MLK3 is a member of MLK sub-family that belongs to the family of MAP kinase kinase kinase (MAP3K). The MLK family members are characterized by the presence of signature sequences of Ser/Thr and Tyr kinases within their catalytic domain. The detailed physiological function of MLK3 is yet to be deciphered fully, however the available data suggest that in neuronal cells, MLK3 plays role of a pro-apoptotic kinase. The function of MLK3 in any type of cancer is not known. Interestingly the C-terminal tail of MLK3 contains consensus sequences for various protein-protein interactions and phosphorylation sites for numerous kinases. The C-terminal tail also contains Ser/Thr-Pro sequence, a consensus sequence for prolyl isomerase (Pin1), which has been reported to be over-expressed in cancer cells, including in breast cancer cells and primary tumors.

Our initial hypothesis was that probably Pin1 binds to MLK3 and regulates MLK3's kinase activity. Our in vivo binding indeed suggested that Pin1 binds to MLK3 via PPI domain robustly and to certain extent through WW domain. The binding of Pin1 did not alter the kinase activity of MLK3. Interestingly, counterintuitive to our initial hypothesis, we observed that MLK3 was able to phosphorylate Pin1 and alter Pin1's isomerase activity. We also identified S138 as a phosphorylation site on Pin1 and raised p-S138 antibody. Since, Pin1 is known to drive G2-M-cell cycle transition, we examined whether phosphorylation of Pin1 promotes cell cycle progression. Our data indicated that phospho-S138 Pin1 increased the G2-M transition of cell cycle. Pin1 is reported to be a nuclear protein; however it is not known whether this translocation of Pin1is regulated by any protein. We observed that Pin1 was translocated to nucleus upon phosphorylation by MLK3 at S138 site and the phospho-S138 Pin1 was highly expressed in breast cancer cell lines and tumors, compared to non-transformed breast epithelial cell line and normal breast tissues respectively. The breast cancer tissue arrays analyzed also showed that phospho-S138 was highly expressed in tumors compared to normal breast tissue and the intensity of staining was significantly higher in tumors as well.

Taken together, our data suggest that phosphorylation of Pin1 by MLK3 drives G2-M transition and promote breast cancer pathogenesis. Thus targeting MLK3 or Pin1 may prove beneficial to control breast cancer tumorigenesis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5065.