Human mitochondrial peptide deformylase, a new anticancer target of actinonin-based antibiotics.

Peptide deformylase activity was thought to be limited to ribosomal protein synthesis in prokaryotes, where new peptides are initiated with an N-formylated methionine. We describe here a new human peptide deformylase (Homo sapiens PDF, or HsPDF) that is localized to the mitochondria. HsPDF is capable of removing formyl groups from N-terminal methionines of newly synthesized mitochondrial proteins, an activity previously not thought to be necessary in mammalian cells. We show that actinonin, a peptidomimetic antibiotic that inhibits HsPDF, also inhibits the proliferation of 16 human cancer cell lines. We designed and synthesized 33 chemical analogs of actinonin; all of the molecules with potent activity against HsPDF also inhibited tumor cell growth, and vice versa, confirming target specificity. Small interfering RNA inhibition of HsPDF protein expression was also antiproliferative. Actinonin treatment of cells led to a tumor-specific mitochondrial membrane depolarization and ATP depletion in a time- and dose-dependent manner; removal of actinonin led to a recovery of the membrane potential consistent with indirect effects on the electron transport chain. In animal models, oral or parenteral actinonin was well tolerated and inhibited human prostate cancer and lung cancer growth. We conclude that HsPDF is a new human mitochondrial enzyme that may provide a novel selective target for anticancer therapy by use of actinonin-based antibiotics.

E2F site in the essential promoter region does not confer S phase-specific transcription of the ABCC10 gene in human prostate cancer cells.

ABCC10 (MRP7) plays a role in cellular detoxification and resistance to anticancer drugs. Since ABCC10 gene transcription in human prostate cancer CWR22Rv1 cells was found dependent on E2F binding sequence motif, ABCC10 expression in G1 and S phases of the cell cycle of CWR22Rv1 cells, was analyzed. The cells were synchronized in G1 phase by double thymidine block and in S phase by thymidine/mimosine double block. ABCC10 mRNA level was found to be similar in S phase-synchronized and asynchronous cell populations. In G1 phase it decreased by 2.4- to 3-fold. It is thus inferred, that ABCC10 expression in CWR22Rv1 cells is not S phase-specific but is primarily associated with cell proliferation.

Drg1, a novel target for modulating sensitivity to CPT-11 in colon cancer cells.

Treatment of the human colon cancer cells Hct116 with SN-38 (an active metabolite of CPT-11) resulted in G2 cell cycle arrest without induction of apoptosis. However, subsequent treatment of SN-38-treated Hct116 cells with flavopiridol induced apoptosis. One of the genes markedly up-regulated during cell cycle arrest by SN-38 and suppressed during apoptosis by SN-38 followed by flavopiridol in Hct116 cells is Drg1. We found that Drg1 had profound effects on SN-38 sensitivity. Inhibition of endogenous Drg1 expression in Hct116 cells by stable expression of an antisense (AS) Drg1 cDNA increased the sensitivity of cells to undergo apoptosis by SN-38. Clonogenic and apoptosis assays with AS Drg1-expressing cells showed a 2-fold decrease in the IC50 and a 4-5-fold increase in induction of apoptosis with SN-38. Conversely, the forced expression of Drg1 in SW620 cells increased the resistance of these cells to SN-38-induced apoptosis by 2-5-fold. Moreover, when xenografted in mice, AS Drg1-expressing Hct116 cells were 3-fold more sensitive to CPT-11 as compared with vector transfected Hct116 cells. Similarly, tumors established from Drg1 overexpressing SW620 cells were more resistant to CPT-11 as compared with tumors established from vector-transfected SW620 cells in mice. Taken together, our data suggest that Drg1 is a novel gene that plays a direct role in resistance to CPT-11. Inhibition of Drg1 may provide a new means to increase the sensitivity of colon cancer cells to CPT-11.

Studies with CWR22 xenografts in nude mice suggest that ZD1839 may have a role in the treatment of both androgen-dependent and androgen-independent human prostate cancer.

PURPOSE: These studies examined the effect of the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor ZD1839 ("Iressa")(3) on CWR22 prostate tumors in nude mice. The effect of ZD1839 was also examined in combination with either bicalutamide ("Casodex") or cytotoxic agents against a hormone-dependent or -independent variant of CWR22, respectively. EXPERIMENTAL DESIGN: The xenografts were grown for 4-7 days, then tumor measurements were made and therapy initiated. ZD1839 and bicalutamide were given p.o. on a once-daily, 5-day schedule for 2 successive weeks. Carboplatin and paclitaxel were given every 3-4 days for a total of four doses. Measurements of tumor volume were made twice weekly during treatment and for 2 weeks after treatment. The effect of ZD1839 on EGFR function was assessed by Western blotting of EGFR and its phosphorylated form in CWR22 and variant tumors before and after treatment with this agent. RESULTS: ZD1839 at its maximum tolerated dose (150 mg/kg) inhibited the growth of androgen-dependent CWR22 by 54%, and the growth of two variants with different degrees of androgen independence and androgen receptor gene expression (CWR22LD1 and CWR22RV1) by 76%. The effects of ZD1839 were similar to those recorded for phosphorylation of EGFR as determined by Western blotting. Coadministration of ZD1839 at its maximum tolerated dose markedly increased the antiproliferative action of the antiandrogen bicalutamide against CWR22LD1. In fact, combining ZD1839 with a suboptimal dose of bicalutamide was more effective than a higher dose of bicalutamide alone. Coadministration of ZD1839, which required a 2-3-fold attenuation of dose to avoid toxicity, also markedly increased the therapeutic activity of carboplatin and paclitaxel against CWR22RV1, bringing about regression to a degree not seen with either agent alone. Tumor-free mice were seen only with the combination of ZD1839 and paclitaxel. CONCLUSIONS: The results obtained in these related and highly relevant models of human prostate cancer suggest that ZD1839 may have a role in enhancing existing treatments of androgen-dependent and -independent forms of this disease in patients.

The cyclin-dependent kinase inhibitor flavopiridol potentiates gamma-irradiation-induced apoptosis in colon and gastric cancer cells.

PURPOSE: Flavopiridol is a cyclin-dependent kinase inhibitor currently under development by the National Cancer Institute both as a single agent and in combination with chemotherapy. There have been numerous reports that flavopiridol potently enhances the induction of apoptosis by chemotherapy. However, the effect of flavopiridol on radiotherapy (RT)-induced apoptosis has been largely untested. RT has become the cornerstone of adjuvant treatment of colorectal and gastric cancer. In view of this, we elected to evaluate the effect of flavopiridol on potentiating RT-induced apoptosis in the human colon cancer cell line HCT-116 and the gastric cancer cell line MKN-74. EXPERIMENTAL DESIGN: The efficacy of combination of gamma-irradiation and flavopiridol was tested in vitro in MKN-74 and HCT-116 cells and correlated to changes in p21 expression. HCT-116 cells were also established as tumors in nude mice and treated with gamma-irradiation and flavopiridol either as single agents or in sequential combinations such that flavopiridol was either given 7 h before, concomitantly, or 3 and 7 h after gamma-irradiation. RESULTS: Flavopiridol significantly enhanced the induction of apoptosis by gamma-irradiation in both cell lines as measured by quantitative fluorescent microscopy, caspase-3 activation, poly(ADP-ribose) polymerase cleavage, and cytochrome c release. To achieve the best effect, it was important to expose the tumor cells to gamma-irradiation before the flavopiridol. This sequence dependence was confirmed in vivo. When gamma-irradiation was administered 7 h before flavopiridol, 42% of the tumor-bearing animals were rendered disease free, compared with no animals treated with either gamma-irradiation or flavopiridol alone. Examination of the p21 status of HCT-116 and MKN-74 cells, after treatment with sequential gamma-irradiation and flavopiridol, indicated a loss of p21 protein expression. Loss of p21 was mainly due to cleavage by caspases. HCT-116 cells that lack p21 (p21(-/-)) also exhibited sensitization to gamma-irradiation and showed an even greater enhancement of gamma-irradiation-induced apoptosis by flavopiridol when compared with the parental HCT-116 cells. CONCLUSIONS: These studies indicate that gamma-irradiation followed by flavopiridol enhances apoptosis and yields significantly increased tumor regressions and cures that are not achievable with radiation alone. These results indicate that flavopiridol can potently enhance the effect of gamma-radiation both in vitro and in vivo and may provide a new means to treat patients with locally advanced gastrointestinal cancers.

Flavopiridol enhances the effect of docetaxel in vitro and in vivo in human gastric cancer cells.

Gastric cancer is one of the leading causes of cancer death throughout the world. It is a disease in desperate need of new therapeutic approaches. Docetaxel, a semisynthetic taxane, has shown potent activity against a broad range of solid tumors. However, in gastric cancer, response rates to docetaxel remain only approximately 20%. In these studies we show that flavopiridol, a cyclin-dependent kinase inhibitor, potentiates docetaxel-induced apoptosis 3-fold in MKN-74 human gastric cells. This effect is sequence dependent, such that flavopiridol must follow docetaxel to induce this effect. Docetaxel induces transient arrest in the M phase of the cell cycle. Cells exit mitosis in a specific time window without cytokinesis with a decrease in cyclin B1/cdc-2 kinase activity and MPM-2 labeling. Flavopiridol treatment of docetaxel-treated cells enhances the exit from mitosis with a more rapid decrease in mitotic markers including MPM-2 labeling and cyclin B1/cdc2 kinase activity. In contrast, pretreatment with flavopiridol prevents cells from entering mitosis by inhibiting cyclin B1/cdc-2 kinase activity, thus antagonizing the docetaxel effect. The testing of this combination against MKN-74 xenografts confirms the sequence dependency. Treatment of MKN-74 tumor-bearing xenografts with docetaxel at a dose of 10 mg/kg followed 3-7 h later by flavopiridol at a dose of 2.5 mg/kg resulted in a 1-18% decrease in tumor volume. In contrast, treatment with docetaxel alone at this same dose resulted in a 394% increase in tumor volume. When flavopiridol was given immediately after docetaxel, the effect was not statistically different from that of docetaxel alone. The reverse combination of flavopiridol followed 7 h later by docetaxel was similar to treatment with docetaxel alone. Flavopiridol alone had no effect in this tumor model. Thus, flavopiridol, when combined with docetaxel in a sequence-specific manner, may provide a completely new therapeutic approach in the treatment of gastric cancer.

Regulation of transcription of the human MRP7 gene. Characteristics of the basal promoter and identification of tumor-derived transcripts encoding additional 5' end heterogeneity.

Studies focusing on the transcriptional regulation of MRP7 (multidrug resistance associated protein 7) gene expression in human tumor cells are described. As shown by real-time RT-PCR, expression of the MRP7 gene compared to the expression of the MRP1, 2 and 3 genes was less variable among the different cell types. MRP1, 2, 3 and 7 gene expression was highest in HepG2 cells compared to expression in CWR22Rv1 and TSU-PR1 cells. MRP7 gene expression was less than expression of the MRP1 and 2 genes in HepG2 cells but similar to MRP3 gene expression in this cell type and similar to or greater than expression of the MRP1, 2 and 3 genes in CWR22Rv1 and TSU-PR1 cells. Functional deletion analysis, in situ mutagenesis and electromobility shift assays (EMSA) showed that basal MRP7 promoter activity relied upon a proximal segment of the 5' flanking region 169 to 257 nt in length bearing an E2F site acting cooperatively with two closely positioned Sp1 sites. Two additional Sp1 sites further downstream were of secondary importance. The sequence of the E2F site was noncanonical and its interaction with E2F protein was confirmed by a competitive EMSA using a consensus E2F oligonucleotide probe and by demonstrating a supershift with the antibody against the E2F4 and E2F5 pocket protein, p107. 5' RACE carried out with CWR22Rv1 and HepG2 cells detected a single transcription start site (tsp) distal to the basal promoter and identified two new MRP7 transcripts with very short 5' UTR sequences compared to transcripts found by others in nontumorous human tissue. This 5' end heterogeneity infers a more complex intron-exon composition than hitherto shown.

Studies with ZD1839 in preclinical models.

ZD1839 (Iressa; AstraZeneca Pharmaceuticals LP, Wilmington, DE) is an orally active, selective inhibitor of epidermal growth factor receptor-tyrosine kinase (EGFR-TK) that blocks signaling pathways responsible for driving proliferation, invasion, and survival of cancer cells. In preclinical studies of cell lines and human tumor xenografts, ZD1839 as single-agent therapy produced growth inhibition in a wide variety of common solid tumor types including lung, prostate, breast, colon, and ovarian cancers. In these models, ZD1839 inhibited growth of tumor xenografts with high, moderate, and low expression of EGFR. The A431 vulvar carcinoma model, which expresses abnormally high levels of EGFR, was particularly sensitive to ZD1839 treatment, leading to tumor regression. When ZD1839 was coadministered with cytotoxic chemotherapy agents or radiotherapy, additive or even synergistic antitumor activity was achieved. The inhibition observed with ZD1839 treatment was not restricted to advanced metastatic tumors, but also extended to early lesions such as breast xenografts of human ductal carcinoma in situ. Inhibition of EGFR-TK has also been shown to delay the onset of tumor development in a transgenic animal model. The diverse and profound antitumor activities attained with ZD1839 treatment in tumor cells and in xenograft tumor models provided the rationale for clinical development of ZD1839. Ongoing preclinical studies continue to support the importance of EGFR-TK activity in the biology of solid tumors.

Randomized phase II study of pulse erlotinib before or after carboplatin and paclitaxel in current or former smokers with advanced non-small-cell lung cancer.

PURPOSE: A prior study demonstrated that addition of continuous daily erlotinib fails to improve response rate or survival in non-small-cell lung cancer (NSCLC) patients treated with carboplatin and paclitaxel. However, preclinical data support the hypothesis that intermittent administration of erlotinib before or after chemotherapy may improve efficacy. We tested this hypothesis in patients with advanced NSCLC. PATIENTS AND METHODS: Eligible patients were former or current smokers with chemotherapy-naive stage IIIB or IV NSCLC. All patients received up to six cycles of carboplatin (area under the curve = 6) and paclitaxel (200 mg/m(2)), with random assignment to one of the following three erlotinib treatments: erlotinib 150 mg on days 1 and 2 with chemotherapy on day 3 (150 PRE); erlotinib 1,500 mg on days 1 and 2 with chemotherapy on day 3 (1,500 PRE); or chemotherapy on day 1 with erlotinib 1,500 mg on days 2 and 3 (1,500 POST). The primary end point was response rate. RESULTS: Eighty-six patients received treatment. The response rates for the 150 PRE, 1,500 PRE, and 1,500 POST arms were 18% (five of 28 patients), 34% (10 of 29 patients), and 28% (eight of 29 patients), respectively. The median overall survival times were 10, 15, and 10 months for the 150 PRE, 1,500 PRE, and 1,500 POST arms, respectively. The most common grade 3 and 4 toxicities were neutropenia (39%), fatigue (15%), and anemia (12%). Grade 3 and 4 rash and diarrhea were uncommon. CONCLUSION: Patients treated on the 1,500 PRE arm had the highest response rate and longest survival, with ranges similar to those reported for carboplatin, paclitaxel, and bevacizumab in a more restricted population. Further evaluation of this strategy in a phase III trial is proposed.

A new human peptide deformylase inhibitable by actinonin.

Peptide deformylases (PDFs) have been investigated as potential specific targets for antibiotics, but the possible existence of a functional human PDF (HsPDF) presents a potential hurdle to the design of specific drugs. We have expression cloned a HsPDF that has deformylase activity, although it is a slower and catalytically less active enzyme than bacterial or plant PDFs. A cobalt-substituted form of HsPDF (but not nickel or zinc) is active, and the enzyme appears to be active at a pH between 6.0 and 7.2, a temperature range of 25-50 degrees C, and in a low KCl ionic strength buffer. Actinonin inhibits HsPDF activity with an IC50 of 43 nM and kills Daudi and HL60 human cancer cell lines with an LC50 of 5.3 and 8.8 microM, respectively. The inhibition of HsPDF may provide an explanation for the mechanism by which actinonin is cytotoxic against various human tumor cell lines.