Fig. 1: Myotubes secrete a cell non-autonomous unfolded protein response (UPR) -inducing lipid signal in response to lipotoxicity.

a C2C12 myotubes were treated with 200 µM palmitate or bovine serum albumin (BSA) vehicle, serum-free media was conditioned on these cells (24 h) and transferred to naïve myocytes. b Activating transcription factor 3 (Atf3), activating transcription factor 4 (Atf4), Heat Shock Protein Family A (Hsp70) Member 5 (Hspa5) and ER Degradation Enhancing Alpha-Mannosidase Like Protein 1 (Edem1) UPR gene expression in C2C12 myotubes receiving conditioned media from myotubes treated with 200 μM palmitate (grey) or BSA (red) (n = 4; two-tailed Student’s t test; Atf4 P = 0.00009, Hspa5 P = 0.005, Edem1 P = 0.000024). c UPR gene expression in human skeletal muscle cells (HSkMCs) receiving conditioned media from HSkMCs treated with 100 μM palmitate (grey) or BSA (red) (n = 3, two-tailed Student’s t test; ATF3 P = 0.005, ATF4 P = 0.0017, Hspa5 P = 0.0088). d UPR gene expression in myotubes receiving boiled media conditioned on C2C12 myotubes treated with 200 μM palmitate or BSA (control = red; control boiled = grey; 200 µM palmitate = blue; 200 µM palmitate boiled = green; n = 3; one-way ANOVA; control media vs 200 µM palmitate media, Atf3 P = 0.017, Atf4 P = 0.005; control vs 200 µM palmitate boiled, Atf3 P = 0.03, Atf4 P = 0.0065; control boiled vs 200 µM palmitate, Atf3 P = 0.016, Atf4 P = 0.005; control boiled vs 200 µM palmitate boiled, Atf3 P = 0.027, Atf4 P = 0.007). e UPR gene expression in myotubes receiving reconstituted lipid extract isolated from media conditioned on C2C12 myotubes treated with 200 μM palmitate (grey) or BSA (red) (n = 4; two-tailed Student’s t test: Atf3 P = 0.05, Atf4 P = 0.0026, Hspa5 P = 0.0097, Edem1 P = 0.0048). *P ≤ 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001. Data are expressed as mean ± SEM with individual data points. Source data are provided as a Source Data file.