The growth factor-like effects of tumor necrosis factor-alpha. Stimulation of glucose transport activity and induction of glucose transporter and immediate early gene expression in 3T3-L1 preadipocytes

Abstract

In the present study, the ability of tumor necrosis factor-alpha (TNF) to stimulate hexose transport in quiescent 3T3-L1 fibroblasts has been examined. Activation of transport occurred in a dose- and time-dependent manner, with maximal stimulation (6-8-fold) observed 16 h after exposure to 2.5 nM TNF. Early activation of hexose transport by TNF (2-fold within 30 min) was associated with increased plasma membrane immunoreactive glucose transporters. Prolonged exposure to TNF (16 h) resulted in a 2-fold increase in glucose transporter content of both plasma and inner membrane compartments. The magnitude of increased glucose transport (6-8-fold) was greater than the increased content of plasma membrane glucose transporters (2-fold), suggesting that the TNF-treatment altered the intrinsic activity of the glucose transporters. Increased transcription of the glucose transporter (GLUT-1) gene, as well as several immediate-early genes (c-fos, c-jun, jun-B, and beta-actin) was observed within 15 min of exposure to TNF. Transcriptional activation of immediate-early genes was tightly coupled to subsequent accumulation of their respective mRNAs. However, increased GLUT-1 mRNA (8 h after TNF treatment) was due to an apparent 3-fold increase in the stability of this message and not to increased transcription. The time course of TNF-induced hexose transport occurred concomitant with a 6-fold increase in total RNA synthesis which preceded a 3-fold increase in protein synthesis. Moreover, TNF induced cell-cycle progression through S-phase, as measured by aphidicolin-sensitive thymidine uptake. Phorbol myristate acetate also stimulated hexose transport as well as expression of the GLUT-1 gene and several immediate-early genes in quiescent 3T3-L1 cells. TNF-induced immediate-early gene expression was intact in PMA-pretreated cells (with the exception of GLUT-1 and beta-actin genes where the response was muted), suggesting the involvement of multiple pathways in TNF signal transduction. Our results indicate that TNF initiates mitogenic events in quiescent 3T3-L1 fibroblasts reminiscent of serum-derived growth factors.