MAPK1 and cancer: CAFs expressing GPER can activate the epidermal growth factor receptor (EGFR) and extracellular signal-regulated kinase 2 (ERK) signaling which leads to (i) estradiol production by CAFs and endocrine therapy resistance [70] and (ii) cancer cell integrin β1 activation by increased CAF-secreted fibronectin [71], which drives EMT and the tamoxifen-resistant phenotype.