Phospho-proteomic analysis of cellular signaling

Reversible protein phosphorylation plays an important role in the regulation of many different processes, such as cell growth, differentiation, migration, metabolism, and apoptosis. Identification of differentially phosphorylated proteins by means of phospho-proteomic analysis provides insight into signal transduction pathways that are activated in response to, for example, growth factor stimulation or toxicant-induced apoptosis. This review summarizes recent advances made in the field of phospho-proteomics and provides examples of how phospho-proteomic techniques can be combined to quantitatively investigate the dynamic changes in protein phosphorylation in time. By linking experimental data to clinical data (e.g., disease progression or response to therapy) new disease markers could be identified, which could then be validated for applications in disease diagnosis and progression or prediction of a response to drugs.

Focal adhesion kinase and protein kinase B cooperate to suppress doxorubicin-induced apoptosis of breast tumor cells

Focal adhesion kinase (FAK) is up-regulated in a variety of cancers, including breast cancer, in association with poor disease prognosis. In the present study, we examined the role of FAK in the control of anticancer drug-induced apoptosis of mammary adenocarcinoma MTLn3 cells. Doxorubicin caused the formation of well defined focal adhesions and stress fibers early after treatment, which was later followed by their loss in association with the onset of apoptosis. Phosphorylation of FAK on tyrosine 397 decreased only during the onset of doxorubicin-induced apoptosis in a Bcl-2 and caspase-independent manner. Doxorubicin also caused an early activation of protein kinase B (PKB). Expression of the dominant-negative acting focal adhesion kinase-related nonkinase (FRNK) sensitized MTLn3 cells to apoptosis caused by doxorubicin. FRNK inhibited the doxorubicin-induced activation of PKB. In addition, inhibition of phosphatidylinositide-3 (PI-3) kinase with wortmannin inhibited the activation of PKB by doxorubicin. Both wortmannin and transient overexpression of the dual lipid/protein phosphatase and tensin homolog deleted on chromosome 10 enhanced doxorubicin-induced cell death. Altogether, these data fit with a model wherein FAK is involved in the doxorubicin-induced activation of the PI-3 kinase/PKB signaling route, thereby suppressing the onset of apoptosis caused by doxorubicin.

Macrophage retinoblastoma deficiency leads to enhanced atherosclerosis development in ApoE-deficient mice

The cellular composition of an atherosclerotic lesion is determined by cell infiltration, proliferation, and apoptosis. The tumor suppressor gene retinoblastoma (Rb) has been shown to regulate both cell proliferation and cell death in many cell types. To study the role of macrophage Rb in the development of atherosclerosis, we used apoE-deficient mice with a macrophage-restricted deletion of Rb (Rb(del) mice) and control littermates (Rb(fl) mice). After 12 wk feeding a cholesterol-rich diet, the Rb(del) mice showed a 51% increase in atherosclerotic lesion area with a 39% increase in the relative number of advanced lesions. Atherosclerotic lesions showed a 13% decrease in relative macrophage area and a 46% increase in relative smooth muscle cell area, reflecting the more advanced state of the lesions. The increase in atherosclerosis was independent of in vitro macrophage modified lipoprotein uptake or cytokine production. Whereas macrophage-restricted Rb deletion did not affect lesional macrophage apoptosis, a clear 2.6-fold increase in lesional macrophage proliferation was observed. These studies demonstrate that macrophage Rb is a suppressing factor in the progression of atherosclerosis by reducing macrophage proliferation.

Peptide-bond modified glutathione conjugate analogs modulate GSTπ function in GSH-conjugation, drug sensitivity and JNK signaling

Glutathione S-transferase pi (GST, E.C.2.5.1.18) overexpression contributes to resistance of cancer cells towards cytostatic drugs. Furthermore, GSTpi is involved in the cellular stress response through inhibition of Jun N-terminal-kinase (JNK), a process that can be modulated by GST inhibitors. GSH conjugates are potent GST inhibitors, but are sensitive towards gamma-glutamyltranspeptidase (gammaGT)-mediated breakdown. In search for new peptidase stable GST inhibitors we employed the following strategy: (1) selection of a suitable (GST inhibiting) peptide-bond isostere from a series of previously synthesized gammaGT stabilized GSH-analogs. (2) The use of this peptidomimetic strategy to prepare a GSTpi selective inhibitor. Two gammaGT stable GSH conjugate analogs inhibited human GSTs, although non-selectively. One of these, a urethane-type peptide-bond is well accepted by GSTs and we selected this modification for the development of a gammaGT stable, GSTpi selective inhibitor, UrPhg-Et(2). This compound displayed selectivity for GSTpi compared to alpha and mu class enzymes. Furthermore, the inhibitor reversed GSTpi-mediated drug resistance (MDR) in breast tumor cells. In addition, short-term exposure of cells to UrPhg-Et(2) led to GSTpi oligomerization and JNK activation, suggesting that it activates the JNK-cJun signaling module through GSTpi dissociation. Altogether, we show the successful use of peptidomimetic glutathione conjugate analogs as GST inhibitors and MDR-modifiers. As many MDR related enzymes, such as MRP1, glyoxalase 1 and DNA-pk are also inhibited by GSH conjugates, these peptidomimetic compounds can be used as scaffolds for the development of multi-target MDR drugs.

Cellular stress responses and molecular mechanisms of nephrotoxicity

Increasing our knowledge on the molecular and cellular mechanisms of acute renal tubular pathologies will lead to potential novel therapeutic strategies either to prevent the initiation of renal failure or to promote the renal regeneration after injury. Currently many genomic- and proteomic-based techniques are available to identify genes, proteins or protein modifications in relation to renal toxicity. Although we are able to identify many genes and proteins at once, the actual role of the genes and proteins with respect to cellular toxicity needs to be defined in order to better understand the molecular basis of renal cell injury and repair. This review will focus on the relationship between changes in gene and protein expression, cellular perturbations, signal transduction, and mechanisms of toxicity. A focus is on the role of stress response proteins in repair of injured renal cells.

Requirement for focal adhesion kinase in the early phase of mammary adenocarcinoma lung metastasis formation

An increased expression of focal adhesion kinase (FAK) in a variety of cancers is associated with a poor disease prognosis. To study the role of FAK in breast tumor growth and metastasis formation, we used conditional doxycycline-regulated expression of a dominant-negative acting splice variant of FAK, FAK-related non-kinase (FRNK), in MTLn3 mammary adenocarcinoma cells in a syngeneic Fischer 344 rat tumor and metastasis model. In cell culture, doxycycline-mediated expression of FRNK inhibited MTLn3 cell spreading and migration in association with reduced formation of focal adhesions and phosphorylation of FAK on Tyr(397), but FRNK did not cause apoptosis. Continuous expression of FRNK decreased the primary tumor growth in the mammary fat pad by 60%, which was not due to induction of apoptosis. Lung metastasis formation was almost completely prevented when FRNK was already expressed 1 day before tumor cell injection, whereas expression of FRNK 11 days after injection did not affect lung metastasis formation. FRNK expression during the first 5 days was sufficient to block metastasis formation, excluding the possibility of FRNK-induced dormancy of tumor cells. Together, these data fit with a model wherein FAK is required for breast tumor cell invasion/migration processes that take place in the early phase of metastasis formation. Our findings suggest that FAK is a good candidate for therapeutic intervention of metastasis formation.

Heat shock protein 27 is the major differentially phosphorylated protein involved in renal epithelial cellular stress response and controls focal adhesion organization and apoptosis

We used two-dimensional difference gel electrophoresis to determine early changes in the stress-response pathways that precede focal adhesion disorganization linked to the onset of apoptosis of renal epithelial cells. Treatment of LLC-PK1 cells with the model nephrotoxicant 1,2-(dichlorovinyl)-L-cysteine (DCVC) resulted in a >1.5-fold up- and down-regulation of 14 and 9 proteins, respectively, preceding the onset of apoptosis. Proteins included those involved in metabolism, i.e. aconitase and pyruvate dehydrogenase, and those related to stress responses and cytoskeletal reorganization, i.e. cofilin, Hsp27, and alpha-b-crystallin. The most prominent changes were found for Hsp27, which was related to a pI shift in association with an altered phosphorylation status of serine residue 82. Although both p38 and JNK were activated by DCVC, only inhibition of p38 with SB203580 reduced Hsp27 phosphorylation, which was associated with accelerated reorganization of focal adhesions, cell detachment, and apoptosis. In contrast, inhibition of JNK with SP600125 maintained cell adhesion as well as protection against apoptosis. Active JNK co-localized at focal adhesions after DCVC treatment in a FAK-dependent manner. Inhibition of active JNK localization at focal adhesions did not prevent DCVC-induced phosphorylation of Hsp27. Overexpression of a phosphorylation-defective mutant Hsp27 acted as a dominant negative and accelerated the DCVC-induced changes in the focal adhesions as well as the onset of apoptosis. Our data fit a model whereby early p38 activation results in a rapid phosphorylation of Hsp27, a requirement for proper maintenance of cell adhesion, thus suppressing renal epithelial cell apoptosis.

Protein kinase C mediates cisplatin-induced loss of adherens junctions followed by apoptosis of renal proximal tubular epithelial cells

Cisplatin is a commonly used antitumor agent in the treatment of various human cancers, with nephrotoxicity as a major side effect. Cisplatin causes the loss of cell-cell contacts of renal proximal tubular epithelial cells prior to the onset of apoptosis. We studied the involvement of protein kinase C in these events in the renal epithelial cell line LLC-PK1. Cisplatin caused apoptosis in LLC-PK1 cells, which was directly related to the activation of caspase-3 and DNA fragmentation. Apoptosis was almost completely inhibited by the protein kinase C inhibitors bisindolylmaleimide (Bis) I and Go6983 [2-[1-(3-dimethylaminopropyl)-5-methoxyindol-3-yl]-3-(1H-indol-3-yl) maleimide], but not by Go6976 [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole]. Also, in primary cultured rat renal proximal tubular cells, inhibition of protein kinase C (PKC) inhibited apoptosis. Cisplatin also caused the early loss of cell-cell adhesions, which was associated with the altered localization of the adherens junction-associated protein beta-catenin in association with PKC-mediated phosphorylation of the actincapping protein adducin. These events preceded and were independent of caspase activation. beta-Catenin did not dissociate from E-cadherin. Cisplatin-induced loss of cell-cell contacts was associated with the increased formation of F-actin stress fibers, which was inhibited by Bis I and Go6983 as well as dominant-negative PKC-epsilon. Also, the loss of cell-cell adhesions by cisplatin was prevented by Bis I and Go6983. Activation of protein kinase C with phorbol esters promoted cisplatin-induced loss of cell-cell adhesions as well as apoptosis. In conclusion, the combined data fit a model whereby protein kinase C mediates the cisplatin-induced loss of cellular interactions. Such a loss of these interactions has a role in the onset of apoptosis.

Differential regulation of doxorubicin-induced mitochondrial dysfunction and apoptosis by Bcl-2 in mammary adenocarcinoma (MTLn3) cells

Various anticancer drugs cause mitochondrial perturbations in association with apoptosis. Here we investigated the involvement of caspase- and Bcl-2-dependent pathways in doxorubicin-induced mitochondrial perturbations and apoptosis. For this purpose, we set up a novel three-color flow cytometric assay using rhodamine 123, annexin V-allophycocyanin, and propidium iodide to assess the involvement of the mitochondria in apoptosis caused by doxorubicin in the breast cancer cell line MTLn3. Doxorubicin-induced apoptosis was preceded by up-regulation of CD95 and CD95L and a collapse of mitochondrial membrane potential (Deltapsi) occurring prior to phosphatidylserine externalization. This drop in Deltapsi was independent of caspase activity, since benzyloxycarbonyl-Val-Ala-dl-Asp-fluoromethylketone did not inhibit it. Benzyloxycarbonyl-Val-Ala-dl-Asp-fluoromethylketone also blocked activation of caspase-8, thus excluding an involvement of the death receptor pathway in Deltapsi dissipation. Furthermore, although overexpression of Bcl-2 in MTLn3 cells inhibited apoptosis, dissipation of Deltapsi was still observed. No decrease in Deltapsi was observed in cells undergoing etoposide-induced apoptosis. Immunofluorescent analysis of Deltapsi and cytochrome c localization on a cell-to-cell basis indicates that the collapse of Deltapsi and cytochrome c release are mutually independent in both normal and Bcl-2-overexpressing cells. Together, these data indicate that doxorubicin-induced dissipation of the mitochondrial membrane potential precedes phosphatidylserine externalization and is independent of a caspase- or Bcl-2-controlled checkpoint.

Pathways of proximal tubular cell death in bismuth nephrotoxicity

Colloidal bismuth subcitrate (CBS), a drug for treatment of peptic ulcers, has been reported in the literature to be nephrotoxic in humans when taken in high overdoses. To investigate the mechanism of bismuth nephropathy, we developed an animal model by feeding rats single doses of CBS containing 3.0 mmol Bi/kg body weight. Terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling assay, immunostaining for active caspase-3, and electron microscopy showed that proximal tubular epithelial cells die by necrosis and not by apoptosis within 3 h after CBS administration. Exposure of the renal epithelial cell lines NRK-52E and LLC-PK1 to Bi(3+) in citrate buffer served as an in vitro model of bismuth nephropathy. NRK-52E cells exposed to 100 microM Bi(3+) or more died by necrosis, as was demonstrated by nuclear staining with Hoechst 33258 and flow cytometry using Alexa(488)-labeled Annexin-V and the vital nuclear dye TOPRO-3. Bismuth-induced cell death of NRK-52E cells was not prevented by the caspase-3 inhibitor z-VAD-fmk, whereas this inhibitor did prevent cisplatinum-induced apoptosis. Mitochondrial dysfunction and induction of free radicals were shown not to be involved in bismuth nephrotoxicity. The early time point of damage induction in vitro as well as in vivo and the early displacement of N-cadherin, as found in previous studies, suggest that bismuth induces cell death by destabilizing the cell membrane. In conclusion, we showed that high overdose of bismuth induced cell death by necrosis in vivo as well as in vitro, possibly by destabilization of the cell membrane.

The membrane attack complex of complement induces caspase activation and apoptosis

Activation of the terminal pathway of the complement system leads to insertion of terminal complement complexes (C5b-9) into the cell membrane, which may induce cytolysis. Recent data indicate that the terminal complement pathway can also result in apoptosis in vivo. To further define the cell death pathway induced by complement, we examined induction of apoptosis by complement in vitro. Rat mesangial cells opsonized with a complement-activating antibody and exposed to rat serum as a complement source underwent apoptotic cell death in a time- and dose-dependent fashion, as demonstrated by membrane exposure of phosphatidylserine and fragmentation of nuclei. No significant apoptosis was detected in either cultures treated with C6-deficient serum or in control cultures. The pan-caspase-inhibitor zVAD-fmk inhibited complement-induced apoptosis completely. In line with this observation, complement induced cleavage and activation of caspase 3. Importantly, cellular exposure to purified cytolytically inactive C5b-9, in the absence of antibody and early complement components, also resulted into caspase activation and apoptosis. Together, these results indicate that C5b-9 is involved in induction of apoptosis via a caspase-dependent pathway. Apoptosis as a consequence of complement-mediated cell damage may provide an explanation for the presence of apoptosis in inflammatory processes, for instance in hyperacute xenograft rejection.