Abstract: Abstract:Objiective To investigate whether hUCMSCs undergo malignant transformation when exposed to MCF-7B breast cancer microenvironment and whether the abnormal activation and over expression of STAT3 play an important role in this transformation. Methods The experiment was divided into three groups:blank group(hUCMSCs were separately cultured),experimental group(hUCMSCs were indirectly co-cultured with MCF-7B breast cancer cells),positive control group(MCF-7B breast cancer cells were separately cultured ).Morphology of cells was detected by invertedmicroscope. Cell cycle was detected by Flow cytometry. The mRNA expressions of STAT3、c-Myc and Bcl-xL were tested by real-time PCR.The protein expressions and locations of p-STAT3、c-Myc and Bcl-xL were detected by immunofluorescence.The protein expressions of p-STAT3、STAT3、c-Myc and Bcl-xL were also measured by Western blot.Results The experimental group cells showed typical morphology features of tumor cell. The cells proportion of experiment group in G1 phase was significantly lower than that of the blank group（P<0.05）,but which in S and G2 phase were significantly higher than those of the blank group（P<0.05）.The mRNA expression levels of STAT3、c-Myc and Bcl-xL in experimental group were significantly higher than those in blank group(P<0.05). p-STAT3、STAT3、c-Myc and Bcl-xL protein were significantly higher than that of the blank group(P<0.05),and they were mainly situated in the nuclei.The protein expressions of STATS also showed significant changes in experimental group. Conclusions hUCMSCs trends to malignant transformations when exposed to MCF-7B breast cancer microenvironment.And the abnormal activation and over expression of STAT3 is one of important factors leading to the malignant transformation of hUCMSCs.

Key words: human Umbilical Cord Mesenchymal Stem Cells, malignant transformation, Signal transducerand activator of transcription 3