ATF4 and breast cancer: Activating transcription factor 4 (ATF4), an endoplasmic reticulum-resident transmembrane protein, and microtubule-associated protein 1 light chain 3B (LC3B), one of the key factors in autophagosome formation, play a critical role in activating autophagy and protecting breast cancer cells from bortezomib-induced cell death, representing another potential mechanism of resistance to bortezomib [121].