In vitro hypoxia-conditioned colon cancer cell lines derived from HCT116 and HT29 exhibit altered apoptosis susceptibility and a more angiogenic profile in vivo

Hypoxia is an important selective force in the clonal evolution of tumours. Through HIF-1 and other transcription factors combined with tumour-specific genetic alterations, hypoxia is a dominant factor in the angiogenic phenotype. Cellular adaptation to hypoxia is an important requirement of tumour progression independent of angiogenesis. The adaptive changes, insofar as they alter hypoxia-induced apoptosis, are likely to determine responsiveness to antiangiogenic strategies. To investigate this adaptation of tumour cells to hypoxia, we recreated in vitro the in vivo situation of chronic intermittent exposure to low-oxygen levels. The colon carcinoma cell lines HT29 and HCT116 were subjected to 40 episodes of sublethal hypoxia (4 h) three times a week. The resulting two hypoxia-conditioned cell lines have been maintained in culture for more than 2 years. In both cell lines changes in doubling times occurred: in HT29 an increase, and in HCT116 a decrease. Cell survival in response to hypoxia and to DNA damage differed strikingly in the two cell lines. The HT29 hypoxia-conditioned cells were more resistant than the parental line to a 24 h hypoxic challenge, while those from HCT116 surprisingly were more sensitive. Sensitivity to cisplatin in vitro was also significantly different for the hypoxia-conditioned compared with the parental lines, suggesting a change in pathways leading to apoptosis following DNA damage signaling. The growth of both conditioned cell lines in vivo as xenografts in immunodeficient (SCID) mice was more rapid than their parental lines, and was accompanied in each by evidence of enhanced vascular proliferation as a consequence of the hypoxia-conditioning. Thus the changes in apoptotic susceptibility were independent of altered angiogenesis. The derivation of these lines provides a model for events within hypoxic regions of colon cancers, and for the acquisition of resistance and sensitivity characteristics that may have therapeutic implications for the use of antiangiogenesis drugs.

Ovarian epithelial cell lineage-specific gene expression using the promoter of a retrovirus-like element

We have isolated 462 bp of sequence termed ovarian-specific promoter 1 (OSP-1) that is part of a retrovirus-like element specifically expressed in the rat ovary. We have evaluated the ability of OSP-1 to activate gene expression in normal and neoplastic cell lines derived from the ovaries of rats and women. We have found that there was marked specificity in the ability of OSP-1 to drive reporter gene expression in an ovarian epithelial cell lineage manner. The expression of herpes simplex virus thymidine kinase (HSV-TK) under OSP-1 control was sufficiently ovarian cancer cell line specific to render ganciclovir approximately 50-fold more toxic in the A2780 human ovarian cancer cell line compared with clones of the HCT-116 and HT-29 colon cancer cell lines. Furthermore, ganciclovir had marked antitumor efficacy in vivo in severe combined immunodeficient mice bearing A2780OSP-1-HSV-TK as a s.c. xenograft. We suggest that these data support the use of OSP-1 as a tool to provide specificity to the gene therapy of ovarian cancer and to drive ovarian-specific oncogene expression for the creation of transgenic mouse models of ovarian cancer.

The phosphatidylinositol 3-kinase/AKT signal transduction pathway plays a critical role in the expression of p21WAF1/CIP1/SDI1 induced by cisplatin and paclitaxel

The cyclin-dependent kinase inhibitor p21WAF1/CIP1/SD11 (p21) plays a crucial role in DNA repair, cell differentiation, and apoptosis through regulation of the cell cycle. A2780 human ovarian carcinoma cells, which are sensitive to cisplatin and paclitaxel, express wild-type p53 and exhibit a p53-mediated increase in p21 in response to the chemotherapeutic agents. Here, we demonstrate that phosphatidylinositol 3-kinase (PI3K) and its downstream targets serine/threonine kinases AKT1 and AKT2 (AKT), are required for the full induction of p21 in A2780 cells treated with cisplatin or paclitaxel. Inactivation of the PI3K/AKT signal transduction pathway either by its specific inhibitor LY294002 or by expression of dominant negative AKT inhibited p21 expression but had no inhibitory effect on the expression of the proapoptotic protein BAX by cisplatin and paclitaxel treatment. In addition, overexpression of wild-type or constitutively active AKT in A2780 cells sustained the regulation of p21 induction or increased the level of p21 expression, respectively. Experiments with additional ovarian carcinoma cell lines revealed that PI3K is involved in the expression of p21 induced by cisplatin or paclitaxel in OVCAR-10 cells, which have wild-type p53, but not in OVCAR-5 cells, which lack functional p53. These data indicate that the PI3K/AKT signal transduction pathway mediates p21 expression and suggest that this pathway contributes to cell cycle regulation promoted by p53 in response to drug-induced stress. However, inactivation of PI3K/AKT signaling did not result in significant alteration of the drug sensitivity of A2780 cells, suggesting that the cell death induced by cisplatin or paclitaxel proceeds independently of cell protective effects of PI3K and AKT.

Use of a surrogate marker (human secreted alkaline phosphatase) to monitor in vivo tumor growth and anticancer drug efficacy in ovarian cancer xenografts

OBJECTIVES

A limitation to preclinical evaluation of possible anticancer therapy is the objective assessment of efficacy, especially in the presence of small tumor burden or inaccessible disease. This study is designed to test whether human secreted alkaline phosphatase (SEAP) could be used as a soluble marker for in vivo tumor burden.

METHODS

A SEAP expression construct under control of the CMV promoter was created. The SEAP activity in the conditioned medium was evaluated at 24 h and 48 h after the A2780 cell line was transiently transfected with the SEAP vector using Superfect reagent. Stable transfection of A2780 was accomplished by selection of transfectants in G418. SEAP activity of the stable transfectant was determined in conditioned medium and its relationship to tumor cell number was examined. A highly expressing stable transfectant was implanted into immunocompromised mice (2 x 10(6) subcutaneously and 5 x 10(6) intraperitoneally) and peripheral blood was obtained by orbital puncture every 5 days. The relationship between blood SEAP activity and tumor burden was studied. The usefulness of this marker in preclinical assessment of anticancer drug efficacy was evaluated by studying the plasma SEAP activity in xenografted mice treated or not treated with paclitaxel.

RESULTS

After transient transfection of the A2780 cell line (5 x 10(5)) with the plasmid, SEAP activity was found in the medium at 24 h (482.0 +/- 2.0 ng/ml) and 48 h (1296.0 +/- 1.0 ng/ml). The in vitro study using a stable transfectant demonstrated that SEAP activity was linearly related to cell numbers (r = 0.99). The in vivo study demonstrated that SEAP was detectable in plasma one day postinjection, long before measurable tumor or detectable intraperitoneal tumor was present. Once detectable SC tumor was present, the SEAP activity correlated well with tumor volume (r = 0. 94-0.97). The plasma SEAP level was reduced after xenografted mice were treated with paclitaxel (20 mg/kg, weekly x5) compared with untreated mice in both SC and IP tumor models (P = 0.05, P = 0.025, respectively).

CONCLUSION

These data suggest that the plasma SEAP activity can be used as an alternative to survival or tumor measurement in evaluating anticancer agents for efficacy, especially in the case of minimal or inaccessible disease.

The proto-oncogene c-myc blocks myeloid differentiation independently of its target gene ornithine decarboxylase

Ornithine decarboxylase (ODC), a rate-limiting enzyme of polyamine biosynthesis, has been shown to be required for entry into and progression through the cell cycle and to be a transcriptional target of the proto-oncogene, c-myc. We show that ODC transcripts and enzyme activity are down-regulated following induction of myeloid differentiation, using M1 myeloblastic leukemic cells and normal cells from bone marrow (BM), and fail to be suppressed when c-myc expression is deregulated. In M1mycer cells, when endogenous c-myc expression has been suppressed following stimulation by interleukin-6 (IL-60), treatment with estrogen and cycloheximide results in induction of ODC transcripts. These data demonstrate that ODC is a c-myc target gene in M1 cells. It was of interest to determine whether deregulated ODC expression would alter the myeloid differentiation program. To answer this question, M1-ODC cell lines constitutively expressing ODC were established. These cells can undergo terminal differentiation and growth arrest following IL-6 stimulation, exactly like parental M1 cells, demonstrating that deregulated ODC expression is not sufficient to block myeloid differentiation. Another question to be answered was whether ODC expression is necessary for the c-myc-mediated block in differentiation. The use of alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC enzyme activity, indicates that ODC is not necessary for the c-myc-mediated differentiation block.

Blocking c-Myc and Max expression inhibits proliferation and induces differentiation of normal and leukemic myeloid cells

Given the central role c-Myc plays in growth control, differentiation and apoptosis, understanding how c-Myc functions will increase our understanding about normal cell development, and how alterations in these processes can lead to malignancy. C-Myc is a negative regulator of terminal myeloid differentiation; therefore, it was of interest to determine what effect blocking c-Myc expression would have on proliferation and differentiation. In this work we showed that blocking expression of either c-Myc or Max, its molecular partner, in myeloblastic leukemia M1 cells activated the differentiation program in the absence of an exogenous source of differentiation inducer; the cells assumed an intermediate stage myeloid morphology. Moreover, when both c-Myc and Max expression was concommitantly blocked, many of the cells underwent terminal differentiation. Finally, extending these studies to myeloblast enriched normal bone marrow (BM) cell has shown that blocking expression of either c-Myc or Max accelerated GM-CSF-induced differentiation along both the granulocytic and monocytic lineages. Thus, it can be concluded that blocking either c-Myc or Max expression in myeloid cells at specific stages of development activates and accelerates the terminal differentiation program.

Dissection of the genetic programs of p53-mediated G1 growth arrest and apoptosis: blocking p53-induced apoptosis unmasks G1 arrest

Employing the myeloblastic leukemia M1 cell line, which does not express endogenous p53, and genetically engineered variants, it was recently shown that activation of p53, using a p53 temperature-sensitive mutant transgene (p53ts), resulted in rapid apoptosis that was delayed by high level ectopic expression of bcl-2. In this report, advantage has been taken of these M1 variants to investigate the relationship between p53-mediated G1 arrest and apoptosis. Flow cytometric cell cycle analysis has provided evidence that activation of wild-type (wt) p53 function in M1 cells resulted in the induction of G1 growth arrest; this was clearly seen in the M1p53/bcl-2 cells because of the delay in apoptosis that unmasked p53-induced G1 growth arrest. This finding was further corroborated at the molecular level by analysis of the expression and function of key cell cycle regulatory genes in M1p53 versus M1p53/bcl-2 cells after the activation of wt p53 function; events that take place at early times during the p53-induced G1 arrest occur in both the M1p53 and the M1p53/bcl-2 cells, whereas later events occur only in the M1p53/bcl-2 cells, which undergo delayed apoptosis, thereby allowing the cells to complete G1 arrest. Finally, it was observed that a spectrum of p53 target genes implicated in p53-induced growth suppression and apoptosis were similarly regulated, either induced (gadd45, waf1, mdm2, and bax) or suppressed (c-myc and bcl-2), after activation of wt p53 function in M1p53 and M1p53/bcl-2 cells. Taken together, these findings show that wt p53 can simultaneously induce the genetic programs of both G1 growth arrest and apoptosis within the same cell type, in which the genetic program of cell death can proceed in either G1-arrested (M1p53/bcl-2) or cycling (M1p53) cells. These findings increase our understanding of the functions of p53 as a tumor suppressor and how alterations in these functions could contribute to malignancy.

Progression of the myeloid differentiation program is dominant to transforming growth factor-beta 1-induced apoptosis in M1 myeloid leukemic cells

Hematopoiesis is a profound example of cell homeostasis that is regulated throughout life. This process requires the participation of many factors, including positive and negative regulators of growth and differentiation, that determine survival, growth stimulation, differentiation, functional activation, and programmed cell death. Understanding the effects of multiple stimuli on specific cells at the molecular and cellular level is crucial towards understanding how the population of blood cells maintains a homeostatic state. Two appropriate stimuli for analysis, both of which are found in bone marrow, are differentiation-inducing cytokines, which induce terminal differentiation associated with growth arrest, ultimately culminating in programmed cell death, and transforming growth factor-beta 1 (TGF-beta 1), which induces rapid growth arrest and apoptosis of hematopoietic cells. Previously, we have shown, using M1 myeloblastic leukemic cells as a model system, that differentiation-inducing cytokines induce terminal differentiation associated with growth arrest and, only after 5 to 7 days, apoptosis, whereas TGF-beta 1 induces rapid growth arrest and apoptosis. In this report, we show that M1 myeloid leukemic cells treated concomitantly with the differentiation inducer interleukin-6 and TGF-beta 1 undergo terminal differentiation, in which modulators of the MyD118 gene product, previously shown to be a positive regulator of TGF-beta 1-induced apoptosis, are implicated to play a role in protecting the cells from TGF-beta 1-induced apoptosis. Furthermore, using M1 cell variants blocked at different stages after induction of differentiation, including M1myb and M1myc, as well as conditionally blocked M1mycer, it has been shown that the dominance of interleukin-6 to TGF-beta 1-induced apoptosis is dependent on the progression of the differentiation program. Further studies with M1 and the genetically engineered M1 cell variants will be instrumental towards molecularly dissecting the interaction of hematopoietic differentiation with a variety of apoptotic pathways.

Immediate early up-regulation of bax expression by p53 but not TGF beta 1: a paradigm for distinct apoptotic pathways

Recently, both Bcl-2, which promotes cell survival, and Bax, which promotes cell death, have been implicated as major players in the control of apoptotic pathways, and it has been suggested that the ratio of Bcl-2 and Bax protein controls the relative susceptibility of cells to death stimuli. We have used M1 myeloid leukemia cells and genetically engineered M1 variants as a model system to study apoptosis induced by two distinct apoptotic stimuli. This includes apoptosis induced by activation of wild type p53 function of a temperature sensitive p53 transgene expressed in M1 cells, which do not express endogenous p53, and apoptosis induced by TGF beta 1. It is shown that the kinetics of apoptosis induced by p53 is more rapid than apoptosis induced by TGF beta 1. It is also shown that ectopic expression of Bcl-2, at levels which blocked TGF beta 1-induced apoptosis of M1 cells, delayed, but did not block, p53-induced apoptosis. Both p53 and TGF beta 1 down-regulated endogenous Bcl-2 expression, but only p53 up-regulated Bax expression, where bax has been identified as a p53 immediate early response gene. Thus, the p53-mediated up-regulation of Bax may provide at least a partial explanation for the more rapid rate of apoptosis induced by p53 compared to by TGF beta 1, as well as for the ineffectiveness of ectopoic Bcl-2 to abrogate p53-mediated apoptosis. These findings provide first insights to the molecular mechanisms which mediate p53-induced apoptosis, identifying bax and bcl-2 as p53 regulated genes, and serve as a paradigm of how the intracellular balance of Bcl-2 to Bax is differentially altered by distinct death stimuli.

The novel primary response gene MyD118 and the proto-oncogenes myb, myc, and bcl-2 modulate transforming growth factor beta 1-induced apoptosis of myeloid leukemia cells

Cell numbers are regulated by a balance among proliferation, growth arrest, and programmed cell death. A profound example of cell homeostasis, controlled throughout life, is the complex process of blood cell development, yet little is understood about the intracellular mechanisms that regulate blood cell growth arrest and programmed cell death. In this work, using transforming growth factor beta 1 (TGF beta 1)-treated M1 myeloid leukemia cells and genetically engineered M1 cell variants, the regulation of growth arrest and apoptosis was dissected. Blocking of early expression of MyD118, a novel differentiation primary response gene also shown to be a primary response gene induced by TGF beta 1, delayed TGF beta 1-induced apoptosis, demonstrating that MyD118 is a positive modulator of TGF beta 1-mediated cell death. Elevated expression of bcl-2 blocked the TGF beta 1-induced apoptotic pathway but not growth arrest induced by TGF beta 1. Deregulated expression of either c-myc or c-myb inhibited growth arrest and accelerated apoptosis, demonstrating for the first time that c-myb plays a role in regulating apoptosis. In all cases, the apoptotic response was correlated with the level of MyD118 expression. Taken together, these findings demonstrate that the primary response gene MyD118 and the c-myc, c-myb, and bcl-2 proto-oncogenes interact to modulate growth arrest and apoptosis of myeloid cells.

Myeloblastic leukemia cells conditionally blocked by myc-estrogen receptor chimeric transgenes for terminal differentiation coupled to growth arrest and apoptosis

Conditional mutants of the myeloblastic leukemic M1 cell line, expressing the chimeric mycer transgene, have been established. It is shown that M1 mycer cells, like M1, undergo terminal differentiation coupled to growth arrest and programmed cell death (apoptosis) after treatment with the physiologic differentiation inducer interleukin-6. However, when beta-estradiol is included in the culture medium, M1 mycer cells respond to differentiation inducers like M1 myc cell lines, where the differentiation program is blocked at an intermediate stage. By manipulating the function of the mycer transgene product, it is shown that there is a 10-hour window during myeloid differentiation, from 30 to 40 hours after the addition of the differentiation inducer, when the terminal differentiation program switches from being dependent on c-myc suppression to becoming c-myc suppression independent, where activation of c-myc has no apparent effect on mature macrophages. M1 mycer cell lines provide a powerful tool to increase our understanding of the role of c-myc in normal myelopoiesis and in leukemogenesis, also providing a strategy to clone c-myc target genes.

Deregulated c-myb disrupts interleukin-6- or leukemia inhibitory factor-induced myeloid differentiation prior to c-myc: role in leukemogenesis

The c-myb proto-oncogene is abundantly expressed in tissues of hematopoietic origin, and changes in endogenous c-myb genes have been implicated in both human and murine hematopoietic tumors. c-myb encodes a DNA-binding protein capable of trans-activating the c-myc promoter. Suppression of both of these proto-oncogenes was shown to occur upon induction of terminal differentiation but not upon induction of growth inhibition in myeloid leukemia cells. Myeloblastic leukemia M1 cells that can be induced for terminal differentiation with the physiological hematopoietic inducers interleukin-6 and leukemia inhibitory factor were genetically manipulated to constitutively express a c-myb transgene. By using immediate-early to late genetic and morphological markers, it was shown that continuous expression of c-myb disrupts the genetic program of myeloid differentiation at a very early stage, which precedes the block previously shown to be exerted by deregulated c-myc, thereby indicating that the c-myb block is not mediated via deregulation of c-myc. Enforced c-myb expression also prevents the loss in leukemogenicity of M1 cells normally induced by interleukin-6 or leukemia inhibitory factor. Any changes which have taken place, including induction of myeloid differentiation primary response genes, eventually are reversed. Also, it was shown that suppression of c-myb, essential for terminal differentiation, is not intrinsic to growth inhibition. Taken together, these findings show that c-myb plays a key regulatory role in myeloid differentiation and substantiate the notion that deregulated expression of c-myb can play an important role in leukemogenicity.

Influence of arecoline on immune system: II. Suppression of thymus-dependent immune responses and parameter of non-specific resistance after short-term exposure

Arecoline, a major alkaloid of arecanut was screened to explore its modulatory influence on cell-mediated immune response in a murine model system. The in vivo and in vitro effects were evaluated at subtoxic concentrations of arecoline. Delayed type hypersensitivity (DTH) reactions to sheep red blood cells (SRBC) were evaluated in male mice. When treated subcutaneously with 20 mg/kg bw (1/5 of LD50) dose of arecoline for 1, 2 or 3 weeks, the DTH reactions were significantly suppressed. At arecoline concentration of 10 mg/kg bw, there was a moderate reduction in DTH response, while no appreciable change was observed at a dosage of 5 mg/kg bw. The effects were not dependent on the duration of treatment. In contrast, treating with arecoline continuously for 4 days following SRBC immunization showed significant suppression in DTH reactions at both 10 and 20 mg/kg bw doses. When treated after 12 h following immunization with 20 mg/kg bw arecoline, significant reduction in DTH reactions were seen. While moderate reduction in response was observed with arecoline dosage of 10 mg/kg bw, there was no alteration in response at the dose level of 5mg/kg bw. Recovery experiments in mice revealed that arecoline mediated effects are of a reversible nature. Arecoline treatment did not appreciably alter the host resistance to endotoxin shock. In vitro experiments revealed both dose-dependent and time-dependent cytotoxic effects of arecoline when spleen cells were incubated with varying concentrations of arecoline. Concomitant exposure of arecoline at concentrations of 10(-6) - 10(-4) M with con A, markedly suppressed both 3H-thymidine incorporation and interleukin-2 production of splenic cells. In contrast, concomitant exposure of arecoline with IL-2 did not alter 3H-thymidine incorporation in the IL-2 dependent cytolytic T-lymphocyte line (CTLL), except at the concentration of 10(-4) M arecoline. From these studies it is concluded that the dose-dependent suppressive effects of arecoline on DTH response to SRBC and on certain in vitro lymphocyte functions are more clear than the host resistance to endotoxin shock.