Characterisation of cyclic adenosine 3':5'-monophosphate phospodiesterase from Walker carcinoma sensitive and resistant to bifunctional alkylating agents

Selective inhibition by bis(2-chloroethyl)methylamine (nitrogen mustard) of the Na+/K+/Cl- cotransporter of murine L1210 leukemia cells

Incubation of L1210 murine leukemia cells in vitro with 10 microM of the bifunctional alkylating agent bis(2-chloroethyl)methylamine (nitrogen mustard, HN2) for 10 min brought about a fall of more than 99.9% in their ability to form colonies when the cells were suspended in 0.5% nutrient agar. Incubation with HN2 also inhibited the influx of the potassium congener 86Rb+ to exponentially proliferating L1210 cells in a concentration-dependent manner. This inhibition was specific and was accounted for by a reduction of a diuretic-sensitive component of 86Rb+ influx, identified in the preceding paper (Wilcock, C. and Hickman, J.A. (1988) Biochim. Biophys. Acta 946, 359-367) as being mediated by a Na+/K+/Cl- cotransporter. Inhibition by 10 microM HN2 was complete after a 3-h incubation. There was no inhibition at this time of the ouabain-sensitive component of 86Rb+ influx, mediated by Na+/K+-ATPase. After 3 h of incubation with 10 microM HN2 there was also no change in the membrane potential of the treated cells as measured by the distribution of the [3H]TPMP+, no decrease in cellular ATP concentration and no change in intracellular pH, and the ability of the cells to exclude the vital dye Trypan blue was not significantly different from control values. These effects of HN2, therefore, appeared to follow lethal damage, but precede cell death. In the stationary phase of L1210 cell growth, the component of HN2 and diuretic-sensitive K+ influx to L1210 cells was reduced, whilst the component constituting the HN2-insensitive ouabain-sensitive sodium pump was increased. The monofunctional alkylating agent MeHN1 (2-chloroethyldimethylamine) which cannot cross-link cellular targets and has no antitumor activity, did not inhibit 86Rb+ influx to L1210 cells when incubated at equimolar or equitoxic concentrations to HN2. Intracellular potassium concentration was maintained close to control values of 138 +/- 10 mM in HN2-treated cells because of an approx. 35% fall in cell volume. The results suggest that the Na+/K+/Cl- cotransporter is a selectively inhibitable target for HN2, and the lesion is discussed with reference to the cytotoxic effects of this agent.

A comparison of cAMP phosphodiesterases in normal, malignant, and somatic cell hybrids

Hybrids (PCM) between a malignant mouse lymphoma suspension cell line (P388F-36) and a normal Chinese hamster fibroblastic cell line (Ch23) have already been isolated in this laboratory. Investigations were carried out on the cAMP phospodiesterases of the parents and two of these hybrids--PCM2 and PCM3. PCM3 shows a rather unusual growth characteristic in that a considerable proportion of the cells exist at any one time either in suspension or only loosely attached to the substratum, the remaining cell population existing in a monolayer form. It was found that each cell line exhibited multiple forms of the enzyme with varying affinities for cAMP. Both parents, although different, contained high-, low-, and extra-high apparent Km forms of the enzyme. The hybrids exhibited characteristics of both parental systems but were different from each other. Neither hybrid exhibited a high-Km enzyme, but both exhibited two low-Km forms. There was also a slight variation between monolayer and suspension cells of PCM3 hybrid. An attempt has been made to explain these phenomena with respect to hybridization and the growth characteristics of the cells.

Growth stimulation of sparse, serum deprived and of confluent, contact inhibited mammalian fibroblasts by a preparation of 3':5'-cyclic AMP phosphodiesterase

Resting mammalian fibroblasts, either sparse and maintained in a serum-free medium, or confluent and contact inhibited, are stimulated to divide by treatment with a preparation of 3':5'-cyclic AMP phosphodiesterase. This enzyme preparation contained a low level of trypsin-like and alpha-chymotrypsin-like activity, but its effect on cell growth could not be mimicked by pure, crystallized trypsin and alpha-chymotrypsin at concentration equivalent to their contamination in the above preparation. Preincubation of the 3':5'-cyclic AMP phosphodiesterase preparation with the protease inhibitor, phenyl methane sulfonyl fulride, did not affect, either, its stimulation of DNA synthesis in fibroblasts, or its enzymatic hydrolysis of cyclic AMP.