Subcellular localization of inositide enzymes in established T-cell lines and activated lymphocytes

The specific activity of phosphatidylinositol 4,5-bisphosphate phospholipase C (PIP2-PLC) and inositol 1,4,5-trisphosphate monophosphatase (IP3-MP) involved in phosphoinositide catabolism was found to be significantly lower in the total homogenate of four human lymphoblastoid cell lines, HSB-2, MOLT-4, CEM and JURKAT, than in resting and activated peripheral blood mononuclear cells, ranging from 0.8 to 3.2 and from 1.3 to 3.7 nmol/min/mg for PIP2-PLC and IP3-MP, respectively. In PHA-stimulated cells, the specific activities were enhanced 25 and 35% respectively over the values (8.02 and 7.83 nmol/min/mg, respectively) measured in resting peripheral blood mononuclear cells. After centrifugation on discontinuous sucrose gradient of cell homogenate, PIP2-PLC and IP3-MP activities were found to be predominantly associated with the cytosol fraction (> 69%) in HSB-2 and MOLT-4 cells, with a distribution similar to that found in PHA-stimulated and in resting lymphocytes. In CEM cells, about half of the total activity remained in this fraction, while in JURKAT lymphoblastic cells more than 45% of the total activity was recovered in the high-density membrane fraction (d = 1.20-1.25), the soluble PIP2-PLC and IP3-MP activity accounting for only 13 and 25%, respectively. Conversely, in less differentiated leukemic cells HSB-2 and MOLT-4, conspicuous activity of the ectoenzymes 5'-nucleotidase (5'-NT) and gamma-glutamyltransferase (gamma-GT) was recovered in the soluble fraction. Growing leukemic cells at a distinct level of differentiation have a general reduction in activity but a characteristic distribution of enzymes involved in the transmission of signals usually targeting the cell surface.