STAT3 mRNA and protein expression in colorectal cancer: effects on STAT3-inducible targets linked to cell survival and proliferation

To evaluate mRNA and protein expression of signal transducers and activators of transcription (STAT)3 in colorectal carcinomas (CRCs) and to define the association of STAT3 activity with the STAT3-inducible targets cyclin D1, survivin, Bcl-xl and Mcl-1.Matching serial sections of normal colonic epithelium and invasive CRCs (n = 32) were subjected to quantitative reverse transcriptase polymerase chain reaction specific to STAT3, cyclin D1, survivin, Bcl-xl and Mcl-1, as well as immunohistochemistry. For STAT3 immunohistochemistry, two antibodies, recognising unphosphorylated (UP-) and phosphorylated (tyr705, P-) STAT3 were used. Ki-67 (MIB-1) staining was included as a proliferation marker.Compared with normal colonic epithelium, UP-STAT3 and P-STAT3 (p = 0.023 and 0.006) protein expression and expression of its associated targets cyclin D1, survivin and Bcl-xl were significantly (all p0.001) increased in carcinoma. In carcinomas, STAT3 (p = 0.019) and Bcl-xl (p = 0.001) mRNAs were correlated with lymph node status. Moreover, nuclear P-STAT3 protein expression (active state) was associated with the expression of its target genes Bcl-xl (p = 0.038) and survivin (p = 0.01) as well as with Ki-67 (p = 0.017). By contrast, cytoplasmic UP-STAT was significantly linked to Bcl-xl mRNA (p = 0.024) and protein (p = 0.001) as well as to cytoplasmic survivin protein expression (p = 0.019).Both inactive (UP-STAT3) and active (P-STAT3) STAT3 proteins are markedly increased in invasive CRCs. This is associated with Bcl-xl and survivin induction, increased proliferation and lymph node metastasis. This study therefore provides the basis for further examination of the prognostic or predictive value of these molecular markers in CRC.