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Endothelin-stimulated Ca2+ mobilization by 3T3-L1 adipocytes is suppressed by tumor necrosis factor-alpha
1Diabetes-Endocrinology Research Center and Veterans Affairs Medical Center, University of Iowa, Iowa City, Iowa, 52245, USA. myorek@icva.gov
Abstract:
The cytokine tumor necrosis factor-alpha (TNFalpha) contributes to metabolic changes in disease states such as insulin resistance. However, the mechanism by which TNFalpha alters cellular function in these conditions is poorly understood. Because changes in intracellular calcium concentration plays a critical role in hormone action we investigated the effect of TNFalpha on calcium homeostasis in 3T3-L1 adipocytes. In these studies we show that TNFalpha causes a concentration- and time-dependent decrease in Na+/myo-inositol cotransporter (SMIT) mRNA levels and myo-inositol accumulation as well as a decrease in myo-inositol incorporation into phosphoinositides. These changes coincided with a decrease in endothelin-1-induced phosphatidylinositol (PI) cycle activity in 3T3-L1 adipocytes chronically exposed to TNFalpha. Endothelin-1-induced mobilization of calcium from intracellular stores was also diminished by TNFalpha. The effect of TNFalpha on endothelin-1-induced PI cycle activity and calcium mobilization was not due to a decrease in endothelin receptors. However, TNFalpha did cause a moderate decrease in phosphatidylinositol 4,5-bisphosphate (PIP2)-specific phospholipase C (PLC) activity in 3T3-L1 adipocytes. Combined, a decrease in phosphoinositide production and PIP2-specific PLC activity could be responsible for altering PI cycle activity and the generation of the second messenger myo-inositol 1,4,5-trisphosphate, thereby reducing calcium mobilization. Such changes in intracellular signaling may contribute to the pathophysiology of insulin resistance associated with TNFalpha.
Insights
Tumor necrosis factor-alpha (TNFalpha) disrupts calcium homeostasis in fat cells, reducing myo-inositol signaling. This impacts cellular function and may contribute to insulin resistance.
Area of Science:
- Cell Biology
- Metabolic Disease Research
- Signaling Pathways
Background:
- Tumor necrosis factor-alpha (TNFalpha) is linked to metabolic dysfunction, including insulin resistance.
- The precise mechanisms by which TNFalpha affects cellular function in metabolic diseases remain unclear.
- Intracellular calcium concentration is crucial for hormone action and cellular signaling.
Purpose of the Study:
- To investigate the impact of TNFalpha on calcium homeostasis in 3T3-L1 adipocytes.
- To elucidate the molecular mechanisms underlying TNFalpha-induced alterations in cellular signaling.
Main Methods:
- 3T3-L1 adipocytes were treated with varying concentrations and durations of TNFalpha.
- Measurements included Na+/myo-inositol cotransporter (SMIT) mRNA levels, myo-inositol accumulation, and phosphoinositide incorporation.
- Endothelin-1-induced calcium mobilization, phosphatidylinositol (PI) cycle activity, and phospholipase C (PLC) activity were assessed.
Main Results:
- TNFalpha decreased SMIT mRNA levels, myo-inositol accumulation, and phosphoinositide incorporation in a dose- and time-dependent manner.
- Chronic TNFalpha exposure reduced endothelin-1-induced PI cycle activity and intracellular calcium mobilization.
- TNFalpha moderately decreased phosphatidylinositol 4,5-bisphosphate (PIP2)-specific PLC activity without affecting endothelin receptor levels.
Conclusions:
- TNFalpha impairs calcium homeostasis and myo-inositol signaling in adipocytes.
- Reduced phosphoinositide production and PLC activity contribute to altered PI cycle activity and diminished calcium mobilization.
- These cellular changes may play a role in the pathophysiology of insulin resistance associated with TNFalpha.