Regulation of Human Endometrial Stromal Proliferation and Differentiation by C/EBPβ Involves Cyclin E-cdk2 and STAT3

During each menstrual cycle, the human uterus undergoes a unique transformation, known as decidualization, which involves endometrial stromal proliferation and differentiation. During this process, the stromal cells are transformed into decidual cells, which produce factors that prepare the uterus for potential embryo implantation. We previously identified the transcription factor CCAAT/enhancer-binding protein (C/EBP)beta as a regulator of endometrial stromal proliferation and differentiation in mice. In this study, we addressed the role of C/EBP beta in human endometrial decidualization. Using small interfering RNA targeted to C/EBP beta mRNA, we demonstrated that C/EBP beta controls the proliferation of primary human endometrial stromal cells (HESCs) by regulating the expression of several key cell cycle-regulatory factors during the G(1)-S phase transition. Additionally, loss of C/EBP beta expression blocked the differentiation of HESCs in response to estrogen, progesterone, and cyclic AMP. Gene expression profiling of normal and C/EBP beta-deficient HESCs revealed that the receptor for the cytokine IL-11 and its downstream signal transducer signal transducer and activator of transcription 3 (STAT3) are targets of regulation by C/EBP beta. Chromatin immunoprecipitation analysis indicated that C/EBP beta controls the expression of STAT3 gene by directly interacting with a distinct regulatory sequence in its 5'-flanking region. Attenuation of STAT3 mRNA expression in HESCs resulted in markedly reduced differentiation of these cells, indicating an important role for STAT3 in decidualization. Gene expression profiling, using STAT3-deficient HESCs, showed an extensive overlap of pathways downstream of STAT3 and C/EBP beta during stromal cell differentiation. Collectively, these findings revealed a novel functional link between C/EBP beta and STAT3 that is a critical regulator of endometrial differentiation in women. (Molecular Endocrinology 26: 2016-2030, 2012)