Regulation of the Ca2+-inhibitable Adenylyl Cyclase Type VI by Capacitative Ca2+ Entry Requires Localization in Cholesterol-rich Domains

Kent A Fagan,Karen E Smith,Dermot M.F Cooper

doi:10.1074/jbc.m001369200

Kent A Fagan, Karen E Smith + Show 1 more

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https://doi.org/10.1074/jbc.m001369200

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Abstract

The endogenous Ca(2+)-inhibitable adenylyl cyclase type VI of C6-2B glioma cells is regulated only by capacitative Ca(2+) entry and not by a substantial elevation of [Ca(2+)](i) from either intracellular stores or via ionophore-mediated Ca(2+) entry (Chiono, M., Mahey, R., Tate, G., and Cooper, D. M. F. (1995) J. Biol. Chem. 270, 1149-1155; Fagan, K. A., Mons, N., and Cooper, D. M. F. (1998) J. Biol. Chem. 273, 9297-9305). The present studies explored the role of cholesterol-rich domains in maintaining this functional association. The cholesterol-binding agent, filipin, profoundly inhibited adenylyl cyclase activity. Depletion of plasma membrane cholesterol with methyl-beta-cyclodextrin did not affect forskolin-stimulated adenylyl cyclase activity and did not affect capacitative Ca(2+) entry. However, cholesterol depletion completely ablated the regulation of adenylyl cyclase by capacitative Ca(2+) entry. Repletion of cholesterol restored the sensitivity of adenylyl cyclase to capacitative Ca(2+) entry. Adenylyl cyclase catalytic activity and immunoreactivity were extracted into buoyant caveolar fractions with Triton X-100. The presence of adenylyl cyclase in such structures was eliminated by depletion of plasma membrane cholesterol. Altogether, these data lead us to conclude that adenylyl cyclase must occur in cholesterol-rich domains to be susceptible to regulation by capacitative Ca(2+) entry. These findings are the first indication of regulatory significance for the localization of adenylyl cyclase in caveolae.

Highlights

These results indicate that the functional colocalization of capacitative Ca2ϩ entry (CCE) channels and adenylyl cyclase activity in C6-2B cells is heavily dependent on the residence of ACVI in caveolae
Effects of Filipin on Adenylyl Cyclase Activity—In order to probe the lipid environment of ACVI, C6-2B cells were treated with the cholesterol-binding agent filipin
Effect of Cholesterol Depletion Using M␤CD on the Ability of CCE to Regulate ACVI—The potential contribution of cholesterol to the functional colocalization of Ca2ϩ-sensitive adenylyl cyclases and CCE sites was assessed by cholesterol depletion experiments using M␤CD

Summary

Introduction

Extraction of the cells with the detergent, Triton X-100, followed by density gradient fractionation revealed that a portion of adenylyl cyclase immunoreactivity comigrated with caveolin immunostaining in buoyant membrane fractions that were consistent with cholesterol-rich membranes. Depletion of membrane cholesterol with methyl-␤-cyclodextrin (M␤CD) resulted in the retention of both adenylyl cyclase immunoreactivity and enzymatic activity in heavier, Triton-soluble fractions. In whole-cell experiments, disruption of caveolae by cholesterol depletion, using M␤CD, eliminated the ability of CCE to regulate adenylyl cyclase, without significantly affecting CCE.

Results

Conclusion