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The oncogenic membrane protein LMP1 sequesters TRAF3 in B-cell lymphoma cells to produce functional TRAF3 deficiency

Pradeep Bangalore-Prakash, Laura L. Stunz, Nurbek Mambetsariev, Amy L. Whillock, Bruce S. Hostager and Gail A. Bishop

Article Figures & Data

Figures

  • Figure 1.

    TRAF3 and LMP1 protein expression in human DLBCL samples. Formalin-fixed, paraffin-embedded tissue samples were stained for TRAF3 protein as described in “Methods.” (A) The top row shows representative samples identified by whole-exome sequencing as having decreased copy number for the TRAF3 gene stained for TRAF3 protein (brown); the bottom row shows representative samples identified as having a normal complement of TRAF3 genes stained for TRAF3 protein. (B) A control sample of normal human tonsil tissue stained for TRAF3 protein. (C) A sample with low/negative TRAF3 protein (previously determined by TRAF3 staining as in A) (left) stained in parallel for LMP1 (brown). An LMP1-negative specimen from the same tissue slide is shown on the right.

  • Figure 2.

    Effect of LMP1 expression upon TRAF3 availability to bind membrane CD40 or BAFFR in human BCL lines. (A-C) Western blots representative of 3 similar experiments. CD40 was immunoprecipitated from lysates of the indicated BCL-derived lines as described in “Methods.” Immunoprecipitates (IP) or whole-cell lysates (WCL) were subjected to SDS-PAGE and western blotting with antibodies specific for the indicated proteins. (D) DAUDI or SKW6.4 cells were untreated (left two lanes) or treated for 2 hours with 50 ng/mL recombinant human BAFF (rBAFF), after which BAFFR was immunoprecipitated from cell lysates. (E) Relative levels (mean ± standard error [SE]) of BAFFR-associated TRAF3 from 3 independent experiments. Statistical significance of differences was calculated as described in “Methods”; *P < .0008.

  • Figure 3.

    Impact of LMP1 expression on levels of proviability kinases in human BCL cell lines. (A) Representative experiments in which whole-cell lysates of the indicated cell lines were subjected to SDS-PAGE and western blotted for NIK, PIM2, LMP1, and actin as described in “Methods.” (B) Relative PIM2 protein levels (presented as mean ± SE from 3 independent experiments as in A), with PIM2 levels in SKW6.4 cells set as 1. (C) Relative NIK protein levels (presented as mean values ± SE from 3 independent experiments as in A), with NIK levels in SKW6.4 cells set as 1. Statistical significance of differences was calculated as described in “Methods”; *P < .006.

  • Figure 4.

    CD40-associated TRAF3 and cellular NIK levels in matched LMP1+vs LMP1clones of mouse BCL lines. (A) The mouse BCL cell line CH12.LX and a subclone stably transfected with IPTG-inducible WT LMP1 were cultured overnight with IPTG as described in “Methods.” CD40 was immunoprecipitated from whole-cell lysates as in Figure 2, and samples subjected to SDS-PAGE were analyzed for CD40-associated TRAF3 by western blot. A representative of 3 similar blots is presented. (B) Relative CD40-associated TRAF3 levels (presented as mean values ± SE from 3 independent experiments as above), with CD40-associated TRAF3 levels in parent CH12.LX cells set at 1. *P < .0001. (C) Effect of LMP1 expression on relative cellular NIK levels. Whole-cell lysates of the indicated cell lines were blotted for NIK and actin as described in “Methods.”

  • Figure 5.

    LMP1 sequestration of TRAF3 requires the major LMP1 cytoplasmic domain TRAF-binding site. (A) A schematic of the CTAR1 cytoplasmic region of LMP1, indicating its 3 TRAF-binding motifs. The SUB2 LMP1 molecule has the indicated mutation in the first of these motifs. (B) CD40 immunoprecipitations (IP) as in prior figures, from whole-cell lysates (WCL) of parent CH12.LX cells or transfected subclones expressing WT or SUB2 LMP1 molecules, induced to express LMP1 as in Figure 4. A representative western blot of 3 similar experiments is shown. (C) Relative CD40-associated TRAF3 levels (presented as mean values ± SE from 3 independent experiments as above), with CD40-associated TRAF3 levels in parent CH12.LX cells set at 1. A 1-way ANOVA was used to analyze results for statistical significance, and the adjusted P value was calculated using Sidak’s multiple comparison test (*P < .0001, **P = .001).

  • Figure 6.

    Influence of LMP1 expression on responsiveness to IL-6 signaling. (A) The indicated cell lines were stimulated with recombinant mouse IL-6 (as described in “Methods”), and whole-cell lysates were subjected to SDS-PAGE and western blot analysis for the indicated proteins. A representative blot of 3 similar experiments is shown. (B) Relative STAT3 phosphorylation following IL-6 stimulation was quantified in 3 independent experiments, and mean values ± SE were calculated. A 2-way ANOVA was used to examine the results for statistical significance, and the adjusted P value was calculated using the Bonferroni method (n = 2 independent experiments; *P = .043, **P = .031).

  • Figure 7.

    Relationship between LMP1 expression and levels of nuclear CREB in BCL cells. (A) Parent and LMP1-expressing clones of the M12.4.1 BCL cell line were stained with fluorescent anti-CREB antibody and analyzed by immunofluorescent confocal microscopy as described in “Methods.” Representative images from 6 fields per slide are shown and are also representative of 3 independent experiments with similar results. (B) Whole-cell lysates from the indicated cells were subjected to SDS-PAGE and western blot analysis of the indicated proteins; a representative blot of 3 similar experiments is shown. (C) Relative CREB protein levels (mean ± SE from 3 independent experiments as in panel B) were quantified, with levels in parent M12.4.1 cells set as 1. *P = .01 (Student t test; see “Methods”).

Tables

  • Table 1.

    TRAF3 deficiency in human DLBCL samples

    ParameterTotal number of samplesTRAF3 deficient, n (%)
    Exome sequencing (of 51 samples tested)516 (11.7) by exome sequencing
    TRAF3 protein staining (277 samples tested)27795 (34.3) by immunohistochemistry
    LMP1 protein positive (of 277 tested)5030 (60.0) by immunohistochemistry
    LMP1 protein negative (of 277 tested)22764 (28.1) by immunohistochemistry
    • LMP1-positive and -negative samples differ significantly in their percentage of TRAF3high and TRAF3low staining, as determined by the Fisher's exact test (P < .0001).