PT  - JOURNAL ARTICLE
AU  - Lobaugh, L A
AU  - Eisfelder, B
AU  - Gibson, K
AU  - Johnson, G L
AU  - Putney, J W
TI  - Constitutive activation of a phosphoinositidase C-linked G protein in murine fibroblasts decreases agonist-stimulated Ca2+ mobilization.
DP  - 1996 Sep 01
TA  - Molecular Pharmacology
PG  - 493--500
VI  - 50
IP  - 3
4099  - http://molpharm.aspetjournals.org/content/50/3/493.short
4100  - http://molpharm.aspetjournals.org/content/50/3/493.full
SO  - Mol Pharmacol1996 Sep 01; 50
AB  - We compared Ca2+ signaling and inositol polyphosphate metabolism in NIH-3T3 cells stably transfected with cDNA encoding either the wild-type G protein G16 alpha subunit or a GTPase-deficient alpha 16 subunit (Q212L-alpha 16). Constitutive activation of phosphoinositidase C (PIC) in cells expressing Q212L-alpha 16 was demonstrated by 1) an increased basal level of [3H]inositol polyphosphates, 2) an enhanced rate of [3H]inositol polyphosphate accumulation in cells treated with 10 mM LiCl, and 3) an increased rate of incorporation of [3H]inositol into cell lipids. Q212L-alpha 16 cells had a diminished cell growth rate. Basal intracellular Ca2+ concentration was equivalent in Fura-2 acetoxymethyl ester-loaded Q212L-alpha 16 cells compared with controls; however, calcium release in Q212L-alpha 16 cells exposed to ionomycin, ATP (a G protein-linked agonist), or platelet-derived growth factor (a tyrosine kinase-linked agonist) was decreased. Permeabilized, 45Ca-loaded Q212L-alpha 16 cells released less 45Ca at each concentration of inositol-1,4,5-trisphosphate than did control cells. Accordingly, the total amount of inositol trisphosphate (IP3) receptor protein was decreased in Q212L-alpha 16 cells relative to controls. These data demonstrate that Q212L-alpha 16 cells maintain physiological levels of cytoplasmic calcium and partially loaded Ca2+ stores in the face of constitutively active PIC. This is accomplished, at least in part, by down-regulation of IP3 receptor number. Thus, diminution in cell growth rate in Q212L-alpha 16 cells seems to be attributable to a combination of at least two effects: a direct effect of PIC activation leading to partial depletion of Ca2+ stores and an indirect, adaptive response resulting in a decreased IP3 receptor number.