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A role for phospholipase C activity in GLUT4-mediated glucose transport
M Van Epps-Fung1, K Gupta, R W Hardy
1Department of Pathology, University of Alabama at Birmingham, 35294, USA.
Abstract:
Overexpression of surrogate receptors [epidermal growth factor (EGF) receptor (EGFR) and platelet-derived growth factor receptor] in adipocytes has demonstrated that multiple signaling pathways may lead to GLUT4-mediated glucose uptake. These implicated pathways function independently of IRS-1 phosphorylation and PI3-kinase activation. In addition, we previously demonstrated that EGFR tyrosyl autophosphorylation is required to stimulate GLUT4-mediated glucose transport in 3T3-L1 adipocytes. This observation suggests that signaling molecules that are dependent on EGFR autophosphorylation, such as phospholipase C (PLC), may lie in the signaling pathway to glucose transport. As PLC has been implicated in glucose transport by several clinical and basic mechanistic studies, we investigated whether EGFR signaling may promote glucose transport via modulation of PLC activity. Activation of EGFR overexpressing 3T3-L1 adipocytes leads to a 3.4 +/- 1.2-fold stimulation of PLC activity over basal levels vs. only 1.06 +/- 0.01-fold stimulation by insulin. Pharmacological inhibition of PLC by 50 microM U73122 reduced phosphoinositide accumulation by 79.2 +/- 16.9% and resulted in a concomitant 56.0 +/- 12.7% decrease in EGF-induced glucose transport. This inhibition of glucose transport by U73122 was specific, because the inactive congener, U73343, failed to block EGF-induced glucose transport. Despite the low levels of insulin-induced PLC activity, insulin-stimulated glucose transport activity was similarly inhibited by U73122 (55.9 +/- 13.1% inhibition). Inhibition of PLC activation did not impair either EGF- or insulin-induced activation of glycogen synthase or incorporation of glucose into lipid, supporting the hypothesis that both EGF- and insulin-induced glucose disposal can be independent of GLUT4-mediated glucose transport. The diminution of glucose transport secondary to inhibition of PLC activity was reflected by a decrease in GLUT4 translocation to the plasma membrane upon either EGF or insulin stimulation. These results are consistent with either a permissive or an active role for PLC activity in the translocation of GLUT4 to the plasma membrane.
Insights
Epidermal growth factor receptor (EGFR) signaling stimulates glucose uptake in adipocytes by activating phospholipase C (PLC). This pathway is crucial for glucose transporter 4 (GLUT4) translocation, independent of insulin signaling pathways.
Area of Science:
- Cellular metabolism
- Molecular signaling pathways
- Adipocyte biology
Background:
- Overexpression of epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor in adipocytes suggests multiple pathways for glucose uptake.
- EGFR autophosphorylation is essential for stimulating glucose transporter 4 (GLUT4)-mediated glucose transport in 3T3-L1 adipocytes.
- Phospholipase C (PLC) is implicated in glucose transport and may be downstream of EGFR signaling.
Purpose of the Study:
- To investigate if EGFR signaling promotes glucose transport through modulation of PLC activity.
- To determine the role of PLC in both EGF- and insulin-stimulated glucose transport and GLUT4 translocation.
Main Methods:
- Overexpression of EGFR in 3T3-L1 adipocytes.
- Measurement of PLC activity following EGF stimulation.
- Pharmacological inhibition of PLC using U73122 and assessment of EGF- and insulin-induced glucose transport, glycogen synthase activation, and glucose incorporation into lipids.
- Analysis of GLUT4 translocation to the plasma membrane.
Main Results:
- EGF stimulation significantly increased PLC activity in EGFR-overexpressing adipocytes, more so than insulin.
- Inhibition of PLC by U73122 reduced EGF-induced glucose transport and GLUT4 translocation, with specificity confirmed by U73343.
- Insulin-stimulated glucose transport was also inhibited by U73122, indicating a role for PLC in both signaling pathways.
- Inhibition of PLC did not affect EGF- or insulin-induced glycogen synthase activation or glucose incorporation into lipids.
Conclusions:
- EGFR signaling activates PLC, which plays a role in promoting glucose transport via GLUT4 translocation in adipocytes.
- PLC activity is involved in both EGF- and insulin-stimulated glucose transport, suggesting a convergence point in signaling.
- The findings support a permissive or active role for PLC in the translocation of GLUT4 to the plasma membrane.