Underexpression of the 43 kDa inositol polyphosphate 5-phosphatase is associated with cellular transformation

The 43 kDa inositol polyphosphate 5-phosphatase (5-phosphatase) hydrolyses the second messenger molecules inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P₄]. We have underexpressed the 43 kDa 5-phosphatase by stably transfecting normal rat kidney cells with the cDNA encoding the enzyme, cloned in the antisense orientation into the tetracycline-inducible expression vector pUHD10-3. Antisense-transfected cells demonstrated a 45% reduction in Ins(1,4,5)P₃ 5-phosphatase activity in the total cell homogenate upon withdrawal of tetracycline, and an ~80% reduction in the detergent-soluble membrane fraction of the cell, as compared with antisense-transfected cells in the presence of tetracycline. Unstimulated antisense-transfected cells showed a concomitant 2-fold increase in Ins(1,4,5)P₃ and 4-fold increase in Ins(1,3,3,5)P₄ levels. The basal intracellular calcium concentration of antisense-transfected cells (170 ± 25 nM) was increased 1.9-fold, compared with cells transfected with vector alone (90 ± 25 nM). Cells underexpressing the 43 kDa 5-phosphatase demonstrated a transformed phenotype. Antisense-transfected cells grew at a 1.7-fold faster rate, reached confluence at higher density and demonstrated increased [³H]thymidine incorporation compared with cells transfected with vector alone. Furthermore, antisense-transfected cells formed colonies in soft agar and tumours in nude mice. These studies support the contention that a decrease in Ins(1,4,5)P₃ 5-phosphatase activity is associated with cellular transformation.