Hexose transport in undifferentiated and differentiated BALB/c 3T3 preadipose cells: effects 12-O-tetradecanoylphorbol-13-acetate and insulin.

Abstract

AbstractThe effect of the phorbol diester 12‐0‐tetradecanoylphorbol‐13‐acetate (TPA) on hexose transport in undifferentiated and differentiated BALB/c 3T3 preadipose cells was studied. Additon of TPA to undifferentiated or fully differentiated cultures resulted in an increased rate of both 2‐deoxyglucose uptake and 3‐0‐methylglucose transport; the time course and maximal stimulation differed for each type of culture and for each hexose. In confluent, undifferentiated cells, half‐maximal stimulation of 2‐deoxyglucose uptake occurred at 3 nM TPA, while the half‐maximal stimulation of 3–0‐methylglucose occurred at 30 nM. Epidermal growth factor and fetal bovine serum increased 2‐deoxyglucose uptake in undifferentiated cells, while insulin did not. Insulin did, however, stimulate 3–0‐methylglucose transport in differentiated cells. From dose‐response curves in differentiated cells, halfmaximally effective concentrations were 0.17 nM for insulin and 30 nM for TPA. At optimal concentrations and incubation times for each, TPA was significantly more effective than insulin in stimulating hexose transport in differentiated cells. It was also shown that insulin could further increase hexose transport in maximally stimulated TPA‐treated cells. Cycloheximide inhibited by 75% the increase in hexose transport by TPA in differentiated cells, while having no effect on the response of these cells to insulin. In differentiated cells, chronic exposure to insulin abolished the ability of these cells to respond acutely to insulin addition but they could still respond to TPA. On the other hand, differentiated cells exposed continuously to TPA for 5 days retained the ability to activate 3–0‐methylglucose transport after either TPA or insulin addition. These results demonstrate that TPA can stimulate hexose transport directly in both undifferentiated and differentiated 3T3 cells and suggest that TPA and insulin affect transport by different mechanisms.