COP1 and type 2 diabetes mellitus: To test the notion that increased proinsulin expression, as in T2D, may exacerbate abnormal disulfide formation, we tested the effect of brefeldin A (BFA), which promotes retrograde COP1 trafficking from the cis-Golgi to the ER to prevent export of secretory proteins to the Golgi, and thus increasing their concentration in the ER.