 lines. We have now investi gated the effects of D-mannoheptulose upon D-glucose metabolism in these tw o cell lines. D-mannoheptulose (1.0-10.0 mM) only caused a minor decrease o f D-glucose metabolism in RINm5F cells, whether at low (1.1 mM) or higher ( 8.3 mM) D-glucose concentration. A comparable situation was found in INS-1 cells examined after more than 20 passages. In both cases, however, the hex aacetate ester of D-mannoheptulose (5.0 mM) efficiently inhibited D-glucose metabolism. In the INS-1 cells, the relative extent of the inhibitory acti on of D-mannoheptulose upon D-glucose metabolism increased from 12.4 +/- 2. 6 to 38.3 +/- 3.8% as the number of passages was decreased from more than 2 0 to 13-15 passages, the latter percentage remaining lower, however, than t hat recorded in INS-1 cells also examined after 13-15 passages but exposed to D-mannoheptulose hexaacetate (66.9 +/- 2.2%). These findings when compar ed to our recent measurements of D-[H-3]mannoheptulose uptake, reinforce th e view that the entry of the heptose into cells and, hence, its inhibitory action on D-glucose metabolism are dictated by expression of the GLUT2 gene .