Mg²⁺- and MgATP-inhibited and Ca²⁺/calmodulin-sensitive TRPM7-like current in hepatoma and hepatocytes

Abstract

Mishra R, Rao V, Ta R, Shobeiri N, Hill CE. Mg 2ϩ -and MgATP-inhibited and Ca 2ϩ /calmodulin-sensitive TRPM7-like current in hepatoma and hepatocytes. Am J Physiol Gastrointest Liver Physiol 297: G687-G694, 2009. First published August 6, 2009 doi:10.1152/ajpgi.90683.2008.-Although understood to be ubiquitously expressed, the functional identification and significance of Mg 2ϩ -inhibited, nonspecific cation currents has been established in only a few cell types. Here we identified an outwardly rectifying nonspecific cation current in quiescent rat hepatocytes and the proliferating and polarized rat hepatoma, WIF-B. Under whole cell recording conditions in which cells were bathed and dialyzed with Nagluconate solutions, the latter Ca 2ϩ and Mg 2ϩ free, current reversed close to 0 mV, was time independent, and was greater than 10 times higher at ϩ120 mV compared with Ϫ120 mV. Outward current at Ϫ120 mV developed slowly, from 17.7 Ϯ 10.3 pA/pF at patch rupture to 106.6 Ϯ 15.6 pA/pF at 12 min in WIF-B cells, and 4.9 Ϯ 2.7 to 20.6 Ϯ 5.6 pA/pF in rat hepatocytes. The nonspecific TRP channel inhibitor, 2-aminoethoxyphenylborate (2-APB), inhibited current (IC 50 ϭ 72 Ϯ 13 M) and caused apoptotic cell death in WIF-B cells. Rat hepatocyte survival was more resistant to 2-APB. Dialysis of WIF-B cells with physiological concentrations of Mg 2ϩ and Mg-ATP, but not ATP alone, inhibited current development, suggesting that Trpm7 rather than Trpm6 underlies this current. RT-PCR demonstrated that both Trpm6 and Trpm7 are expressed at similar levels in both cell types, suggesting that the functional differences noted are not transcript dependent. Intracellular Ca 2ϩ (IC50 ϭ 125 Ϯ 35 nM) also inhibited current development, and this could be partially relieved by the calmodulin and Ca 2ϩ /calmodulin-dependent kinase inhibitors W-7, staurosporine, or (2,3,13,17,26,35,39), and pathophysiological situations such as anoxic cell death and ischemia-reperfusion injury (1, 41). Although a role for TRPC1 in stimulus-induced Ca 2ϩ influx in the major liver epithelial cell, the hepatocyte, has been suggested (5), and both TRPM6 and TRPM7 are expressed in relatively high amounts in liver tissue (7, 9, 22), Mg 2ϩ -sensitive nonselective cation currents have not been demonstrated in these cells. Nevertheless, 2-aminoethoxyphenylborate (2-APB), albeit a relatively nonspecific inhibitor of TRPM and TRPC channels and other transporters, inhibited bile generation in perfused rat livers (10), and reduced hepatic store-operated Ca 2ϩ current and ischemia-reperfusion injury (15,27).TRPM7 currents are outwardly rectifying and exhibit very small inward currents under divalent-free intracellular recording conditions due to block of inward current by external Mg 2ϩ (26, 34). These currents develop over the course of minutes following the initiation of intracellular dialysis due to the dilution of cytosolic Mg 2ϩ . Dialysis with Mg 2ϩ -containing solutions also induces a slow inactivation, or "rundown," of TRPM7 current [IC 50 0.1-0.7 mM (26,31,40)]. T...

Cholesterol-induced activation of TRPM7 regulates cell proliferation, migration, and viability of human prostate cells

“…To understand the ion channel(s) responsible for cholesterol-mediated increase in Ca 2+ entry, current recordings (whole cell) were performed in both control and cancer cells. Importantly, an inward current (that reversed around zero mV) was observed in both control RWPE1 and DU145 cells and application of cholesterol in bath solution showed an increase in inward Ca 2+ currents in both control RWPE1 (data not shown) and in DU145 cells (Figure 1E-G), which was similar as observed in MagNuM currents 19, 32 To identify the pharmacological properties of these currents, we next studied the effects of 2-APB and Mg-ATP on these cells. Mg-ATP has been shown to inhibit these MagNuM currents, whereas 2-APB has been shown to inhibit TRPM7 function, but potentiates TRPM6 function 19, 32 , that also shows similar electrophysiological properties.…”

Increase in Serum Ca2+/Mg2+ Ratio Promotes Proliferation of Prostate Cancer Cells by Activating TRPM7 Channels

“…It has been shown previously that TRPM6 currents are potentiated by the addition of 2APB, whereas TRPM7 currents are significantly inhibited by addition of similar concentrations of 2APB (32, 33). Extracellular addition of 500 μ m 2APB dramatically decreased MagNuM current amplitude in control cells (Fig.…”

Magnesium in Man: Implications for Health and Disease

“…Mg 2ϩ uptake via divalent cation channel TRPM7 has been suggested to stimulate tumor cell proliferation (203,264). TRPM7 expression is upregulated in hepatoma, pancreatic adenocarcinoma, gastric cancer, and breast cancer tissue (203,278,346,581). Although TRPM7 has been primarily described as a Mg 2ϩ channel, it is also permeable for other divalent cations (349).…”