ATF4 and obesity due to melanocortin 4 receptor deficiency: Acute exposure (24 h) to PAL resulted in a significant elevation in ER stress in a concentration-dependent gradual manner; higher levels (500 μM), typical and reflecting blood levels in human obesity, resulted in higher elevation in ER stress markers compared with exposure to lower levels (250 μM), as indicated by UPR transcript and protein levels of ATF4, ATF6 and XBP1 and in XBP1 splicing levels.