Adenovirus–Mediated Overexpression of Follistatin Enlarges Intact Liver of Adult Rats

Abstract

Under normal physiologic conditions, liver size is under strict regulatory control. Activin, a member of the transforming growth factor ␤ (TGF-␤) superfamily, is expressed in the intact adult liver and is an inhibitor of hepatocyte growth. However, the exact role played by endogenous activin in maintaining the size of a normal adult liver has yet to be completely examined in vivo. Here, we report the development of an adenoviral vector (AdexCAFS288) that expressed human follistatin-288, which binds to activin and neutralizes its biologic activities. AdexCAGFP, a control virus, expressed green fluorescent protein. AdexCAFS288 effectively expressed follistatin-288, as measured both in HepG2 cell lysate and conditioned medium and blocked activin signaling and its biologic functions in vitro. Intraperitoneal injection of AdexCAFS288 in vivo resulted in significant liver growth (146% of control) in intact liver of adult male rats 12 days following treatment without significant dysfunctions. The increase in liver size was attributed to increased hepatocyte proliferation, as monitored by the mitotic index. Furthermore, there was a significant correlation between serum follistatin levels and liver weight. In conclusion, our results suggest that activin plays a critical role in maintaining optimal liver size and implicates the endogenous activin system as a therapeutic target in the treatment of liver disease. (HEPATOLOGY 2003;38:1107-1115 T he liver has an optimal functional volume that is regulated in relation to body size. 1 When part of the liver is surgically removed, the remaining liver rapidly responds to this alteration by growing back to its original size. 2 Interestingly, the opposite is also true. A transplanted liver that is larger than the optimal size for a particular recipient undergoes apoptosis of hepatocytes and decreases in size until optimal liver volume is reached. 3 These observations suggest that the liver possesses the ability to maintain its optimal functional volume regardless of whether initial liver volume was smaller or larger than standard liver volume. Numerous studies have elucidated mechanisms that initiate and promote liver growth, but few have focused on factors that inhibit liver growth, or decrease liver mass. 4 Activin, a member of the transforming growth factor ␤ (TGF-␤) family of growth and differentiation factors, was first purified as a factor that stimulates follicle stimulating hormone (FSH) release from the pituitary gland. 5 Further work has revealed that activin is widely expressed during development and throughout life and acts as a multipotential growth and differentiation factor in many systems. The administration of exogenous activin inhibited epidermal growth factor-induced growth of primary cultured hepatocytes. 6,7 Previously, we have demonstrated that activin and its signaling partners were expressed in hepatocytes, and endogenous activin negatively regulated hepatocyte proliferation using antisense oligonucleotides directed against activin in vitro. 8 Furthermore, t...

Therapeutic Potential of Follistatin to Promote Tissue Regeneration and Prevent Tissue Fibrosis

“…Hence, activins may tonically inhibit growth of hepatocytes even in normal conditions. In agreement with these observations, adenovirus-mediated overexpression of follistatin initiates hepatocyte proliferation in intact liver and enlarges the liver [19]. Taken together, activin A not only exerts tonic inhibition on hepatocyte proliferation in regenerating liver but also blocks growth potential of hepatocytes in intact liver.…”

Activins and follistatins: Emerging roles in liver physiology and cancer

“…In addition to binding activins A, B, AB, and E, follistatin was also shown to bind and antagonize myostatin as well as BMPs 2, 4, 6, and 7 [88,[116][117][118][119] . Follistatin administration by intraportal infusion or adenovirusmediated overexpression caused DNA synthesis and liver growth in the rat, presumably by antagonizing tonic inhibition of hepatocyte proliferation by activin A [120,121] . Following partial hepatectomy, follistatin expression was up-regulated after 24-48 h, the time period in which hepatocyte replication was increased [42] .…”

Activins and activin antagonists in hepatocellular carcinoma

“…In addition to binding activins A, B, AB, and E, follistatin was also shown to bind and antagonize myostatin as well as BMPs 2, 4, 6 and 7 [16,[102][103][104][105] . Follistatin administration by intraportal infusion or adenovirus-mediated overexpression caused DNA synthesis and liver growth in normal rat livers presumably by antagonizing tonic inhibition of liver growth by activin A [106,107] . Following partial hepatectomy follistatin expression was up-regulated after 24-48 h, the time period in which hepatocyte replication was increased [50] .…”