Cytosolic Multiple Inositol Polyphosphate Phosphatase in the Regulation of Cytoplasmic Free Ca[sup 2+] Concentration.

Multiple inositol polyphosphate phosphatase (MIPP) is an enzyme that, in vitro, has the interesting property of degrading higher inositol polyphosphates to the Ca[sup 2+] second messenger, inositol 1,4,5-trisphosphate (Ins(1,4,5)P[sub 3]), 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) P[sub 3]dependent intracellular Ca[sup 2+] homeostasis. To optimize our ability to judge the impact of cyt-MIPP on intracellular Ca[sup 2+] concentration ([Ca[sup 2+]][sub i]) we chose a poorly responsive β-cell line (HIT M222.2) with an abnormally low [Ca[sup 2+]][sub 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)P[sub 3] concentration. This is achieved without a significant interference from other cyt-MIPP-derived inositol phosphates. Furthermore, the low basal [Ca[sup 2+]][sub 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 Ca[sup 2+] response of HIT M2.2.2 cells was enhanced dramatically by mechanisms related to this increase in basal [Ca[sup 2+]][sub i]. These data support the use of cyt-MIPP as an important tool in investigating Ins(1,4,5)P[sub 3]dependent Ca[sup 2+] homeostasis and suggest a close link between Ins(1,4,5)P[sub 3] concentration and basal [Ca[sup 2+]][sub i], the latter being an important modulator of Ca[sup 2+] signaling in the pancreatic β-cell. [ABSTRACT FROM AUTHOR]