NPM-ALK Converts JUNB from a Tumor Suppressor to an Oncogene.

High expression of the tumor necrosis factor receptor CD30 and the AP-1 transcription factor JunB are the hallmark of anaplastic large cell lymphoma (ALCL). In contrast to the prototypic AP-1 factor c-Jun, JunB exerts an antioncogenic function in most cell types. Its functional role in ALCL remains uncertain. In about 50% of nodal ALCL the balanced chromosomal rearrangement t(2;5)(p23;q35), generating the fusion protein nucleophosmin-anaplastic lymphoma kinase (NPM-ALK), can be detected. Expression of this fusion kinase induces malignancy in mice and also leads to IL-3 independent outgrowth of the murine hematopoietic cell line Ba/F3.

Using NPM-ALK transduced Ba/F3 cells, we show that NPM-ALK induces JunB expression and activation as verified by quantitative RT-PCR, immunoblot and electro-mobility supershift assay. Interestingly, NPM-ALK transduced Ba/F3 cells also express CD30, which is undetectable in the corresponding wild type cells. Since NPM-ALK induces JunB and CD30 and also leads to growth factor independent proliferation in Ba/F3 cells, these cells mimic conditions present in ALCL.

Knock down of JUNB in NPM-ALK expressing cells using RNA interference leads to downregulation of CD30. Moreover, this partial loss of JunB induces upregulation of p16INK4a and downregulation of CCND1, which directly affect the cell cycle at the G1/S transition. These observations indicate that JunB is an essential factor for CD30 regulation and for neoplastic transformation.

To test if JunB by itself is sufficient to induce CD30 expression and IL-3 independence, we stably transduced Ba/F3 cells with JUNB. In Ba/F3 wild type (WT) cells, JunB expression leads to reverse effects compared to that observed in NPM-ALK transduced Ba/F3. Ba/F3 WT cells do not become IL-3 independent. In addition, compared to vector control, JUNB-transduced Ba/F3 cells show a decrease in proliferation. Furthermore, an induction of p16INK4a and a decrease of CCND1 expression are observed. Moreover, aberrant JunB expression does not trigger CD30 expression in this system.

Taken together, we show that both NPM-ALK and JunB are essential to induce CD30 expression. Furthermore the opposing effects of JunB on p16INK4a and CCND1 in the presence or absence of NPM-ALK indicate that NPM-ALK converts JUNB from a tumor suppressor to an oncogene.