Cholesterol modulates ion channels via down-regulation of phosphatidylinositol 4,5-bisphosphate

Ubiquitously expressed Mg²⁺-inhibitory cation (MIC) channels are permeable to Ca²⁺ and Mg²⁺ and are essential for cell viability. When membrane cholesterol level was increased by pre-incubating cells with a water-soluble form of cholesterol, the endogenous MIC current in HEK293 cells was negatively regulated. The application of phosphatidylinositol 4,5-bisphosphate (PIP₂) recovered MIC current from cholesterol effect. As PIP₂ is the direct modulator for MIC channels, high cholesterol content may cause down-regulation of PIP₂. To test this possibility, we examined the effect of cholesterol on two exogenously expressed PIP₂-sensitive K⁺ channels: human Ether-a-go-go related gene (HERG) and KCNQ. Enrichment with cholesterol inhibited HERG currents, while inclusion of PIP₂ in the pipette solution blocked the cholesterol effect. KCNQ channel was also inhibited by cholesterol. The effects of cholesterol on these channels were blocked by pre-incubating cells with inhibitors for phospholipase C, which may indicate that cholesterol enrichment induces the depletion of PIP₂ via phospholipase C activation. Lipid analysis showed that cholesterol enrichment reduced γ-³²P incorporation into PIP₂ by approximately 35%. Our results suggest that cholesterol may modulate ion channels by changing the levels of PIP ₂. Thus, an important cross-talk exists among two plasma membrane-enriched lipids, cholesterol and PIP₂.