Fig. 3

The bistability of the system correlates with the sign of the PLS-PAI1 feedback loop. a KLF2 affects the sign of PLS-PAI1 feedback loop. We designed an exogenous addition of PLS into the system using a step function for the level of PLS over time (top panel, black curve). Stimulating the TGF-β1 activation model with exogenous PLS caused two different effects in silico, depending on the KLF2 status. In the absence of KLF2 (red curve on left), the stimulus caused positive (double-negative) feedback between PAI1 and PLS, which can occur via the red arrows shown. In the presence of KLF2 (blue curve on right), exogenous PLS treatment caused a positive effect on PAI1 and the negative feedback loop (blue arrows) was restored. b Methods to calculate the 2d bistable region. We did bifurcation analysis of kp2 for a series of kp1 value (100 equally spaced values between kp1 = 1.2 and kp1 = 0) for the TGF-β1 activation model. The stable s.s. on the bifurcation curve were plotted as solid blue line, while the unstable s.s. were plotted as dotted red line. The Limit Points (LP), which tell the bistable interval of the bifurcation parameter (kp2) were denoted by black dots on the bifurcation curve. c The series of KLF2 levels cross the boundary of the bistable region in kp1-kp2 phase plane. The 2d bistable region (dark gray) in kp1-kp1 phase plane can be constructed based on the kp2 coordinates of LPs in (b) and their corresponding kp1 values. KLF2 levels are represented as diamond dots. d Overlap between bistable region and PLS-PAI1 positive feedback region in the kp1-kp2 phase plane. Bistable region is denoted by solid dark gray, while PLS-PAI1 positive feedback region is denoted by dotted light gray. The x and y axis are all in log scale. e Empirically, PLS can down-regulate PAI1 gene expression in a co-culture of hepatocytes and HSC-T6 cells. Hepatic stellate cells (HSC-T6) were cultured with primary rat hepatocytes in a 7:1 ratio overnight, creating a fibrosis-like state with high TGF-β1 level. The next day, the medium was changed to non-serum medium with different doses of PLS. Cells were collected 24 h later