Diseased cells and pH

Altered proton extrusion in cells adapted to growth at low extracellular pH

Intracellular pH (pHi) homeostasis is crucial to cell survival. Cells that are chronically exposed to a low pH environment must adapt their hydrogen ion extrusion mechanisms to maintain their pHi in the physiologic range. An important component of the adaptation to growth at low pH is the upregulation of pHi relative to the extracellular pH (pHe). To test the ability of low pHe adapted cells to respond to a pHi lowering challenge, a fluorescence assay was used that directly monitors proton removal as the rate of change of pHi during recovery from cytosolic acidification. Two cell lines of Chinese hamster origin (ovarian carcinoma and ovary fibroblastoid cells) were compared, both of which showed altered proton extrusion after adaptation to growth at low pHe = 6.70. In the ovarian carcinoma (OvCa) cell line, the pattern was consistent with an upregulation by means of an increase in the number of functional proton transporters in the plasma membrane. In the ovary fibroblastoid (CHO-10B) cell line, pHi was consistently elevated in adapted cells as compared with cells grown at normal pHe = 7.30 without an increase in maximum extrusion rate. This upregulation was consistent with a shift in the activating pHi of proton transporters without an increase in the number of transporters, i.e., a change in substrate affinity of the transporter. In OvCa cells, recovery from acidification could be blocked by amiloride, an inhibitor of Na+/ H+ exchange. In contrast, a more modest effect of amiloride on CHO cells was observed but a complete inhibition was seen with the Cl-/HCO(-3)exchange inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS). These data indicate that the two cell lines rely to different degrees on the two major pathways for pH regulation during recovery from cytosolic acidification.

Alkaline pH facilitates the exchange of guanine nucleotides: a possible mechanism for modulation of the kinetics of responses mediated by guanine nucleotide-binding proteins

GTP gamma S1 binding experiments in a particulate preparation from Xenopus oocytes revealed two binding sites at pH = 6.9: A high affinity site (Kd = 77 +/- 4 nM) and a low affinity site (Kd = 8.74 +/- 0.05 microM). Alkaline pH (8.5) caused a significant increase in the dissociation constants of both sites (160 +/- 46 nM and 30.7 +/- 1.6 microM, respectively). In purified plasma membrane preparation, alkaline pH increased the rate of dissociation of GTP gamma S. We have previously proposed that the activation of a G-protein by the agonist-occupied receptor is rate-limiting in the kinetics of hormone-induced responses (Lipinsky et al., 1993; Pflugers Arch., 425:140-149). We have, therefore, assayed the latencies of responses evoked by TRH at different pH, in oocytes expressing the TRH receptor. A change in the medium pH was reflected by an approximately tenfold smaller change in cellular pH (pHi). Alkalinization of the medium (from pH 7.4 to 8.5) caused a shortening of latency (by 45%), whereas acidification to pH = 6.0 prolonged it (by 87%). Moreover, alkalinization decreased the latency and increased the rate of responses to microinjected GTP gamma S, but did not change the latency of responses to microinjected InsP3. These results show that activation of plasma membrane receptors coupled to G-proteins, concurrent with a change in pHi, can alter the kinetic pattern of physiological responses, thus affecting the ultimate physiological output of the cell. This finding suggests that a change of pH, is a novel potential mechanism for modulation of responses mediated by G-proteins.

The interaction among glucose transport, hexokinase, and glucose-6-phosphatase with respect to 3H-2-deoxyglucose retention in murine tumor models

The development of new diagnostic/therapeutic modalities for cancer requires a specific understanding of how tumors differ from normal tissues. Though the key components involved in the selective accumulation of 2-deoxy-D-glucose (2-DG) analogs in tumors are known, the relative importance of each is controversial. For this reason glucose transport protein (GLUT) density, hexokinase/glucose-6-phosphatase (GP) activity, and 2-DG biodistribution were measured together in four tumor models and normal murine tissues. Direct binding studies with 3H-cytochalasin B showed that GLUT density was elevated 20-fold in LX-1 tumors. Immunohistochemically in all tumors, the expression of GLUT-1 was highest in the necrotic/ perinecrotic foci and similar in cells not adjacent to necrotic foci. As the retention of 3H-2-DG was similar in all tumors, these data suggest that the GLUT-1 in perinecrotic tumor cells were not rate limiting for 3H-2-DG uptake. Kidney, liver, and lung had high GP activity and rapid clearance of 3H-2-DG. Sodium orthovanadate (5 mumol), a GP inhibitor, increased the concentration of 3H-2-DG in these tissues, suggesting that GP is a rate-limiting enzyme for 3H-2-DG clearance. All tumor homogenates had low GP activity, and hexokinase activity was not elevated compared to normal tissues. Thus, in the tumors studied, the selective accumulation of 3H-2-DG consistently occurred in the absence of significant GP activity without the marked overexpression of hexokinase or GLUT.

Thermotolerance and intracellular pH in two Chinese hamster cell lines adapted to growth at low pH

As an in vitro model for the low extracellular pH (pHe) which has frequently been observed in tumors, cell lines have been grown in a low-pH medium in order to allow cell adaptation to that milieu. Two Chinese hamster cell lines [Chinese hamster ovary (CHO) and Chinese hamster ovarian carcinoma (OvCa)] were compared, both of which acquired thermotolerance during 42 degrees C heating in pHe = 7.3 buffer, but not in pHe = 6.7 medium unless grown at that pH long enough to become adapted. CHO cells, even when acutely acidified, showed higher intracellular pH (pHi) values in a suspension assay than OvCa cells, which confirmed the danger of comparing absolute values of pHi between cell lines. Despite this fundamental difference, relative changes in pHi were similar in that both lines showed a higher pHi in adapted than in unadapted cells, over the range of pHe values tested. The upregulation of pHi was statistically significant, but the two lines differed in the time frame over which adaptation occurred. OvCa cells acquired an enhanced ability to develop tolerance to 42 degrees heat at pHe = 6.7 in 4 days, but the CHO cells acquired this ability more progressively, achieving a maximum ability at approximately 100 days. In contrast, both lines were able to upregulate their pHi within 4 hours of being exposed to pH 6.7 medium. A further indication of different biochemical mechanisms at work was the opposite effects seen on pHi in the two cell lines upon the removal of extracellular CO2/HCO3-. The differential between adapted and unadapted OvCa cells was enhanced by removal of bicarbonate, whereas CHO cells seemed less stable and the data with greater scatter failed to show any difference between adapted and unadapted cells.