Phospholipase C activation by Na+/Ca2+ exchange is essential for monensin-induced Ca2+ influx and arachidonic acid release in FRTL-5 thyroid cells

Xiao Dong Wang, Juliann G. Kiang, L. William Scheibel, Robert C. Smallridge

Research output: Contribution to journalArticle

11 Scopus citations

Abstract

Background: Monensin, a Na+ ionophore, can increase cytosolic Ca2+ ([Ca2+]i) by reversing the Na+/Ca2+ exchange mechanism. This study provided additional insights into the mechanism of this Na+ ionophore-induced increase in [Ca2+]i, and emphasized the critical role of phospholipase C (PLC) in amplifying Na+/Ca2+ exchange-induced Ca2+ influx and subsequent arachidonic acid (AA) release in FRTL-5 thyroid cells. The possible involvement of protein kinase C (PKC), mitogen-activated protein kinase (MAPK), and GTP-binding (G) protein in mediating monensin-induced AA release was also explored. Methods: FRTL-5 thyroid cells were maintained in Coon's modified Ham's F-12 medium supplemented with a 6-hormone (6H) mixture. Cytosolic Ca2+ was measured by using indo-1 AM and a dual-wave-length spectrofluorometer. Release of 3H-labeled inositol trisphosphates and arachidonic acid were determined by a scintillation counter. Results: In Hank's balanced salt solution with Ca2+ (HBSS+), monensin (100 μmol/L) induced a 2.3-fold sustained Ca2+ increase associated with IP3 generation and a 6-fold increase in AA release. Deletion of extracellular Ca2+, or replacement of Na+ by choline chloride in the medium, reduced the [Ca2+]i increase by 77% and completely prevented the monensin-induced rise in AA release. Similar inhibitory effects were observed in cells pretreated with a Na+ channel blocker, or Na+/Ca2+ exchange inhibitors. In HBSS without Ca2+ (HBSS-), monensin induced a 1-fold transient [Ca2+]i increase but did not increase the AA. This Ca2+ increase was not suppressed by U-73122, a PLC inhibitor. In HBSS+, U-73122 did not affect the monensin-induced initial transient peak increase of [Ca2+]i, but reduced the sustained second phase of [Ca2+]i from 400 nmol/L to 250 nmol/L, and completely blocked AA release. A phospholipase A2 (PLA2) inhibitor blocked the monensin-induced AA release without affecting the [Ca2+]i increase. Inhibition of PKC prevented 87% to 94% of the monensin-stimulated AA release. The monensin-induced AA release was also inhibited 94% by pertussis and 51% by a MAP kinase cascade inhibitor. Conclusions: The results suggest that monensin initiates an increase in [Ca2+]i via a Na+/Ca2+ exchange mechanism that triggers more pronounced and sustained [Ca2+]i increase via activation of PLC and Ca2+ influx. The PLC activation, followed by sustained Ca2+ influx and PKC activation, is a prerequisite for PLA,-mediated processes in monensin-challenged FRTL-5 thyroid cells.

Original languageEnglish (US)
Pages (from-to)388-396
Number of pages9
JournalJournal of Investigative Medicine
Volume47
Issue number8
StatePublished - Sep 1999

Keywords

  • Cell calcium
  • FRTL-5 thyroid cells
  • MAP kinase
  • Phospholipases
  • Protein kinase C

ASJC Scopus subject areas

  • Biochemistry, Genetics and Molecular Biology(all)

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