277915 Modeling the Dynamics of Acute Phase Protein Expression in HepG2 Cells Stimulated by IL-6
Interleukin-6 (IL-6) triggers the human body’s acute phase response to trauma or inflammation mainly by stimulation of two signaling pathways: the first pathway involves the activation of the Janus-associated kinases (JAK) and signal transducers and activators of transcription (STAT3), while the second pathway comprises the activation of mitogen-activated protein kinases (MAPK) and CCAAT (cytidine-cytidine-adenosine-adenosine-thymidine)/enhancer binding protein β (C/EBPβ) (Heinrich et al., 2003). The expression of genes of certain acute phase proteins, such as albumin, haptoglobin, and fibrinogen, is regulated by the transcription factors associated with IL-6 signaling, i.e., nuclear STAT3 dimer and C/EBPβ (Alam et al., 1992; Fuller and Zhang, 2001; Ruminy et al., 2001). The expressed acute phase proteins in turn participate in the human body’s response to trauma or inflammation. Hence, an accurate mathematical model for predicting the dynamics of acute phase protein expression in hepatocytes can facilitate the treatment of trauma or inflammation. Although mathematical models for IL-6 signaling pathways have previously been developed (Huang et al., 2009; Moya et al., 2011; Singh et al., 2006), reactions that describe the expression of acute phase proteins have not been included in these models. To address this deficiency, a recent model of IL-6 signaling (Moya et al., 2011) is extended in this work to predict the dynamics of acute phase protein expression in IL-6-stimulated HepG2 cells (a human hepatoma cell line). A sensitivity analysis is then performed to identify reactions in IL-6 signaling that play an important role in regulation of the dynamics of acute phase protein expression.
The starting point for the model used in this work is the IL-6 signal transduction model presented in Moya et al., 2011. This model is based upon the model structure proposed by Singh et al., 2006, whereas the dynamic model of Jak-STAT signaling is adopted from Yamada et al., 2003, and parts of the detailed kinetic model of MAPK signaling proposed by Schoeberl et al., 2002 are also used. In the present work, reactions that describe the expression of haptoglobin, fibrinogen, and albumin were added into the IL-6 signaling model. C/EBPβ, nuclear STAT3 dimer, and C/EBPα are the transcription factors regulating the transcription of haptoglobin (Alam et al., 1992), fibrinogen (Zhang and Fuller, 2001), and albumin (Alam et al., 1992; Ruminy et al., 2001), respectively. For example, C/EBPβ down-regulates the expression of albumin by inhibiting the activation of C/EBPα (Ruminy et al., 2001). Following this approach, a system of 75 coupled ordinary differential equations was developed based on mass action kinetics and Michaelis-Menten kinetics. Experimental data for HepG2 cells stimulated by 2 nM IL-6 continuously for seven hours (Karlsson et al., 1998) were used to estimate the values of parameters involved in the newly added reaction equations. The extended model was then validated against independent experimental data for HepG2 cells under three-hour exposure to three different IL6 concentrations (Karlsson et al., 1998). Experimental data for a pulse-chase stimulation with 2 nM IL-6 (Karlsson et al., 1998) were used to further validate the extended model. In addition, the predicted steady-state accumulation rates of fibrinogen and albumin for various IL-6 concentrations were compared to the experimental data reported by Heinrich et al., 1990, and found to be consistent with the published measurements. Finally, a sensitivity analysis was performed to identify reactions in IL-6 signaling pathway that play an important role in regulating the expression of haptoglobin, fibrinogen, and albumin, respectively. Binding of STAT3C to (IL6–gp80–gp130–JAK)2* and expression of SOCS3 were identified as essential steps for regulating the secretion rate of fibrinogen, while the conversion of Ras–GDP to Ras–GTP, the unbinding process of SHP2*–Grb2–SOS with (IL6–gp80–gp130–JAK)2*, and the deactivation of Raf* by phosphatase Phosp1 were determined as the key processes that regulate the dynamics of haptoglobin and albumin secretion from HepG2 cells.
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