TGFB1 and diabetic kidney disease: PKCβ1 expressed in the proximal tubule is critical for inducing the expression of transforming growth factor-β1 (TGFβ1), which is an essential mediator of the switch from proliferation to hypertrophy in renal proximal tubular cells during the early stage of DKD through the extracellular signal-regulated kinase (ERK) and p38 pathway.