Modulation of glioma BK channels via erbB2

Abstract

Glioma cells show up‐regulation and constitutive activation of erbB2, and its expression correlates positively with increased malignancy. A similar correlation has been demonstrated for the expression of gBK, a calcium‐sensitive, large‐conductance K⁺ channel. We show here that glioma BK channels are a downstream target of erbB2/neuregulin signaling. Tyrphostin AG825 was able to disrupt the constituitive erbB2 activation in a dose‐dependent manner, causing a 30‐mV positive shift in gBK channel activation in cell‐attached patches. Conversely, maximal stimulation of erbB2 with a recombinant neuregulin (NRG‐1β) caused a 12‐mV shift in the opposite direction. RT‐PCR studies reveal no change in the BK splice variants expressed in treated glioma cells. Furthermore, isolation of surface proteins through biotinylation did not show a change in gBK channel expression, and probing with phospho‐specific antibodies showed no alteration in channel phosphorylation. However, fura‐II Ca²⁺ fluorescence imaging revealed a 35% decrease in the free intracellular Ca²⁺ concentration after erbB2 inhibition and an increase in NRG‐1β‐treated cells, suggesting that the observed changes most likely were due to alterations in [Ca²⁺]_i. Consistent with this conclusion, neither tyrphostin AG825 nor NRG‐1β was able to modulate gBK channels under inside‐out or whole‐cell recording conditions when intracellular Ca²⁺ was fixed. Thus, gBK channels are a downstream target for the abundantly expressed neuregulin‐1 receptor erbB2 in glioma cells. However, unlike the case in other systems, this modulation appears to occur via changes in [Ca²⁺]_i without changes in channel expression or phosphorylation. The enhanced sensitivity of gBK channels in glioma cells to small, physiological Ca²⁺ changes appears to be a prerequisite for this modulation.