Leonard Lothstein

Taxol: Mechanisms of Action and Resistance

Information on the mechanisms of action and of resistance to Taxol, as well as new data from our laboratory on the promoter regions of the genes that encode P-glycoprotein in a murine Taxol-resistant cell line, is discussed. Taxol induces the formation of stable bundles of microtubules, thereby interfering with the normal function of cellular microtubules. The drug can induce the multidrug-resistance (MDR) phenotype that includes the overproduction of P-glycoprotein, a membrane glycoprotein that acts as a drug efflux pump. In human tumors resistant to Taxol, P-glycoprotein could be responsible for maintaining the drug below cytotoxic levels. Analyses of the MDR promoters that are involved in P-glycoprotein expression and overproduction revealed an interesting recombination event in a Taxol-resistant cell line. As an important new clinical agent for the treatment of malignancies, Taxol requires further mechanistic investigations at the preclinical level.

ATM and the Catalytic Subunit of DNA-Dependent Protein Kinase Activate NF-κB through a Common MEK/Extracellular Signal-Regulated Kinase/p90rsk Signaling Pathway in Response to Distinct Forms of DNA Damage

ABSTRACT We have identified a novel pathway of ataxia telangiectasia mutated (ATM) and DNA-dependent protein kinase (DNA-PK) signaling that results in nuclear factor κB (NF-κB) activation and chemoresistance in response to DNA damage. We show that the anthracycline doxorubicin (DOX) and its congener N-benzyladriamycin (AD 288) selectively activate ATM and DNA-PK, respectively. Both ATM and DNA-PK promote sequential activation of the mitogen-activated protein kinase (MAPK)/p90 rsk signaling cascade in a p53-independent fashion. In turn, p90 rsk interacts with the IκB kinase 2 (IKK-2) catalytic subunit of IKK, thereby inducing NF-κB activity and cell survival. Collectively, our findings suggest that distinct members of the phosphatidylinositol kinase family activate a common prosurvival MAPK/IKK/NF-κB pathway that opposes the apoptotic response following DNA damage.

Phosphorylation of mitochondrial phospholipid scramblase 3 by protein kinase C-δ induces its activation and facilitates mitochondrial targeting of tBid

Phospholipid scramblase 3 (PLS3) is a member of the phospholipid scramblase family present in mitochondria. PLS3 plays an important role in regulation of mitochondrial morphology, respiratory function, and apoptotic responses. PLS3 is phosphorylated by PKC‐δ at Thr21 and is the mitochondrial target of PKC‐δ‐induced apoptosis. Cells with overexpression of PLS3, but not the phosphoinhibitory mutant PLS3(T21A), are more susceptible to apoptosis induced by AD198, an extranuclear targeted anthracycline that activates PKC‐δ. Here we report that the phosphomimetic mutant of PLS3(T21D) by itself can induce apoptosis in HeLa cells. Using proteoliposomes with addition of pyrene‐labeled phosphatidylcholine (PC) at the outer leaflet, we measured the lipid flip‐flop activity of PLS3 and its phosphorylation mutant. PLS3(T21D) is more potent than wild‐type PLS3 or PLS3(T21A) to transfer pyrene‐PC from the outer leaflet to the inner leaflet of liposomes. Based on our previous finding that PLS3 enhances tBid‐induced mitochondrial damages, we tested the hypothesis that PLS3 enhances cardiolipin translocation to mitochondrial surface and facilitates tBid targeting. Fluorescein‐labeled tBid(G94E) was used as a probe to quantify cardiolipin on the surface of mitochondria. Mitochondria from cells treated with AD198 or cells expressing PLS3(T21D) had a higher level of tBid‐binding capacity than control cells or cells expressing wild‐type PLS3. These findings indicate that phosphorylation of PLS3 by PKC‐δ induces PLS3 activation to facilitate mitochondrial targeting of tBid and apoptosis. J. Cell. Biochem. 101:1210–1221, 2007.

Involvement of PKC delta (PKCδ) in the resistance against different doxorubicin analogs

Doxorubicin is an anti-tumor antibiotic widely used in the management of cancer patients. Its main mechanism of action involves the generation of DNA damage and the inhibition of topoisomerase II, promoting apoptosis. AD 198 is a novel doxorubicin analog devoid of DNA binding and topoisomerase II inhibitory capacities. It has been proposed that AD 198 induces apoptosis by activating protein kinase C delta (PKCδ); a PKC isoform described as growth inhibitory in a large number of cell types. We have previously demonstrated that PKCδ overexpression in NMuMG cells induced the opposite effect, promoting proliferation and cell survival. In this study, we found that PKCδ overexpression confers an enhanced cell death resistance against AD 198 cytotoxic effect and against AD 288, another doxorubicin analog that preserves its mechanism of action. These resistances involve PKCδ-mediated activation of two well-known survival pathways: Akt and NF-κB. While the resistance against AD 198 could be abrogated upon the inhibition of either Akt or NF-κB pathways, only NF-κB inhibition could revert the resistance to AD 288. Altogether, our results indicate that PKCδ increases cell death resistance against different apoptosis inductors, independently of their mechanism of action, through a differential modulation of Akt and NF-κB pathways. Our study contributes to a better understanding of the mechanisms involved in PKCδ-induced resistance and may greatly impact in the rationale design of isozyme-specific PKC modulators as therapeutic agents.

Interferon-resistant Daudi Cell Line with a Stat2 Defect Is Resistant to Apoptosis Induced by Chemotherapeutic Agents

Interferon-α (IFNα) has shown promise in the treatment of various cancers. However, the development of IFN resistance is a significant drawback. Using conditions that mimic in vivo selection of IFN-resistant cells, the RST2 IFN-resistant cell line was isolated from the highly IFN-sensitive Daudi human Burkitt lymphoma cell line. The RST2 cell line was resistant to the antiviral, antiproliferative, and gene-induction actions of IFNα. Although STAT2 mRNA was present, STAT2 protein expression was deficient in RST2 cells. A variant STAT2 mRNA, which resulted from alternative splicing within the intron between exon 19 and 20, was expressed in several human cell lines but at relatively high levels in RST2 cells. Most importantly, the RST2 line showed an intrinsic resistance to apoptosis induced by a number of chemotherapeutic agents (camptothecin, staurosporine, and doxorubicin). Expression of STAT2 in RST2 cells not only rescued their sensitivity to the biological activities of IFNs but also restored sensitivity to apoptosis induced by these chemotherapeutic agents. The intrinsic resistance of the RST2 cells to IFN as well as chemotherapeutic agents adds a new dimension to our knowledge of the role of STAT2 as it relates to not only biological actions of IFN but also resistance to chemotherapy-induced apoptosis.

Protection from Doxorubicin-Induced Cardiomyopathy Using the Modified Anthracycline N-Benzyladriamycin-14-valerate (AD 198)

The anthracycline doxorubicin (Dox) is an effective antitumor agent. However, its use is limited because of its toxicity in the heart. N-Benzyladriamycin-14-valerate (AD 198) is a modified anthracycline with antitumor efficacy similar to that of Dox, but with significantly less cardiotoxicity and potentially cardioprotective elements. In the present study, we investigated the possibility of in vivo protective effects of low-dose AD 198 against Dox-induced cardiomyopathy. To do this, rats were divided into four groups: vehicle, Dox (20 mg/kg; single injection day 1), AD 198 (0.3 mg/kg per injection; injections on days 1, 2, and 3), or a combination treatment of Dox + AD 198. Seventy-two hours after beginning treatment, hearts from the Dox group had decreased phosphorylation of AMP kinase and troponin I and reduced poly(ADP-ribose) polymerase, β-tubulin, and serum albumin expression. Dox also increased the phosphorylation of phospholamban and expression of inducible nitric-oxide synthase in hearts. Each of these Dox-induced molecular changes was attenuated in the Dox + AD 198 group. In addition, excised hearts from rats treated with Dox had a 25% decrease in left ventricular developed pressure (LVDP) and a higher than normal increase in LVDP when perfused with a high extracellular Ca2+ solution. The Dox-induced decrease in baseline LVDP and hyper-responsiveness to [Ca2+] was not observed in hearts from the Dox + AD 198 group. Thus Dox, with well established and efficient antitumor protocols, in combination with low levels of AD 198, to counter anthracycline cardiotoxicity, may be a promising next step in chemotherapy.

N-benzyladriamycin-14-valerate (AD 198) activates protein kinase C-δ holoenzyme to trigger mitochondrial depolarization and cytochrome c release independently of permeability transition pore opening and Ca2+ influx

Unlike nuclear-targeted anthracyclines, the extranuclear-targeted doxorubicin congener, N-benzyladriamycin-14-valerate (AD 198), does not interfere with normal topoisomerase II activity, but binds to the C1b regulatory domain of conventional and novel isoforms of protein kinase C (PKC). The resulting interaction leads to enzyme activation and rapid apoptosis in a variety of mammalian cell lines through a pathway involving mitochondrial events such as membrane depolarization (Δψm) and cytochrome c release. Unlike other triggers of apoptosis, AD 198-mediated apoptosis is unimpeded by the expression of Bcl-2 and Bcl-XL. We have further examined AD 198-induced apoptosis in 32D.3 mouse myeloid cells to determine how the anti-apoptotic effects of Bcl-2 are circumvented. The PKC-δ inhibitor, rottlerin, and transfection with a transdominant-negative PKC-δ expression vector both inhibit AD 198 cytotoxicity through inhibition of Δψm and cytochrome c release. While the pan-caspase inhibitor Z-VAD-FMK blocks AD 198-induced PKC-δ cleavage, however, it does not inhibit Δψm and cytochrome c release, indicating that AD 198 induces PKC-δ holoenzyme activation to achieve apoptotic mitochondrial effects. AD 198-mediated Δψm and cytochrome c release are also unaffected by cellular treatment with either the mitochondrial permeability transition pore complex (PTPC) inhibitor cyclosporin A or the Ca2+ chelators EGTA and BAPTA-AM. These results suggest that AD 198 activates PKC-δ holoenzyme, resulting in Δψm and cytochrome c release through a mechanism that is independent of both PTPC activation and Ca2+ flux across the mitochondria. PTPC-independent mitochondrial activation by AD 198 is consistent with the inability of Bcl-2 and Bcl-XL expression to block AD 198-induced apoptosis.

N-Benzyladriamycin-14-Valerate (AD198) Induces Apoptosis through Protein Kinase C-δ–Induced Phosphorylation of Phospholipid Scramblase 3

Phospholipid scramblase 3 (PLS3) is an enzyme that plays a critical role in mitochondrial morphology, functions, and apoptotic response. During apoptosis, activated protein kinase C-delta (PKC-delta) translocates to mitochondria and phosphorylates PLS3. Here, we utilize an extranuclear-targeted anthracycline N-benzyladriamycin-14-valerate (AD198), a PKC-delta activator, to investigate the mechanism of PLS3 phosphorylation by PKC-delta. Overexpression of PLS3 enhanced, whereas down-regulation of PLS3 by small interfering RNA decreased, the sensitivity of AD198-induced apoptosis. Overexpression of PKC-delta, but not the kinase-defective PKC-delta, and AD198 treatment enhanced threonine phosphorylation of PLS3. The phosphorylated threonine was mapped to Thr21 of PLS3. Mutation of Thr21 to alanine did not affect mitochondrial localization of PLS3 but abolished threonine phosphorylation by PKC-delta in vitro and AD198-induced PLS3 phosphorylation in vivo. Expression of PLS3(T21A) in cells could not enhance AD198-induced apoptosis compared with expression of the wild-type PLS3. Using benzyloxycarbonyl-Val-Ala-Asp-(OMe) fluoromethyl ketone and cyclosporine A, we also showed that AD198-induced PLS3 phosphorylation occurs upstream of caspase activation and independent of mitochondrial permeability transition. These studies establish that AD198-activated PKC-delta induces phosphorylation of mitochondrial PLS3 at Thr21 and that PLS3 is a critical downstream effector of PKC-delta in AD198-induced apoptosis.

Cytotoxicity and intracellular biotransformation of N-benzyladriamycin- 14-valerate (AD 198) are modulated by changes in 14-O-acyl chain length

N-benzyladriamycin-14-valerate (AD 198) is pharmacologically superior to Adriamycin (ADR) based upon comparable cytotoxicity, decreased cardiotoxicity and the ability of AD 198 to circumvent multidrug resistance conferred by either Pglycoprotein overexpression or reduced topoisomerase II activity. AD 198, however, suffers from systemic lability of the 14-O-valerate moiety to enzymatic and non-enzymatic cleavage to yield N-benzyladriamycin (AD 288), which is more similar to ADR in activity. The purpose of this study was to determine whether stability of the ester linkage could be achieved while preserving the favorable characteristics of AD 198 by using a series of N-benzylated ADR congeners containing 14-O-acyl substitutions of incrementally shorter carbon chain lengths. Results from this study indicate that the linear five-carbon valerate substitution is the minimum length necessary to circumvent P-glycoprotein and prevent inhibition of topoisomerase II activity. In addition, although AD 198 is not a pro-drug of AD 288, intracellular 14-O-acyl cleavage appears to contribute to the cytotoxicity of AD 198.

Hydroxylation at C-3′ of doxorubicin alters the selected phenotype of cellular drug resistance

Abstract Hydroxylation at C-3′ of doxorubicin (DOX) yields the uncharged congener hydroxyrubicin, which circumvents P-glycoprotein-mediated drug resistance without loss of topoisomerase II inhibitory activity. Hydroxyrubicin-resistant cells exhibit a phenotype that is uniquely different from DOX resistance by expressing non-functional P-glycoprotein and hypersensitivity to anti-mitotic drugs.