CAFs and Endocrine Therapy Resistance in Hormone Receptor-Positive Breast Cancer.

The development of endocrine resistance represents a major obstacle when treating hormone receptor-positive breast cancer. The tumor microenvironment (TME), represented by cancer-associated fibroblasts (CAFs) in this context, has recently been proposed as a key mediator significantly contributing to resistance against currently available endocrine therapies. The exact mechanisms behind this interaction are not fully understood; specific breast CAF subtypes have been linked to it, such as CAFs lacking the expression of the glycoprotein CD146 or maintaining the expression of CD63. Other proposed mechanisms include signaling pathways aberrantly activated in CAFs, epigenetic modifications mainly in the form of long non-coding RNAs (lncRNAs) and microRNAs (miRNAs), and paracrine signaling, all limiting endocrine modulation effectiveness. Strategies aiming to simultaneously target CAFs and endocrine signaling in luminal breast cancer are currently being developed. Fibroblast growth factor receptor (FGFR) targeting in combination with endocrine inhibition has already entered the clinical trial landscape. However, CAFs are a highly diverse and heterogeneous cell population, making their targeting complex and difficult to implement in clinical practice.