Trans-Acting Factors in circRNA Biogenesis—Potential Biomarkers and Therapeutic Targets for Receptive and Cancerous Endometrium

Dysregulated circRNA expression is considered one of the main molecular regulatory mechanisms for the processes in an organism, among which are endometrial cancerogenesis and the establishment of endometrial receptivity. The cancerous and receptive endometrium have been described as opposites—the first characterized by intense proliferation and the lowest differentiation, while the latter is regarded as the most highly differentiated. CircRNAs are non-coding RNA molecules, represented by a closed loop with no 5′ cap and 3′ poly(A) tail structure, which makes them undistinguishable from most endonucleases and ensures them a longer half-life than their linear cognates, influencing the expression of miRNA and genes. CircRNA biogenesis is regulated by cis-molecules and trans-acting factors (TAFs). Our attention was on TAFs—key molecular switches in circRNA biogenesis, less noted in the literature and involved in studies concerning the endometrium, which can change the bias between circular and linear counterparts, affecting the cellular structure and function. Today, over 2600 TAFs are involved in the ubiquitous disorders of transcriptional programs. TAFs participate in cellular proliferation, differentiation, epithelial–mesenchymal transition (EMT), and angiogenesis. They play a pivotal role in developmental and oncogenic pathways and influence both cancer development and progression, and the establishment of endometrial receptivity. An in-depth and detailed analysis of the TAFs could facilitate both diagnostics and treatment of EC (endometrial cancer) and ER (endometrial receptivity) for RIF (repeated implantation failure) cases’ solution. Future research is necessary to broaden the knowledge about the application capacity of TAFs as circRNA modulators in EC and ER, and to study their potential as biomarkers and therapeutic molecules.