The expression and activity of P-glycoprotein (pGP) play a role in the multidrug resistance of tumors. Because solid-growing tumors often show pronounced hypoxia or extracellular acidosis, this study attempted to analyze the impact of an acidic environment on the expression and activity of pGP and on the cytotoxicity of chemotherapeutic agents. For this, prostate carcinoma cells were exposed to an acidic extracellular environment (pH 6.6) for up to 24 hours. pGP activity was more than doubled after 3 to 6 hours of incubation in acidic medium, whereas cellular pGP expression remained constant, indicating that increased transport rate is the result of functional modulation. In parallel, the cytotoxic efficacy of daunorubicin showed pronounced reduction at low pH, an effect that was reversible on coincubation with a pGP inhibitor. A reduction of intracellular Ca2+ concentration by 35% under acidic conditions induced a higher transport rate of pGP, an effect comparable to that found on inhibition of protein kinase C (PKC). These data indicate that pGP activity is increased by low extracellular pH presumably as a result of lowered intracellular calcium levels and inhibition of PKC. These findings may explain the reduced cytotoxicity of chemotherapeutic agents in hypoxic/acidic tumors.

Toxic effects of platinum complexes on renal epithelia depend on the platinum complex used and the site of application. We conclude that cell polarity and basolateral transport mechanisms are essential in nephrotoxicity of platinum drugs.

We used proximal tubule-derived opossum kidney (OK) cells to determine the dependence of albumin endocytosis on regulation by protein kinases and on the cytoskeleton. Uptake was observed only across the apical but not the basolateral membrane and exceeded uptake in collecting duct-derived Madin-Darby canine kidney cells 14-fold. Inhibition of endocytosis via clathrin-coated vesicles but not via caveolae abolished uptake. Cytochalasin D reduced uptake to < 5% of control, and inhibition of microtubule polymerization by nocodazole reduced uptake to approximately 55% of control. Activation of protein kinase A (PKA) by adenosine 3',5'-cyclic monophosphate, forskolin, or parathyroid hormone (PTH) reduced uptake to approximately 65% of control. Protein kinase C (PKC) activation did affect uptake to a similar extent as PKA activation but with a certain delay. Stimulation of PKG by guanosine 3',5'-cyclic monophosphate did not affect albumin endocytosis. The inhibitor of tyrosine kinases (TRK), genistein, induced an increase of uptake to approximately 160% of control. Reexocytosis of albumin was enhanced by PKC activation but not by PKA activation. TRK inhibition reduced the rate of reexocytosis. We conclude that albumin endocytosis in OK cells requires the integrity of the actin cytoskeleton. Microtubules facilitate endocytosis. Uptake is regulated by PKA, PKC, and TRK, yet with different time course and by different mechanisms, e.g., reexocytosis. Possibly TRK activity serves in a negative feedback loop to limit albumin endocytosis via a stimulation of reexocytosis.

No abstract

Ochratoxin A (OTA) is a ubiquitous fungal metabolite with nephrotoxic, carcinogenic, and apoptotic potential. Toxicokinetics make the kidney the primary target organ for OTA. Due to its widespread occurrence in improperly stored foodstuff the complete and safe avoidance of OTA for humans is impossible. There are several reports showing a significant correlation between OTA exposure and certain forms of nephropathies. At nanomolar concentrations OTA leads to specific changes of function and phenotype in renal cells. The toxin interacts with certain cellular "key-molecules" (e. g., mitogen-activated protein (MAP) kinases, Ca2+), thereby disturbing cellular signalling and regulation events as well as mitochondrial function. Moreover, OTA has the ability to modulate physiological signals (e. g., angiotensin II or TNFalpha) and thereby influences cell function and cell growth and may even stable re-program the cells (e. g., altered distribution of chromosomes). This review concentrates on the effects of OTA in the nanomolar range and its interactions with cellular signalling networks in different renal cells proposing OTA to act as a signal modulator.