PPARG and metabolic dysfunction-associated steatotic liver disease: Comparing different animal models of lean NAFLD, the most common mechanisms of liver fibrosis include the overexpression of TGF-β, α-SMA, and collagen type I. Additional factors that contribute to liver fibrogenesis in lean NAFLD models include MMPs, TIMPs, PPARγ, TLR2, and TLR4 and the overactivation of HSCs.