Frequent Expression of MUC1, a Membrane-Associated Mucin, in Adult T-Cell Leukemia Cells.

Abstract 1922

Poster Board I-945

Adult T-cell Leukemia (ATL), an aggressive neoplasm etiologically associated with human T-cell lymphotropic virus type-1 (HTLV-1), is resistant to anti-cancer therapies. ATL is sub-classified into 4 clinical subtypes of acute, chronic, smoldering and lymphoma and is known to have the diversity in clinico-pathological features. Mucins are high molecular weight glycoprotein expressed at the luminal surface of a variety of reproductive tract epithelia. MUC1 represents a type I transmembrane glycoprotein and often becomes highly overexpressed in epithelial cancer cells and a limited types of hematological malignant cells such as myeloma cells. Since ATL cells frequently show multi-organ invasions, we analyzed the expression profiles and the functions of MUC1 on ATL cells.

We first analyzed mRNA expression of MUC1 by quantitative RT-PCR in various hematological cell lines: 8 ATL-related, 2 T-cell leukemia, 5 B-cell lineage, and 3 myeloid leukemia. All of ATL-related cell lines had quite high levels of MUC1 mRNA (4-20 folds) compare to the other cell lines. In primary ATL cells, acute-type (n=52) and chronic-type (n=12) ATL cases had significantly higher levels of MUC1 mRNA (p<0.01) compared with PBMC's from healthy donors (n=10). Flowcytometric analysis and western blotting confirmed that ATL-related cell lines and primary ATL cells had high levels of MUC1 protein.

Clinical observations in solid tumor have demonstrated that high levels of MUC1 expression are indicative of a more aggressive stage in tumor progression and poor survival. Hence, although the anti-adhesive property of MUC1 has not been clearly elucidated, it have long been implicated that MUC1 on the whole cell membrane destabilize cell-cell adhesion and permit cancer cells to migrate and metastasize. The effect of MUC1 on tumor growth also has not been clearly demonstrated. We assessed prognosis between high MUC1 mRNA expression cases (n=8) and low cases (n=8) of acute-type ATL. Despite the tendency that high MUC1 mRNA expression cases had poor prognosis, there was no significant difference (p=0.33).

Of note, many of ATL-related cell lines proliferate showing auto-aggregation. Among them, we focused on KK1 and KOB cells with high levels of MUC1 expression and performed knockdown experiment of MUC1 by siRNA. After 96 hours incubation, cell growth of si-MUC1 cells was repressed in comparison with si-control cells and growth inhibition in KK1 and KOB cells were approximately 30% and 25%, respectively. Next, we investigated the pattern of aggregation in KK1 and KOB cells. Interestingly, si-MUC1 cells significantly reduced aggregation after 72hours incubation in comparison with si-control cells. In the KK1 cells, si-MUC1 cells significantly reduced aggregation and the percentages of the non-aggregated cells became from 18% in si-control cells to 56%, suggesting that MUC1 expression is a key molecule for cell aggregation. Similar effects were also observed in KOB cells.

We previously found that ATL cells are resistant to TRAIL though they had death-receptors and we further investigated whether MUC1 contribute to the inhibition of apoptosis. After treatment with TRAIL for 24 hours, Annexin-V positive cells of si-control cells were about 20% which increased in si-MUC1 cells to 42% in the KK1 cells. Similar effects were also observed in KOB cells. In addition, when we used a more potent apoptosis inducer MG132, si-MUC1 cells also increased the percentage of annexin-V posistive cells in KK1 and KOB cells.

Consequently, we found for the first time that ATL cells had unusually high levels of MUC1 as leukemia cells both in mRNA and protein level. si-MUC1 experiments unexpectedly revealed that, in addition to cell proliferation, MUC1 contributes to the cell aggregation in a certain types of ATL cells. Furthermore, we demonstrated that MUC1 play a role in the resistance of apoptosis in ATL cells. These findings may contribute to discover unknown function of MUC1 in leukemia cells.