Cytosolic multiple inositol polyphosphate phosphatase in the regulation of cytoplasmic free Ca2+ concentration.
Multiple inositol polyphosphate phosphatase (MIPP) is an enzyme that, in vitro, has the interesting property of degrading higher inositol polyphosphates to the Ca2+ second messenger, inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), independently of inositol lipid breakdown. We hypothesized that a truncated cytosolic form of the largely endoplasmic reticulum-confined MIPP (cyt-MIPP) could represent an important new tool in the investigation of Ins(1,4,5)P3-dependent intracellular Ca2+ homeostasis. To optimize our ability to judge the impact of cyt-MIPP on intracellular Ca2+ concentration ([Ca2+]i) we chose a poorly responsive beta-cell line (HIT M2.2.2) with an abnormally low [Ca2+]i. Our results show for the first time in an intact mammalian cell that cyt-MIPP expression leads to a significant enhancement of Ins(1,4,5)P3 concentration. This is achieved without a significant interference from other cyt-MIPP-derived inositol phosphates. Furthermore, the low basal [Ca2+]i of these cells was raised to normal levels (35 to 115 nm) when they expressed cyt-MIPP. Noteworthy is that the normal feeble glucose-induced Ca2+ response of HIT M2.2.2 cells was enhanced dramatically by mechanisms related to this increase in basal [Ca2+]i. These data support the use of cyt-MIPP as an important tool in investigating Ins(1,4,5)P3-dependent Ca2+ homeostasis and suggest a close link between Ins(1,4,5)P3 concentration and basal [Ca2+]i, the latter being an important modulator of Ca2+ signaling in the pancreatic beta-cell.
Yu
Jia
J
Department of Molecular Medicine, The Rolf Luft Center for Diabetes Research, L3, Karolinska Institutet, Karolinska Hospital, Stockholm SE-171 76, Sweden.
Leibiger
Barbara
B
Yang
Shao-Nian
SN
Caffery
James J
JJ
Shears
Stephen B
SB
Leibiger
Ingo B
IB
Barker
Christopher J
CJ
Berggren
Per-Olof
PO
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D003593:Cytoplasm / Q000378:metabolism; D003600:Cytosol / Q000201:enzymology* / Q000378:metabolism; D004594:Electrophysiology; D005947:Glucose / Q000494:pharmacology; D015544:Inositol 1,4,5-Trisphosphate / Q000032:analysis / Q000378:metabolism; D007295:Inositol Phosphates / Q000032:analysis; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism / Q000502:physiology*; D051381:Rats; D017384:Sequence Deletion
D007295:Inositol Phosphates; D015544:Inositol 1,4,5-Trisphosphate; D010744:Phosphoric Monoester Hydrolases; C064119:multiple inositol-polyphosphate phosphatase; D005947:Glucose; D002118:Calcium