Polyamines secreted by cancer cells possibly account for the impairment of the human erythrocyte sodium pump activity.

Using an original microcalorimetric method, we previously showed that in erythrocytes from cancer patients, the sodium pump activity was decreased and returned to normal in patient in remission. In addition we suggested that a plasma-borne factor probably secreted by cancer cells accounted for this impairment of the sodium transporter. In the present study we sought to identify this factor as well as its mechanism of action. First we determined the effect of culture media from undifferentiated and differentiated colon cancer cell lines (Caco-2 and HT29-D4) on the sodium pump activity of normal human erythrocytes. The inhibitory powers of culture media from undifferentiated cells were higher than those of differentiated cells (38.6 +/- 3.5% vs 6.9 +/- 4.6%, p<0.05 for Caco-2 and 45.8 +/- 6.2% vs 9.0 +/- 5.0%, <0.05 for HT29-D4). The use of alpha difluoro-methylomithine (2 mM) to inhibit ornithine decarboxylase, the rate-limiting enzyme for polyamine biosynthesis, dramatically reduced the sodium pump inhibition induced by the two undifferentiated cell lines (75% for Caco-2 and 89% for HT29-D4). Polyamines secreted by undifferentiated cells and then taken up by human erythrocytes thus appeared as inhibitors of sodium pump of these red blood cells. Putrescine, spermidine, and spermine (the main polyamines) exerted a similar inhibitory effect (33 +/- 2%). Tested in vitro on Na,KATPase, these polyamines (3 mM) were inhibitors (putrescine = 23 +/- 2%; spermidine= 48 +/- 3%; spermine= 55 +/- 2%) when assay condition for the ATPase reaction was suboptimal (Na+ = 10 mM; K+ = 1 mM). The inhibitory effect appeared to be related to their charge and their aliphatic chain length. The effect of spermidine and spermine on the ionic substrates and ATP-Mg showed that molecules decreased the affinity (Km) of the Na,K-ATPase for Na+ (11.24 +/- 0.49 mM for control vs 23.51 +/- 1.53 mM for spermine and 18.86 +/- 0.98 mM for spermidine), indicating that polyamines exerted their inhibitory effect in a competitive manner.

Sodium pump and Na+/H+ antiport restoration in erythrocytes from cancer patients in remission.

We previously showed that in erythrocytes from cancer patients, the sodium pump is decreased and the optimal intracellular pH for Na+/H+ antiport activity is shifted toward an acidic value. We now have studied these sodium transporters in erythrocytes from patients in remission. Moreover, we intended to explain why the transporters were impaired in erythrocytes, which have no apparent bearing on cancer tissues. The sodium pump was studied through a microcalorimetric method, and the Na+/H+ antiport by a titrimetric method. In patients in remission the sodium pump activity returned to normal: 15.10 +/- 6.00 vs. 14.12 +/- 5.28 mW/l cells for remission and control, respectively. The optimal intracellular pH for Na+/H+ antiport activity was identical in remission and control: 6.09 +/- 0.23 vs. 6.10 +/- 0.10. Restoration of sodium pump activity and optimal intracellular pH for Na+/H+ antiport activity in erythrocytes were thus linked to remission. Moreover, we showed that the impairments of the sodium transporters were due to the presence of plasma-borne factors, the existence of which explained why the sodium transporters were impaired in erythrocytes.

Impairment of sodium pump and Na+/H+ antiport in erythrocytes isolated from cancer patients.

We sought to determine whether the impairment of sodium pump activity and Na+/H+ exchange reported in tumorigenic cells was specific to these cells or more general. Sodium pump activity and Na+/H+ exchange were measured in erythrocytes from 49 cancer patients and 51 healthy subjects. Cancer patients with a newly detected cancer or in relapse and without associated pathologies known to modify these sodium transporters were included in this study. Two sodium pump statuses reflecting its physiological modulation were evidenced for healthy subjects (10.3 +/- 0.2 and 19.4 +/- 0.8 mW/liter of cells). In cancer patients, only one basal status lower than those of controls was observed (8.3 +/- 0.5 mW/liter of cells; P < 0.001). Cooperativity of the Na+/H+ antiporter is the same in cancer patients and controls (2.58 +/- 0.27 versus 2.60 +/- 0.15). The intracellular pH (pHi) dependence curve of the antiporter was shifted toward more acidic values, and optimal pH1 was lower in cancer patients than in controls (5.80 +/- 0.03 versus 6.08 +/- 0.02; P < 0.0001). The mean maximal rate and the Km of H+ for the Na+/H+ antiporter were higher: 8.4 +/- 1.2 versus 4.6 +/- 0.4 mmol H+/liter of cells/h (P < 0.01) and 514 +/- 12 versus 322 +/- 16 nM (P < 0.05), respectively. Alterations of these Na+ transporters, therefore, were not restricted to cancerous cells. Among the alterations, the acidic shift in the pHi dependence of Na+/H+ exchange appears associated with cancer because this behavior has never been reported in other pathologies.