ATF4 and osteochondritis dissecans: Gln-free incubation triggers a clear-cut nutritional stress response in OD cells with the increased phosphorylation of eIF2α, a general reduction of protein synthesis, and the induction of ATF4 targets such as the sodium-dependent Gln transporter SNAT2 and the pro-apoptotic protein CHOP (Figure 4a,b, [44,45]).