Photoaffinity labeling of anion transport components in hepatocyte plasma membranes

Defective drug uptake contributing to multidrug resistance in hepatoma cells can be evaluated in vitro

In clinical practice the acquired or de novo resistance of tumors to antitumor chemotherapy remains a big problem. However, in the past few years some progress has been made in understanding the two principal mechanisms: metabolic alterations leading to a reduced cytostatic or cytotoxic effect of drugs, and reduced accumulation of drugs within the tumor cells [15, 34, 35]. The second phenomenon is usually attributed to the ability of tumor cells to accelerate the efflux of various xenobiotics. This phenomenon is considered primarily responsible for the development of multidrug resistance (MDR). However, loss or impairment of drug uptake by the tumor cells may also contribute to resistance to antitumor drugs. This paper focuses on recent findings with hepatoma cells, which support this view.

Anion transport systems in the plasma membrane of vertebrate cells

In the case of the red blood cell, anion transport is a highly specific one-for-one exchange catalyzed by a major membrane protein known as band 3 or as capnophorin. This red cell anion-exchange system mediates the Cl-(-)HCO3- exchange responsible for most of the bicarbonate transport capacity of the blood. The rapidly expanding knowledge of the molecular biology and the transport kinetics of this specialized transport system is very briefly reviewed in Section III. Exchange diffusion mechanisms for anions are found in many cells other than erythrocytes. The exchange diffusion system in Ehrlich cells has several similarities to that in red cells. In several cell types (subsection IV-B), there is evidence that intracellular pH regulation depends on Cl-(-)HCO3- exchange processes. Anion exchange in other single cells is described in Section IV, and its role in pH regulation is described in Section VII. Anion exchange mechanism operating in parallel with, and only functionally linked to Na+-H+ or K+-H+ exchange mechanisms can also play a role in cell volume regulation as described in Section VII. In the Ehrlich ascites cell and other vertebrate cells, electroneutral anion transfer has been found to occur also by a cotransport system for cations and chloride operating in parallel with the exchange diffusion system. The cotransport system is capable of mediating secondary active chloride influx. In avian red cells, the cotransport system has been shown to be activated by adrenergic agonists and by cyclic AMP, suggesting that the cotransport is involved in regulatory processes (see subsection V-A.). In several cell types, cotransport systems are activated and play a role during volume regulation, as described in Section V and in Section VII. It is also likely that this secondary active cotransport of chloride plays a significant role for the apparently active extrusion of acid equivalents from certain cells. If a continuous influx of chloride against an electrochemical gradient is maintained by a cotransport system, the chloride disequilibrium can drive an influx of bicarbonate through the anion exchange mechanism, as described in Section VII. Finally, even the electrodiffusion of anions is shown to be regulated, and in Ehrlich cells and human lymphocytes an activation of the anion diffusion pathway plays a major role in cell volume regulation as described in Section VI and subsection VII-B.(ABSTRACT TRUNCATED AT 250 WORDS)

[Pathogenic significance of bile acids (author's transl)]

Because of their amphiphilic properties, bile acids have important physiological functions. However, they can also be pathogenetically active. Some recent findings on the biochemistry and enterohepatic circulation of bile acids are presented. In contrast to the adult liver where the only primary bile acids formed are cholic- and chenodeoxycholic acid, the foetal liver is able to synthesise a variety of "atypical" bile acids. Under certain circumstances, a retrograde differentiation is possible in the adult. The very effective transport systems in gut and in the sinusoidal and canalicular membrane of the liver cell limit the bile acids almost exclusively to the enterohepatic circulation. During transport in blood, through biomembranes and in the liver cytosol, bile acids are bound to carrier proteins. The carrier has been detected using photoaffinity labelling. Following biotransformation (sulphation and glucuronidation) pathogenetically active bile acids can be converted into derivatives which can be rapidly eliminated. Disturbances of these mechanisms result in functional defects and diseases. The pathological significance of bile acids in hepato-biliary diseases is represented with regard to the cholestatic and proliferative effect of individual bile acids. The significance of bile acids in chologenic diarrhea, steatorrhea and enteral hyperoxaluria are presented as examples of the pathogenetic effects of bile acids on the gut. In these diseases it is possible to recognise the specific effects of certain bile acids on the colon mucosa. Recent studies have demonstrated that bile acids are possibly of pathogenetic significance in the case of epidemiologically proven relationship between colon carcinoma and high fat, high cholesterol and low fibre diets.