Insertional hypermutation in mineral oil-induced plasmacytomas.

Unless stimulated by a chronic inflammatory agent, such as mineral oil, plasma cell tumors are rare in young BALB/c mice. This raises the questions: What do inflammatory tissues provide to promote mutagenesis? And what is the nature of mutagenesis? We determined that mineral oil-induced plasmacytomas produce large amounts of endogenous retroelements--ecotropic and polytropic murine leukemia virus and intracisternal A particles. Therefore, plasmacytoma formation might occur, in part, by de novo insertion of these retroelements, induced or helped by the inflammation. We recovered up to ten de novo insertions in a single plasmacytoma, mostly in genes with common retroviral integration sites. Additional integrations accompany tumor evolution from a solid tumor through several generations in cell culture. The high frequency of de novo integrations into cancer genes suggests that endogenous retroelements are coresponsible for plasmacytoma formation and progression in BALB/c mice.

The use of animal models in multiple myeloma.

In myeloma, the understanding of the tissular, cellular and molecular mechanisms of the interactions between tumor plasma cells and bone cells have progressed from in vitro and in vivo studies. However none of the known animal models of myeloma reproduce exactly the human form of the disease. There are currently three types of animal models: (1) injection of pristane oil in BALB/c mice leads to intraperitoneal plasmacytomas but without bone marrow colonization and osteolysis; (2) injection of malignant plasma cell lines in immunodeficient mice SCID or NOD/SCID; the use of the SCID-hu or SCID-rab model allows the use of fresh plasma cells obtained from MM patients; (3) injection of allogeneic malignant plasma cells (5T2MM, 5T33) in the C57BL/KalwRij mouse induces bone marrow proliferation and osteolytic lesions. These cells did not grow in vitro and can be propagated by injection of plasma cells isolated from bone marrow of a mouse at end stage of the disease into young recipient mice. The 5TGM1 is a subclone of 5T33MM cells and can grow in vitro. Among the different models, the 5TMM models and SCID-hu/SCID-rab models were extensively used to test pathophysiological hypotheses and to assess anti-osteoclastic, anti-osteoblastic or anti-tumor therapies in myeloma. In the present review, we report the different types of animal models of MM and describe their interests and limitations.

Inhibition of plasmacytoma development in BALB/c mice by indomethacin.

Indomethacin given continuously in the drinking water (20 micrograms/ml) to BALB/cAn pi mice during the latent period of pristane-induced plasmacytoma development dramatically reduced the plasmacytoma incidence from 34.9 to 2.2%. Additionally, indomethacin given from day 0 to 120 or begun as late as 60 d after a single injection of 1.0 ml pristane was also highly effective in reducing the development of plasmacytomas. Indomethacin treatment did not prevent the formation of a peritoneal inflammatory exudate or peritoneal oil granulomatous tissue, although it had a mild inhibitory effect on the intensity of the cellular inflammation, particularly after extensive treatment of greater than 100 d. Indomethacin treatment reduced the incidence of arthritis by 50%. A major effect of indomethacin treatment was a reduction in the appearance of microscopic plasmacytomas that appear in the oil granuloma before plasmacytomas can be detected by routine sampling of the peritoneal exudate. Between days 116 and 181, 16 of 20 mice given 0.5 ml pristane were found to have foci of plasmacytoma cells, while only 2 of 20 indomethacin-treated mice had foci-containing plasmacytoma cells. The number of mice with microscopic foci in the pristane-treated group greatly exceeded the expected incidence of plasmacytomas (22%) at this dose of pristane. The growth of primary plasmacytomas in transplant that is dependent on the pristane-conditioned peritoneal environment was not inhibited by indomethacin treatment. The role of indomethacin in inhibiting plasmacytoma development was not established; two possibilities are that it inhibits production of mutagenic and tissue destructive oxidants by inflammatory cells, and it inhibits prostaglandin synthesis and intracellular production of oxidant biproducts.

Disruption of transforming growth factor beta signaling by a novel ligand-dependent mechanism.

Transforming growth factor (TGF)-beta is the prototype in a family of secreted proteins that act in autocrine and paracrine pathways to regulate cell development and function. Normal cells typically coexpress TGF-beta receptors and one or more isoforms of TGF-beta, thus the synthesis and secretion of TGF-beta as an inactive latent complex is considered an essential step in regula-ting the activity of this pathway. To determine whether intracellular activation of TGF-beta results in TGF-beta ligand-receptor interactions within the cell, we studied pristane-induced plasma cell tumors (PCTs). We now demonstrate that active TGF-beta1 in the PCT binds to intracellular TGF-beta type II receptor (TbetaRII). Disruption of the expression of TGF-beta1 by antisense TGF-beta1 mRNA restores localization of TbetaRII at the PCT cell surface, indicating a ligand-induced impediment in receptor trafficking. We also show that retroviral expression of a truncated, dominant-negative TbetaRII (dnTbetaRII) effectively competes for intracellular binding of active ligand in the PCT and restores cell surface expression of the endogenous TbetaRII. Analysis of TGF-beta receptor-activated Smad2 suggests the intracellular ligand-receptor complex is not capable of signaling. These data are the first to demonstrate the formation of an intracellular TGF-beta-receptor complex, and define a novel mechanism for modulating the TGF-beta signaling pathway.

In vitro induction of tumor-specific immunity. I. Parameters of activation and cytotoxic reactivity of mouse lymphoid cells immunized in vitro against syngeneic and allogeneic plasma cell tumors.

Induction of tumor-specific immunity in vitro was accomplished by cocultivation of cortisone-resistant murine thymocytes or spleen cells with irradiated syngeneic plasma cell tumors (PCT). The cytotoxic activity generated could be detected in a short-term (51)Cr-release assay. Optimal cytotoxic activity against PCT-associated transplantation antigens (TATA) was generated after 7 days in culture. Unlike cytotoxic responses to tumor allografts in which the cytotoxic activity was directed against allogeneic transplantation antigens, the cytotoxic activity obtained in the syngeneic tumor system was specific to the immunizing syngeneic PCT. Similar parameters of induction of cytotoxic responses in in vitro tumor allograft responses and in the syngeneic tumor system suggested that both reactions are cell-mediated cytotoxic immune responses. With regard to the magnitude of cytotoxic responses obtained, allogeneic transplantation antigens induced about a 30-fold higher cytotoxic immune response than plasma cell TATA. The results are consistent with the concept that in vitro tumor allograft responses and in vitro responses against TATA of PCT are similar in quality, but differ in the magnitude of the cytotoxic response provoked.

Three IgA myeloma immunoglobulins from the BALB/ mouse: precipitation with pneumococcal C polysaccharide.

Three of 64 IgA immunoglobuilins, derived from plasma cell tumors induced by mineral oil in BALB/c mice, precipitated with species-specific pneumococcus C polysaccharide. A related antigen was also found in group O and some group H streptococci. A difference in ability to precipitate a C polysaccharide from a pneumococcus type XIV was demonstrated between protein 603 which did precipitate and protein 167 which did not precipitate this polysaccharide. Studies of the 167 and 603 proteins showed differences in electrophoretic mobility and polypeptide chains. The antigen-combining site of the 167 and 603 proteins resided on the papain-digestion Fab fragment.

Rapid viral induction of plasmacytomas in pristane-primed BALB-c mice.

Strain BALB/c mice were injected intraperitoneally with 0.5 milliliter of pristane, and 39 to 56 days later they were infected with Abelson murine leukemia virus, which is a lymphosarcomagenic variant of Moloney virus. Fifty-eight percent of the mice developed lymphosarcoma, and 28 percent developed immunoglobulin-producing plasmacytomas within 20 to 93 days (77 to 149 days after the pristane injection). Two of 57 control mice developed plasmacytomas at days 138 and 166 after a single injection of pristane; no plasmacytomas were found in mice treated with virus alone.

Efficiency alleles of the Pctr1 modifier locus for plasmacytoma susceptibility.

The susceptibility of BALB/c mice to pristane-induced plasmacytomas is a complex genetic trait involving multiple loci, while DBA/2 and C57BL/6 strains are genetically resistant to the plasmacytomagenic effects of pristane. In this model system for human B-cell neoplasia, one of the BALB/c susceptibility and modifier loci, Pctr1, was mapped to a 5.7-centimorgan (cM) chromosomal region that included Cdkn2a, which encodes p16(INK4a) and p19(ARF), and the coding sequences for the BALB/c p16(INK4a) and p19(ARF) alleles were found to be polymorphic with respect to their resistant Pctr1 counterparts in DBA/2 and C57BL/6 mice (45). In the present study, alleles of Pctr1, Cdkn2a, and D4Mit15 from a resistant strain (BALB/cDAG) carrying DBA/2 chromatin were introgressively backcrossed to the susceptible BALB/c strain. The resultant C.DAG-Pctr1 Cdkn2a D4Mit15 congenic was more resistant to plasmacytomagenesis than BALB/c, thus narrowing Pctr1 to a 1.5-cM interval. Concomitantly, resistant C57BL/6 mice, from which both gene products of the Cdkn2a gene have been eliminated, developed pristane-induced plasma cell tumors over a shorter latency period than the traditionally susceptible BALB/cAn strain. Biological assays of the p16(INK4a) and p19(ARF) alleles from BALB/c and DBA/2 indicated that the BALB/c p16(INK4a) allele was less active than its DBA/2 counterpart in inducing growth arrest of mouse plasmacytoma cell lines and preventing ras-induced transformation of NIH 3T3 cells, while the two p19(ARF) alleles displayed similar potencies in both assays. We propose that the BALB/c susceptibility/modifier locus, Pctr1, is an "efficiency" allele of the p16(INK4a) gene.

Genetics of susceptibility to plasmacytoma induction. I. BALB/cAnN (C), C57BL/6N (B6), C57BL/Ka (BK), (C times B6)F1, (C times BK)F1, and C times B recombinant-inbred strains.

Plasmacytomas were found in 58% of 373 BALB/cAnN (C) mice given three 0.5-ml doses of mineral oil (Bayol F or light mineral oil) or 2,6,10,14-tetramethylpentadecane (pristane) ip. The incidence of plasmacytomas in C57BL/6N (B6), C57BL/Ka (BK), (C times B6)F1 and (C times BK)F1 was 6.4, 0, 11.5, and 16.5%, respectively. The plasmacytomas occurred in old B6 mice, in contrast to their early appearance in strain C mice. The incidence of plasmacytomas in mineral oil-treated or pristane-treated C times B recombinant-inbred (Rl) strain mice was 28.3% in C times BD, 17.5% IN C times BE, 36.5% IN C times BG, 0% in C times BH, 2.9% in C times Bl, 48% in C times BJ, and 4.3% in C times BK. C times BD, C times BG, and C times BJ strains were considered susceptible to plascytoma induction by mineral oil or pristane; C times BE had a low susceptibility, and C times BH, C times Bl, and C times BK were resistant. The results suggested that there were only a few gene difference between C and B6 or BK that determined susceptibility or resistance to plasmacytoma induction, and that B6 and BK have at least one dominant resistance gene. The distribution pattern of susceptibility and resistance in the C times B Rl strains suggested the presence of a resistance gene on chromosome 9, linkage group II.

Induction of plasmacytomas with silicone gel in genetically susceptible strains of mice.

BACKGROUND: Plasmacytomas can be induced in high frequency in susceptible strains of mice by the intraperitoneal introduction of plastics or paraffin oils, including the chemically defined oil pristane (2,6,10,14-tetramethylpentadecane). These materials persist in the peritoneal cavity, where they induce chronic inflammation during the long periods before plasmacytomas develop. Such plasmacytomas appear to arise from B cells carrying chromosomal translocations that affect c-myc transcription. PURPOSE: Because silicone gels are in widespread medical use and share many of the characteristics of other materials known to be inducers of plasmacytomas, we wished to determine their capacity to induce plasmacytomas in mice. METHODS: In a series of parallel experiments, corn oil, pristane, silicone oil (dimethylpolysiloxane), or silicone gel from commercially obtained mammary implants was injected intraperitoneally into plasmacytoma-susceptible BALB/cAnPt-A and congenic BALB/cAnPt.DBA/2-Idh1-Pep3 mice, as well as into plasmacytoma-resistant C57BL/6N, C3H/HeJ, DBA/2N, and (BALB/c x DBA/2)F1 mice. Mice were examined at least once every 2 weeks for signs of abdominal tumor or weight loss and screened every 4-6 weeks for peritoneal plasmacytoma cells by peritoneal lavage. Tissues were examined by histologic and immunohistochemical techniques. Metaphase chromosome spreads were made from ascitic plasmacytomas without Colcemid treatment, and metaphase plates were G-banded according to standard techniques. The t(12;15) or t(6;15) translocation chromosomes were identified under the microscope in at least five metaphase plates of high banding quality. Mice were autopsied 125-400 days after the injection of test material. Gas chromatography and mass spectrometry were utilized to determine the composition of the silicone oil and silicone gel used in the injections. RESULTS: The silicone gels tested induced plasmacytomas in BALB/cAnPt-A and BALB/cAnPt.DBA/2-Idh1-Pep3 mice. Neither corn oil used as a control nor 1000-centistoke or 12,500-centistoke dimethylpolysiloxane induced plasmacytomas in these mice. The plasmacytomas were transplantable in syngeneic hosts. Cytogenetic studies of 41 silicone-induced plasmacytomas showed that 30 had t(12;15) translocations, eight had t(6;15) translocations, and three had no translocations. CONCLUSIONS: The silicone gels used in mammary implants, which contain a complex mixture of different siloxanes, induced peritoneal plasmacytomas in genetically susceptible mice. Silicone gels provide new chemically defined materials that are effective inducers of plasmacytomas in BALB/cAnPt-A and BALB/cAnPt.DBA/2-Idh1-Pep3 mice. Further studies will be required to determine which of the components of these gels are the active materials.

Peritoneal plasmacytomagenesis in mice: comparison of different pristane dose regimens.

Plasmacytomas were induced in inbred BALB/c pi mice by the ip injection of pristane with 4 different dose schedules. Three 0.5-ml doses (1.5 ml) given at 2-month intervals gave an average yield of 61% plasmacytomas in 6 experimental groups with a range from 51 to 71%; a single 1-ml dose gave an average yield of 42% plasmacytomas in 5 experimental groups with a range from 37 to 45%; and a single 0.5-ml dose gave an average of 22% from 3 experiments involving young mice with a range from 14 to 26%. Two 0.5-ml doses given at various intervals from 14 to 300 days gave yields of plasmacytomas that usually but not always were greater than that obtained with a single 0.5-ml dose of pristane. When the second injection of pristane was delayed as long as 180 days, a strong additive effect over that observed with 0.5 ml alone was obtained. The plasmacytomas developed in mice given the second dose 180 days after the first, with virtually the same latent period as observed with a single 1-ml dose. No plasmacytomas were found in 200 BALB/c pi mice inoculated with corn oil, aluminum hydroxide, or very small doses of pristane (i.e., 0.05 ml). The minimal latent period for plasmacytoma development is about 120 days. The median latent period ranged from 180 to 250 days in the groups of mice that received 3 0.5-ml injections of pristane. In a single experiment pristane freed of UV-absorbing materials was as potent as commercial grade pristane in inducing plasmacytomas.

Lipopolysaccharide and pristane-induced peritoneal exudate requirements for plasma tumor cell colony formation.

The role for lipopolysaccharide (LPS) and a pristane-induced peritoneal exudate (PIPE) on the in vitro development of murine plasmacytoma was studied. MOPC-315 cell suspensions showed little tendency for colony formation in a soft agar culture medium. Additions of LPS and PIPE were required for maximal colony formation. The LPS effect was dose-dependent down to nanogram quantities. PIPE prepared from inbred strains of mice not susceptible to pristane-induced plasmacytoma (DBA/2) or from normal peritoneal washings was ineffective. PIPE activity was radioresistant and not transferable by cell-free conditioned medium. Three strains of transplantable plasmacytomas showed colony formation stimulation by LPS plus PIPE, but LPS and PIPE were ineffective with lymphosarcoma P1798.

Specific chromosome translocation in pristane-induced plasmacytomas of NZB mice.

The karyotypes of 3 pristane (2,6,10,14-tetramethyl-pentadecane)-induced NZB mouse plasmacytomas producing IgA (plasmacytoma TEPC-3660), IgG3K (plasmacytoma TEPC-6583), and IgG2aK plus IgG3K (plasmacytoma TEPC-6906) were studied in early transplantation generations by the combined Hoechst 33258-quinacrine mustard banding method. The two IgG producers (TEPC-6583 and TEPC-6906) showed diploid chromosome number, whereas the IgA producer (TEPC-3660) showed hypotetraploid chromosome number. Banding analysis revealed that these 3 plasmacytomas consistently had the t(12;15) translocation with the specific breakpoints at 12F2 and 15D2. Taken together with the previous observation of t(12;15) in plasmacytomas of BALB/c and C3H mice, these results suggest that the t(12;15) is specific for murine plasmacytomas and that the putative gene(s) on the t(12;15) may participate in the genesis of murine plasmacytomas.

Enhancement and retardation of spontaneous reticulum cell neoplasm development in SJL/J mice.

The enhancement or delay in the appearance of spontaneous reticulum cell neoplasms (RCN-B) in SJL/J mice was tested. Immunosuppressive treatment with antithymocyte serum and prolonged antigenic stimulation by repeated injections of sheep erythrocytes, leukemogenic cell-free centrifugates, and multichain synthetic polypeptides had no effect on the incidence or latency of spontaneous tumor appearance. Treatment with mineral oil or incomplete Freund adjuvant markedly enhanced tumor appearance (though the abundance of plasma cells induced by these treatments did not evoke the development of plasmacytomas, nor did it affect the incidence of serum paraproteinemia among mice with spontaneous tumors). Treatment with cortisone acetate, however, markedly retarded spontaneous tumor development. Tumors occurring early due to mineral oil and adjuvant treatment all had the characteristics of "RCN-B plasma cell type," and tumors occurring late due to cortisone treatment were all of the "RCN-B reticulum cell type." The possible involvement of the stem cell pool size in the enhancement or delay of spontaneous tumor development was discussed.

Detection of immunoglobulin/c-myc recombinations in mice that are resistant to plasmacytoma induction.

Interchromosomal recombinations between c-myc and immunoglobulin sequences can be found in preneoplastic lesions (oil granulomata) during pristane-induced plasmacytoma development in susceptible BALB/cAn mice. In this study we used a more sensitive approach, hybridization-enriched templates with nested PCR, to detect microclones with Ig alpha/c-myc recombinations in oil granulomata of susceptible and resistant mice. Recombinations were detected in as many as 73% (32/44) of plasmacytoma-susceptible BALB/cAn mice 30 days after an injection of pristane. Mice that are resistant to plasmacytoma induction can also harbor recombination-positive cells, but these are less frequent [2/20 DBA/2N, 8/20 (BALB/cAn x DBA/2N)F1, hereafter called CD2F1]. The clones in DBA/2N mice were small (< 400 cells), whereas in BALB/cAn, the oil granuloma harbored up to several thousand of these cells. We conclude that the molecular machinery for generating characteristic interchromosomal recombinations can be found in all strains of mice. Both the frequency of generating Ig alpha/c-myc recombinations and the expansion of recombination-positive cells are greater in susceptible mice than in resistant strains.

BCL2 accelerates inflammation-induced BALB/c plasmacytomas and promotes novel tumors with coexisting T(12;15) and T(6;15) translocations.

Previous studies on peritoneal plasmacytomas (PCTs) in BALB/c (C) mice suggested that the enforced expression of the death repressor BCL2 in B cells might facilitate the malignant transformation of aberrant B cells containing Myc-activating T(12;15) translocations, generating an improved model of plasmacytomagenesis. To investigate this hypothesis, we backcrossed a human BCL2 transgene onto strain C and performed a PCT induction study with pristane in the newly generated C.BCL2 congenics. In specific pathogen-free-maintained C.BCL2 mice, PCT incidence (19 of 34, 56%) was 24 times higher than in specific pathogen-free-maintained C mice (1 of 44, 2.3%), and tumor onset (113 days) was half that of conventionally maintained C mice (220 days). BCL2 transgenic PCT harbored T(12;15) translocations (12 of 12 tumors) with an unusual clustering of translocation breakpoints in the near 5' flank of Myc (4 of 5 tumors, 80%). Five tumors contained coexisting T(12;15) and T(6;15) translocations (not observed in >300 karyotyped PCTs from conventionally maintained C mice). BCL2 transgenic C57BL/6 mice exclusively developed B lymphomas (11 of 20, 55%) that also contained T(12;15) translocations (11 of 11 cases) with breakpoints in the near 5' flank of Myc (five of five tumors). We conclude that BCL2 accelerates PCT with novel Myc-activating translocations independently of environmental antigen stimulation. Accelerated plasmacytomagenesis in strain C.BCL2 may be useful for designing and testing BCL2 inhibition strategies in human plasma cell tumors overexpressing BCL2, such as Waldenström's macroglobulinemia and multiple myeloma.

Association of elevated mutagenesis in the spleen with genetic susceptibility to induced plasmacytoma development in mice.

Using the phage lambdaLIZ-based transgenic in vivo mutagenesis assay, mean mutant rates were determined in the spleen of mice exposed to sustained oxidative stress and were found to be increased approximately 3-fold in plasmacytoma-susceptible BALB/c and C.D2-Idh1-Pep3 mice, but not in plasmacytoma-resistant DBA/2N mice. This finding suggests a correlation between the genetic susceptibility to inflammation-induced peritoneal plasmacytomagenesis and the phenotype of increased mutagenesis in lymphoid tissues, raising the possibility that plasmacytoma resistance genes may inhibit tumor development by minimizing oxidative mutagenesis in B cells.

Endotoxin enhancement of plasma cell tumor development in mice given injections of mineral oil.

Bacterial endotoxins administered to BALB/c mice given i.p. mineral oil cause an increased incidence of plasma cell tumors, compared with mice given either oil or antigens alone, or oil plus antigen other than endotoxin. Endotoxin in ng doses was more effective than in mug doses.

Importance of Lyt 2+ T-cells in the curative effectiveness of a low dose of melphalan for mice bearing a large MOPC-315 tumor.

We have previously demonstrated that the curative effectiveness of a low dose (2.5 mg/kg) of melphalan (L-phenylalanine mustard; L-PAM) for mice bearing a large s.c. (approximately 20 mm in diameter) MOPC-315 tumor and extensive metastases requires the participation of T-cell-dependent antitumor immunity in tumor eradication (S. Ben-Efraim et al., Cancer Immunol. Immunother., 15: 101-107, 1983). Here we show that the Lyt 2+ T-cells, and not the L3T4+ T-cells, participate in the cure of such tumor-bearing mice by a low dose of L-PAM. Specifically, depletion of Lyt 2+ T-cells from mice bearing a large MOPC-315 tumor by treatment with monoclonal anti-Lyt 2.2 antibody abolished the curative effectiveness of the low dose of drug. In contrast, depletion of L3T4+ T-cells from mice bearing a large MOPC-315 tumor by treatment with monoclonal anti-L3T4 antibody did not reduce significantly the curative effectiveness of the low dose of drug. Histological examination of tumor nodules on various days following low-dose L-PAM therapy revealed widespread lymphocytic infiltration by Day 5 following the chemotherapy, and this infiltration was drastically reduced when the L-PAM-treated tumor bearers were treated with either anti-Thy 1.2 or anti-Lyt 2.2 antibody but not with anti-L3T4 antibody. The antitumor immunity exhibited by Lyt 2+ T-cells derived from mice which were in the process of eradicating a large MOPC-315 tumor following low-dose L-PAM therapy was exploited successfully to confer systemic antitumor immunity to mice bearing a barely palpable tumor. Specifically, the adoptively transferred Lyt 2+ splenic T-cells, in conjunction with a subcurative dose of L-PAM, brought about the cure of most mice. The Lyt 2+ splenic T-cells from L-PAM-treated MOPC-315 tumor bearers were also found to be capable of exerting a direct potent lytic effect against MOPC-315 tumor cells in an antigen-specific manner. Thus, it is conceivable that the direct cytotoxic activity of Lyt 2+ T-cells for MOPC-315 tumor cells is responsible, at least in part, for the ability of the Lyt 2+ T-cells from L-PAM-treated MOPC-315 tumor bearers to bring about the eradication of the tumor burden not eradicated through the direct antitumor effects of the low dose of drug.

Activation of insulin-like growth factor I receptor signaling pathway is critical for mouse plasma cell tumor growth.

Plasma cell neoplasia in humans generally occurs as multiple myeloma, an incurable form of cancer. Tumors with marked similarity can be induced in mice by a variety of agents, including chemicals, silicone, and oncogene-containing retroviruses, suggesting the use of murine tumors as an informative model to study plasma cell disease. Herein, we have focused on the role of insulin-like growth factor I receptor (IGF-IR) signaling in the development of plasma cell disease. The insulin receptor substrate 2/phosphatidylinositol 3'-kinase/p70S6K pathway was found to be either constitutively or IGF-I-dependently activated in all plasma cell tumors. Biological relevance was demonstrated in that plasma cell lines with up-regulated IGF-IR expression levels exhibited mitogenic responses to IGF-I. More importantly, expression of a dominant-negative mutant of IGF-IR in these lines strongly suppressed tumorigenesis in vivo. Taken together, these results demonstrate that up-regulation and activation of IGF-IR and the downstream signaling pathway involving insulin receptor substrate 2, phosphatidylinositol 3'-kinase, and p70S6K may play an important role in the development of a broad spectrum of plasma cell tumors.