P815 Tumor Cells Research Articles

New sesquiterpene hydroquinones from a Taiwanese marine sponge Polyfibrospongia australis.

Two new sesquiterpene hydroquinones, polyfibrospongols A (1) and B (2), were isolated from the Taiwanese marine sponge Polyfibrospongia australis Lendenfeld (Spongiidae) in addition to the known metabolites, dictyoceratin A (3), ilimaquinone (4), and 5-epi-ilimaquinone (9). The structures of these compounds have been determined mainly on the basis of spectral and chemical methods. Biological testing revealed that compounds 1 and 2 exhibited significant cytotoxicity against human KB-16, A-549, and murine P-388 tumor cell lines.

Evidence for the Participation of Interleukin-2 (IL-2) and IL-4 in the Regulation of Autonomous Growth and Tumorigenesis of Transformed Cells of Lymphoid Origin

AbstractIn the current study, we investigated the role of interleukin-2 (IL-2) and IL-4 as autocrine growth factors responsible for autonomous growth of four murine tumor cell lines: LSA, a radiation leukemia virus-induced T-cell lymphoma; EL-4, a chemically triggered T-cell lymphoma; PE-3T, a T-cell line that underwent spontaneous transformation ex vivo; and P815, a mastocytoma. All tumor cell lines screened constitutively expressed IL-2 receptor (IL-2R) and IL-4R genes. However, only LSA and PE-3T cells expressed IL-2 and IL-4 genes constitutively, whereas EL-4 and P815 tumor cells expressed only IL-4 but not IL-2. Monoclonal antibodies (MoAbs) against IL-2, IL-4, or a combination of these, as well as MoAbs against IL-2R significantly inhibited the proliferation of LSA but not that of other tumor cell lines ex vivo. To exclude the possibility that, in other tumor cell lines, the autocrine growth factor may interact with its receptor within the cell, the ability of antisense phosphorothioate oligonucleotides to inhibit the growth of the tumor cells was tested. The antisense phosphorothioate oligonucleotides specific for IL-2, IL-4, IL-2Rβ, or IL-2Rγ chains, added in culture, could markedly inhibit the growth of LSA but not that of the other tumor cell lines screened. Inasmuch as IL-2Rβ and IL-2Rγ subunits also serve as a component of the receptors for IL-4, IL-7, IL-9, and IL-15, the above data suggested that such cytokine redundancy was not responsible for autonomous growth of the other tumor cell lines. Addition of exogenous IL-2 or IL-4 to the tumor cell cultures caused significant enhancement in the proliferation of PE-3T cells, whereas other cell lines were either not significantly affected or slightly inhibited from growing. Interestingly, the LSA tumor growth in nude mice was significantly inhibited after treatment of these mice with a combination of MoAbs against IL-2 and IL-4. Together, our studies show for the first time that IL-2 and IL-4 may serve as autocrine growth factors in the autonomous proliferation of tumor cells, particularly those that are retrovirally induced. Second, some tumor cell lines, despite expressing certain cytokines and their receptors constitutively, may not depend exclusively on such factors for autocrine growth.

Regulatory Role of CD8 in Major Histocompatibility Complex-Unrestricted Tumoricidal Activity of Mouse T Cells Activated with Anti-CD3 Monoclonal Antibody

Antigen-nonspecific CD8+ cytotoxic T cells induced with anti-CD3 monoclonal antibody (mAb) are able to kill tumor cells in a major histocompatibility complex (MHC)-unrestricted fashion. However, the role of CD8 in the MHC-independent tumoricidal activity of anti-CD3-activated killer T (AK-T) cells has not been investigated. Here we show that anti-CD8α mAb inhibits, in a dose-dependent fashion, lysis of P815 and YAC-1 tumor cells by mouse AK-T cells. The inhibition of MHC-unrestricted cytotoxicity by anti-CD8α mAb cannot be attributed to interference with an adhesion-like function of CD8 towards class I MHC molecules on the target cells because anti-CD8α mAb (i) had equal inhibitory effects on the cytolysis of tumor target cells regardless of their relative level of class I MHC molecule expression and (ii) did not interfere with the formation of conjugates between AK-T cells and class I MHC-bearing P815 tumor cells. However, anti-CD8α mAb abrogated AK-T cell granule exocytosis in the presence of P815 tumor cells, indicating a regulatory role for CD8 in the signal transduction events which result in lysis of the tumor target cell. Immunoblot analysis of the post-nuclear fraction of lysates from AK-T cells exposed to P815 tumor cells in the presence of anti-CD8α mAb revealed reduced phosphorylation of tyrosine residues on a protein with an Mr of -62 kDa. Taken together, these data suggest that CD8 is able to affect the tumoricidal activity of MHC-unrestricted AK-T cells independent of class I MHC molecules on the target cell.

Synthesis andin vitro antitumor activity of isoazamitosene and isoiminoazamitosene derivatives

Seven isoazamitosene derivatives, mitomycin analogues, were synthesized and tested for cytotoxicities against leukemia and gastric cancer cell lines. Preparation of a pyrrolo[1,2-a]benzimidazole (3) (azamitosene ring system) was completed by utilizing the Lewis acid-catalized cyclization, witho-chloronitrotoluene as the starting material. Nitration of3 produced a mixture of two isomers (5-nitro isomer (4) and 7-nitro isomer (5)) in product ratio of 36∶52.4 was directly converted into quinone (7) by reduction and Fremy oxidaton. Finally, quinone derivatives (8, 9, 10, and11) were synthesized by 1,4-addition of7 with cyclic secondary amines. From above-mentioned5, 8-nitro compound (15) was prepared in 4 steps. At pH 3, Fremy oxidation of15 produced quinone (16), whereas iminoquinone derivatives (17a and17b) at pH 7. Isoazamitosene derivatives (8, 9, 10, and11), containing cyclic amino groups at the 7-position, showed potent cytotoxicity on P388, SNU-1, and KHH tumor cell lines. Among them,8 had stronger cytotoxicity against SNU-1 cell line than mitomycin and adriamycin. Considering these results, isoazamitosene derivatives may had unique cytotoxicity profiles. However, isoiminoazamitosene derivatives (17a and17b) revealed very weak cytotoxicity.

In vivo suppressive effects of copoly(glycolic/L-lactic acid) microspheres containing CDDP on murine tumor cells

Microspheres composed of a L-lactic acid/glycolic acid copolymer and containing cisplatin (CDDP) (CDDP-MS) were prepared for the sustained release of CDDP with the solvent evaporation method using oil-in-oil emulsion. CDDP was released in vitro from the CDDP-MS over 20 days following about 25% release during the first day. The CDDP-MS implanted into the femoral muscles of rats released CDDP around the implantation site over a few months. The weight loss of mice given the intraperitoneal (ip) injection of CDDP-MS was small compared with that of free CDDP, indicating a reduction in the systemic toxicity of CDDP by its microsphere incorporation. This led to an enhanced suppressive effect of CDDP on the growth of murine tumor cells. The ip injection of CDDP-MS prolonged the survival period of mice with ascitic P815 tumor cells to a significant extent compared with that of free CDDP. In addition, the concentration of blood urea nitrogen was low in the tumor-bearing mice receiving the CDDP-MS injection compared with free CDDP injection, although the CDDP dose was 4 times or more higher than its LD 50 value. It was concluded that the CDDP-MS is a promising drug-carrier system which enhances the antitumor effect of CDDP and reduces its systemic toxicity.

Prevention of lethal graft-versus-host disease following allogeneic bone marrow transplantation in mice by short course administration of LF 08-0299.

We investigated the ability of LF 08--0299, a new immunosuppressive compound, to prevent murine graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (BMT). A short term LF 08--0299 treatment at optimal dosage protected more than 75% of recipient mice from lethal GVHD induced either across minor antigens alone or the full H2 barrier. Furthermore, LF 08--0299 still prevented lethal GVHD when treatment was delayed to 10 days post-BMT. Long-term LF 08--0299-treated survivors were free of clinical signs of GVHD, and histopathologic examination of liver, skin, and intestines was normal, demonstrating that recipient mice did not develop chronic GVHD. We assessed the immunocompetence of long-term surviving recipient mice. Results from MLR and CTL assays were weak whereas responses against unrelated H2 antigens were reduced but still preserved. Moreover, in vivo transfer experiments demonstrated that spleen cells from long-term survivors were unable to induce lethal GVHD in irradiated recipients of host origin, while spleen cells injected in irradiated recipients of a host-unrelated H2 were fully competent to induce a lethal GVHD. Together these results indicate that stable chimeric recipient mice were specifically tolerant to host antigens. We further showed that while LF 08--0299 can protect recipient mice from lethal GVHD, it also preserved a graft-versus-leukemia effect when mice were inoculated with P815 tumor cells. These data suggest that LF 08--0299 may be a novel pharmaceutical agent that would prevent GVHD in human unrelated bone marrow transplantation.

Murine model of cancer gene therapy

In order to examine how the adenovirus-mediated gene therapy induce anti-adenoviral immunity, we set up experiments for a murine cancer model. We first administered adenovirus carrying LacZ (AxCALacZ) gene to DBA/2 mice intraperitoneally, then two weeks after administration of the virus, P815 tumor cells infected with adenovirus carrying LacZ gene were inoculated intraperitoneally to the AxCALacZ immune mice. The mice rejected the P815 tumor infected with AxCALacZ and survived for long periods. The peritoneal T cells in adenovirus immune mice had a strong killing effect of adenovirus-infected P815 cells. Further, parental P815 tumor cells were inoculated into the mice, which rejected the P815 tumor infected with AxCALacZ. All mice rejected P815 tumor cells and survived for long periods. These mice developed P815 specific tumor immunity, which was confirmed by a cytotoxicity assay.

B cell lymphoma-2 transfected P815 cells resist reactive nitrogen intermediate-mediated macrophage-dependent cytotoxicity.

Activated murine peritoneal macrophage cytotoxicity against P815 tumor cells has been shown to be mediated by the reactive nitrogen intermediates (RNI) produced by macrophages from L-arginine through nitric oxide (NO) synthase. Previous results from this laboratory indicated that NO-dependent killing of P815 fulfilled the criteria for apoptotic death. Work by others, in turn, demonstrated that the product of the bcl-2 gene confers protection against various inducers of apoptosis, including reactive oxygen intermediates. Experiments were performed to determine whether Bcl-2 could equally protect sensitive cells from RNI-dependent apoptosis within the context of a relevant biologic system such as the delivery of such RNI by activated macrophages. Results demonstrated that transfection of P815 cells with the human bcl-2 gene confers immunity from RNI-dependent, macrophage-mediated cytotoxicity. In contrast with wild-type or mock-transfected P815 cells, which do not contain detectable Bcl-2, bcl-2-transfected cells showed minimal DNA fragmentation and cell membrane failure when cocultured with activated macrophages. Additional findings indicate that Bcl-2 affords the transfected cells almost complete resistance to the DNA-fragmenting effects of chemically generated NO or H202 and partial protection from their cytolytic effects. These findings are consistent with the hypothesis that tumor cells expressing bcl-2 may escape destruction by macrophage-dependent immune surveillance mechanisms.

Macrophages Can Recognize and Kill Tumor Cells Bearing the Membrane Isoform of Macrophage Colony-Stimulating Factor

NBXFO hybridoma cells produced both the membrane and secreted isoforms of macrophage colony-stimulating factor (M-CSF). Murine bone marrow cells stimulated by the secreted form of M-CSF (sM-CSF) became Mac1+, Mac2+, Mac3+, and F4/80+ macrophages that inhibited the growth of NBXFO cells, but not L1210 or P815 tumor cells. In cytotoxicity studies, M-CSF activated macrophages and freshly isolated macrophages killed NBXFO cells in the presence of polymyxin B, eliminating the possibility that contaminating lipopolysaccharide (LPS) was responsible for the delivery of the cytotoxic signal. Retroviral-mediated transfection of T9 glioma cells with the gene for the membrane isoform of M-CSF (mM-CSF), but not for the secreted isoform of M-CSF, transferred the ability of macrophages to kill these transfected T9 cells in a mM-CSF dose-dependent manner. Macrophage-mediated killing of the mM-CSF transfected clone was blocked by using a 100-fold excess of recombinant M-CSF. Catalase, superoxide dismutase, and the nitric oxide inhibitor, N-omega-nitro-arginine methyl ester (NAME), did not effect macrophage cytotoxicity against the mM-CSF transfectant T9 clones. T9 parental cells when cultured in the presence of an equal number of the mM-CSF transfectant cells were not killed, indicating specific target cell cytotoxicity by the macrophages. Electron microscopy showed that macrophages were capable of phagocytosizing mM-CSF bearing T9 tumor cells and NBXFO hybridoma cells; this suggested a possible mechanism of this cytotoxicity. This study indicates that mM-CSF provides the necessary binding and triggering molecules through which macrophages can initiate direct tumor cell cytotoxicity.

The use of the divalent calcium-ionophore A23187 as a biochemical tool in pharmacological and in vitro toxicological studies.

The use of A23187 as a biochemical tool is described. A23187 was used to elucidate the mechanisms of intracellular transport and secretion of N-hydroxylated aniline metabolites in rat hepatocyte primary culture. Results indicate a membrane-bound intracellular transport. Also, A23187 induced macrophages in their anti-tumour cytostatic activity using P815 tumour cells in in vitro co-cultures of macrophages and tumour cells. Results indicate the activating role of A23187 in macrophage leukotriene C4 release and enhanced macrophage anti-tumour activity. In conclusion, A23187 proved to be a useful tool in studying calcium dependent metabolic processes.

Keenamide A, a bioactive cyclic peptide from the marine mollusk Pleurobranchus forskalii.

Keenamide A (1), a new cytotoxic cyclic hexapeptide, was isolated from the notaspidean mollusk Pleurobranchus forskalii. Its structure was determined by NMR spectral data interpretation and chiral amino acid analysis. Keenamide A (1) exhibited significant activity against the P-388, A-549, MEL-20, and HT-29 tumor cell lines, but was inactive when evaluated against the D6 and W2 Plasmodium falciparum malarial clones.

A new cytotoxic sterol methoxymethyl ether from a deep water marine sponge Scleritoderma sp. cf. paccardi.

24(R)-Methyl-5 alpha-cholest-7-enyl 3 beta-methoxymethyl ether (1), a new sterol ether, has been isolated from a deep-water marine sponge Scleritoderma sp. cf. paccardi. Compound 1 exhibited in vitro cytotoxicity against the cultured murine P-388 tumor cell line with an IC50 of 2.3 micrograms/mL. The isolation and structure elucidation of 1 by NMR spectroscopy is described.

Phospholipase A stimulation in tumor cells by subtoxic concentration of tert-butyl hydroperoxide

Previous studies have shown an increase in the intracellular free arachidonic acid content associated with a disturbance in phospholipid metabolism in P815 tumor cells exposed to subtoxic concentration of tert-butyl hydroperoxide. The present study was to determine the respective contribution of the major phospholipid-metabolizing enzymes that could be involved in this process. The enzymes (phospholipase A, lysophospholipase, acylCoA:lysophosphatidylcholine acyltransferase and acylCoA synthetase) were studied under their respective optimal conditions. When P815 cells were treated with 50 μM of tert-butyl hydroperoxide, a significant stimulation ( x 2.5) of phospholipase A was observed after 15 min of treatment. The activity of the acyltranferase tended to be higher in cells treated by tert-butyl hydroperoxide while the other enzyme activities (lysophospholipase and acyl CoA synthetase) were not affected. t-BHP did not significantly induce higher levels of lipid peroxides in P815 cells. These results show that, in the tumor cell line P815, the disturbance of phospholipid and arachidonate metabolism induced by t-BHP is linked to phospholipase A, the activation of which seems independent of oxidative stress.

Induction of lymphokine-activated killer activity in mice by prothymosin alpha.

We have recently demonstrated that prothymosin alpha (ProT alpha) when administered intraperitoneally (i.p.) protects DBA/2 mice against the growth of syngeneic leukemic L1210 cells through the induction of tumoricidal peritoneal cells producing high levels of tumor necrosis factor alpha (TNF alpha) [Papanastasiou et al. (1992) Cancer Immunol Immunother 35: 145]. In this report we tested further immunological alterations that may be caused by the administration of ProT alpha in vivo. We demonstrate that i.p. injections of ProT alpha enhance natural killer (NK) cell activity and induce lymphokine-activated (LAK) activity in vivo. Thus, splenocytes from ProT alpha-treated DBA/2 animals exhibited significantly higher cytotoxic activity (up to threefold) against the NK-sensitive YAC cell line and the NK-resistant P815 and L1210 syngeneic tumor cells, as compared to splenocytes from syngeneic control mice. The enhancement of the cytotoxic profile of DBA/2 splenocytes was associated with increased percentages of CD8+ cells, NK cells and activated CD3+ cells. The ProT alpha-induced effect persisted for 30 days after the end of the ProT alpha treatment period and returned to normal levels 20 days later. Splenocytes from non-treated DBA/2 animals generated high NK and LAK activities in response to ProT alpha in vitro. The ProT alpha-induced NK and LAK activities reached 84% and 75% respectively of what was obtained with interleukin-2 (IL-2). High concentrations of TNF alpha and IL-2 were generated in response to ProT alpha in LAK cultures. These findings suggest that ProT alpha may provide an overall protective effect against tumor growth in vivo through induction of NK and LAK activities possibly indirectly via the production of IL-2 and TNF alpha in the spleen, peritoneal cavity and probably other lymphoid organs.

FAILURE OF ANTI-T CELL MONOCLONAL ANTIBODIES TO PREVENT ACUTE TUMOR ALLOGRAFT REJECTION IS ASSOCIATED WITH THEIR INABILITY TO DEPLETE OR INACTIVATE T CELLS AT THE SITE OF REJECTION1

The ability of anti-T cell mAb therapy to prevent acute rejection of a tumor allograft was studied. Injection of both anti-CD4 and anti-CD8 rat Ig2b mAbs 1 day before intraperitoneal implantation of P815 tumor cells into AB6F1 mice prevented tumor rejection in most of the mice. However, injection of the same mAbs on day 8 of tumor growth, i.e., 2 days before the onset of tumor rejection, failed to prevent rapid elimination of P815 cells from the peritoneal cavity. Flow cytometric analysis revealed that whereas splenic T cells remained depleted in these mice for up to 6 days after mAb injection, peritoneal T cells returned to control levels in 4-6 days. Moreover, a significant number of rat IgG+ cells were found in the peritoneal cavity 2 days after mAb administration, thus demonstrating that T cells were not depleted but coated with the mAbs at that time. The antitumor activity of the mAb-coated T cells was not impaired, in that CD8+ peritoneal cells from mice rejecting allogeneic P815 tumor on day 10, and rat IgG+ peritoneal cells taken from mice 2 days after giving them anti-CD8 mAb on day 8, were similarly highly cytotoxic against P815 cells in vitro. Taken together, the results demonstrate that the failure of anti-T cell mAb therapy to prevent the acute rejection of allogeneic P815 tumor was associated with resistance of T cells at the site of rejection to the action of anti-T cell mAbs in vivo.

Cytotoxic Terpenoids and Flavonoids fromArtemisia annua

The cytotoxic activity of nine terpenoids and flavonoids isolated from Artemisia annua was tested in vitro on several human tumor cell lines. These compounds are artemisinin, deoxyartemisinin, artemisinic acid, arteannuin-B, stigmasterol, friedelin, friedelan-3 beta-ol, artemetin, and quercetagetin 6,7,3',4'-tetramethyl ether. Friedelane-type triterpenoids were isolated for the first time from this plant. Artemisinin and quercetagetin 6,7,3',4'-tetramethyl ether showed significant cytotoxicity against P-388, A-549, HT-29, MCF-7, and KB tumor cells.

TISSUE DISTRIBUTION OF IL-10 mRNA IN NORMAL MICE

Murine IL-10, initially identified as a product of Th2 CD4+ T cell clones, is known to be produced by a variety of hematopoietic cells. The cellular and tissue expression of IL-10 in vivo is not known and could be relevant to understanding its functions. We examined in vivo, expression of IL-10 mRNA using RT-PCR in various normal and mutant mice and after irradiation. In addition, expression was studied during rejection of an i.p. injection of P815 tumor cells. Total RNA was extracted from whole organs; standard RT-PCR, semiquantitative PCR, and RNase protection assays were performed. In normal mice IL-10 mRNA was detectable in all tissues surveyed. Semiquantitative PCR allowed an estimation of the relative levels of IL-10 mRNA in tissues. IL-10 mRNA in spleen and kidney of nude mice and SCID mice was detectable in normal amounts. Expression of IL-10 mRNA was increased in spleen and kidney in a dose-dependent fashion after irradiation. During allogeneic stimulation IL-10 mRNA was increased in spleen and kidney as demonstrated by the PCR and RNase protection assays. In conclusion, IL-10 mRNA is detectable by PCR in many organs of normal mice and is largely T and B cell-independent. The increase of IL-10 mRNA in spleen and kidney during an intraperitoneal T cell response, at a time when IFN-gamma mRNA is known to be increased in the same organs, suggests a complex systemic interaction between IL-10 and other cytokines during rejection. Moreover, the ubiquitous tissue distribution and the increased levels of steady state IL-10 mRNA after irradiation suggest that this molecule plays a general role in the biology of all tissues and may explain some of the immunosuppressive effects of ionizing radiation.

TISSUE DISTRIBUTION OF IL-10 mRNA IN NORMAL MICE

Murine IL-10, initially identified as a product of Th2 CD4+ T cell clones, is known to be produced by a variety of hematopoietic cells. The cellular and tissue expression of IL-10 in vivo is not known and could be relevant to understanding its functions. We examined in vivo, expression of IL-10 mRNA using RT-PCR in various normal and mutant mice and after irradiation. In addition, expression was studied during rejection of an i.p. injection of P815 tumor cells. Total RNA was extracted from whole organs; standard RT-PCR, semiquantitative PCR, and RNase protection assays were performed. In normal mice IL-10 mRNA was detectable in all tissues surveyed. Semiquantitative PCR allowed an estimation of the relative levels of IL-10 mRNA in tissues. IL-10 mRNA in spleen and kidney of nude mice and SCID mice was detectable in normal amounts. Expression of IL-10 mRNA was increased in spleen and kidney in a dose-dependent fashion after irradiation. During allogeneic stimulation IL-10 mRNA was in creased in spleen and kidney as demonstrated by the PCR and RNase protection assays. In conclusion, IL-10 mRNA is detectable by PCR in many organs of normal mice and is largely T and B cell-independent. The increase of IL-10 mRNA in spleen and kidney during an intraperitoneal T cell response, at a time when IFN-γ mRNA is known to be increased in the same organs, suggests a complex systemic interaction between IL-10 and other cytokines during rejection. Moreover, the ubiquitous tissue distribution and the increased levels of steady state IL-10 mRNA after irradiation suggest that this molecule plays a general role in the biology of all tissues and may explain some of the immunosuppressive effects of ionizing radiation.

Adenoviral interleukin-2 gene transfer into P815 tumor cells abrogates tumorigenicity and induces antitumoral immunity in mice.

The murine mastocytoma cell line P815 was used as a model to evaluate the effect on its tumorigenic capacity following interleukin-2 (IL-2) gene transfer into the tumor cells using a replication-defective adenovirus vector. The data show that P815 cells infected in vitro with this recombinant adenovirus secreted significant amounts of functional IL-2 as tested on CTL-L2 cells. Furthermore, when injected into syngeneic DBA/2 mice, the tumorigenic phenotype is lost in up to 80% of the animals. The rejection of the infected cells was host dependent, because co-injection at the same site or concomitant injection at the opposite side of the animal with a tumorigenic dose of noninfected P815 cells did not lead to tumor development in 50-70% of the mice. Moreover, protected animals developed a long-lasting state of immunization against the P815 tumor cells and their splenocytes were able to transfer the immunity to syngeneic naive recipients.

Activation of the mouse mdr3 gene by insertion of retroviruses in multidrug-resistant P388 tumor cells.

In multidrug-resistant (MDR) derivatives of the mouse lymphoid tumor P388, the emergence of MDR is associated with overexpression and transcriptional activation of the mdr3 gene, either in the absence of (P388/VCR-10) or concomitant with (P388/ADM-2) gene amplification. In both instances, Northern (RNA) blotting analyses have suggested the presence of altered mdr3 transcripts in these cells, possibly originating from novel transcription initiation sites. The mechanisms underlying mdr3 overexpression in these cells have been investigated. In P388/VCR-10 cells, Southern blotting analyses together with genomic DNA cloning and nucleotide sequencing have demonstrated the presence of an intact mouse mammary tumor virus (MMTV) within the boundaries of intron 1 of mdr3. cDNA cloning and nucleotide sequencing indicated that this integration event results in the synthesis and overexpression of a hybrid MMTV-mdr3 mRNA which initiates within the U3 region of the 5' long terminal repeat (LTR) of the provirus. Consequently, this mRNA lacks the normal exon 1 of mdr3 but contains (i) MMTV LTR-derived sequences at its 5' end, (ii) a novel mdr3 exon, mapping within the boundaries of intron 1 downstream of the MMTV integration site and generated by alternative splicing, and (iii) an otherwise intact 3' portion of mdr3 starting at exon 2. A similar type of analysis of P388/ADM-2 cells revealed that mdr3 overexpression in these cells is associated with the integration of an intracisternal A particle (IAP) within an L1Md repetitive element, immediately upstream of mdr3. The IAP insertion results in the overexpression of hybrid IAP-mdr3 mRNA transcripts that initiate within the 3' LTR of the IAP and which contain IAP LTR-derived sequences at the 5' end spliced 14 nucleotides upstream of the normal exon 1 of mdr3. Taken together, these results indicate that independent retroviral insertions were the initial mutagenic event responsible for mdr3 overexpression and survival during drug selection of these cell lines. Amplification of the rearranged and activated mdr3 gene copy occurred during further selection for high-level drug resistance in P388/ADM-2 cells.