ABSTRACT The central goal of this R01 proposal is to understand how molecular and cellular interactions of heparan sulfate proteoglycans (HSPGs) modulate the pathogenesis of acetaminophen (APAP)-induced liver injury (AILI). Accidental or intentional misuse of APAP is the leading cause of acute liver failure in the Western world. While mechanisms that trigger AILI are well known, those that facilitate liver recovery are less understood. HSPGs bind and regulate various tissue injury factors through their heparan sulfate (HS) chains, but the significance and mechanisms of HSPGs in tissue injury and repair in vivo remain largely unknown. We examined the role of syndecan-1 (Sdc1), the major cell surface HSPG of hepatocytes, in AILI. Deletion of Sdc1 in mice led to unopposed progression of liver injury in APAP liver disease. However, direct APAP hepatoxicity at early times after APAP overdose was unaffected by Sdc1 deletion, suggesting that Sdc1 regulates later mechanisms that affect the progression and outcome of APAP liver disease. The exuberant AILI phenotypes of Sdc1 null (Sdc1-/-) mice were traced to an exaggerated innate immune response in the liver and a deficiency in pro-survival Akt signaling in hepatocytes and hepatocyte proliferation, which led to amplification of liver damage. Administration of purified Sdc1 or heparan compounds containing 2-O-sulfate motifs rescued Sdc1-/- mice from AILI by inhibiting innate immune responses, and by potentiating hepatocyte proliferation and liver repair. Furthermore, HS showed a significantly prolonged therapeutic efficacy as compared to N-acetylcysteine (NAC), the clinical antidote for APAP overdose. These findings suggest that Sdc1 and HS, either alone or in combination with NAC, could provide a new therapeutic strategy to combat AILI, especially in treating patients admitted after NAC treatment is no longer effective. Based on these preliminary data, we propose that Sdc1 is a critical endogenous factor that halts the perpetuation of liver injury and facilitates liver repair in AILI. This hypothesis will be tested in 3 specific aims. Aim 1 will define how Sdc1 is released from hepatocytes during AILI and establish that discrete structural motifs in Sdc1 HS provide protection against AILI. Aim 2 will elucidate the biological mechanisms of how Sdc1 halts the progression of AILI. Aim 3 will determine how Sdc1 enhances hepatocyte proliferation and facilitates liver repair in AILI. These studies are expected to establish a new integrated pathway in liver injury and repair.