Characterization of a second hamster lung fibroblast mutant with defects in phosphatidylinositol-specific...

We report here further characterization of the Chinese hamster lung fibroblast mutant D1-9b displaying elevated agonist-induced phosphatidylinositol (PI) turnover responses relative to CCL39, the parental cell line (Rath, H. M., Doyle, G. A. R., and Silbert, D. F. (1989) J. Biol. Chem. 264, 13387-13390). These differences in PI turnover responses are further enhanced by long term (24 h) pretreatment of cells with the phorbol ester 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA). Short term pretreatment (15 min) with PMA attenuates the agonist-induced PI turnover response of both wild type and mutant D1-9b cell lines to that of unstimulated controls, suggesting that PMA-induced desensitization responses in D1-9b are intact. Both cytosolic and membrane/particulate fractions prepared from mutant D1-9b have decreased phospholipase C (PLC) activities relative to wild type when assayed with either of four different PI/phosphatidylinositol 4,5-bisphosphate-mixed micelle/vesicle substrate combinations. Thermolability studies, Mono Q anion-exchange chromatography, and Western blot studies identified the source of cytosolic PLC deficiency in D1-9b as being due to the absence of PLC delta 1, one of at least two PLC isozymes previously shown to be in wild type CCL39 cytosol (Rath, H. M., Fee, J. A., Rhee, S. G., and Silbert, D. F. (1990) J. Biol. Chem. 265, 3080-3087). We also report here the presence of two peaks of PLC activities (i.e. peaks mA and mB) following Mono Q chromatography of wild type CCL39 membrane/particulate extracts; membrane/particulate extracts prepared from mutant D1-9b are missing the first of these two peaks of activities (peak mA). Immunoblot analysis confirms the presence of PLC beta 1, but not PLC gamma 1 or PLC delta 1, in peak mB. Comparisons are made between D1-9b and the previously characterized mutant D1-6b, which displays diminished agonist-induced PI turnover responses yet in vitro biochemical deficiencies similar to those of mutant D1-9b. Somatic cell hybridization experiments suggest that the defects found in mutant D1-9b are codominant relative to wild type phenotype and that mutant D1-9b belongs to the same genetic complementation group as mutant D1-6b. Possible explanations to explain these different agonist-induced responses in light of the two mutants similar in vitro biochemical deficiencies are addressed.