Acute Monocytic Leukemia Associated Antigen MLAA-34 up-Regulates JAK2/STAT3 Expression and JAK2/STAT3 Enhances MLAA-34 Activation in a Positive Feedback Loop

Backgroud: The MLAA-34 gene (GenBank no. AY288977.2) was first discovered in acute monocytic leukemia (M5) in an effort to identify monocytic leukemia-associated antigens by serologic analysis of a recombinant cDNA expression library (SEREX). Previous study showed that high MLAA-34 levels were independently associated with a poorer relapse-free survival and overall survival in AML patients. The MLAA-34 is located on 13q14.2 and has been confirmed to be a novel splice variant of CAB39L (calcium binding protein 39-like). Both mRNA and protein levels of MLAA-34 were found to be higher in U937 cells and M5 patients. The study confirmed that MLAA-34 plays a role in the antiapoptosis of U937 cells, and has the function of oncogenes.

Objective: This study is to explore the relationship of MLAA-34 and JAK2/STAT3 signaling pathway so as to further clarify antiapoptotic mechanisms of MLAA-34.

Method: Potential binding sites for STAT3 was identified by computer-assisted analysis of the core promoter of MLAA-34 gene. We analyzed the role of STAT3 in the regulation of MLAA-34 gene expression in U937 cells by site-specific mutation, chromatin immunoprecipitation (ChIP) and electrophoretic mobility shift assay (EMSA). The expression of MLAA-34 was detected by RT-PCR and western blot after over-expressing and interfering STAT3. Through the different concentrations of AG490 (JAK2 inhibitors) and different concentrations of IL- 6 (JAK2 activator) study JAK2/STAT3 signaling pathway upregulated MLAA-34 expression. The gene chip and co-IP were applied to study the MLAA-34-induced JAK2/STAT3 activation. Peripheral blood mononuclear cells and bone marrow (BM) cells of M5 patients were obtained. In M5 patients, mRNA and protein expression of JAK-2, STAT3, p-STAT3 and MLAA-34 were determined by quantitative RT-PCR and western blot respectively to verify the forward feedback regulation pathway.

Result: The reporter assay showed that the activity of reporter gene was downregulated after the mutation of STAT3 binding sites. ChIP assay and EMSA showed that STAT3 can directly bind to MLAA-34 gene promoter. The expression vectors of MLAA-34 as well as siRNA eukaryotic expression vectors respectively targeting STAT3 were successfully constructed. RT-PCR and western blot results showed that STAT3 can increase the level of MLAA-34. Research of administration of different concentrations of AG490 and different concentrations of IL-6 showed that JAK2/STAT3 signaling pathway could upregulate MLAA-34 expression. AML-M5 NOD / SCID mice leukemia model was successfully constructed,.Konckdown of MLAA-34 gene can promote apoptosis of leukemia cells, inhibit tumor growth and prolong survival time. Total RNA from shRNA-MLAA-34/U937 and Vec/U937 cells were analyzed by Human Genome U133 chip. Heat-map showed reduced expression of JAK2/STAT3 pathway genes. The result was verified by qPCR and western blot. CO-IP showed that MLAA-34 could form a complex with endogenous JAK2 under the enhanced role of IL-6. Thereby activating JAK2 may directly or indirectly dependent on the presence of MLAA-34. Furthermore, we detected JAK-2, STAT3, P-STAT3 and MLAA-34 gene and protein level in M5 patients, the results showed that they have a positive correlation.

Conclusion: JAK2/STAT3 pathway up-regulates MLAA-34 transcription, and MLAA-34 enhances JAK2/STAT3 pathway activation. The forward feedback regulation may have profound therapeutic implications for M5 and could help invent novel approaches in treatment for acute monocytic leukemia.