Role of transforming growth factor beta signaling and expansion of progenitor cells in regenerating liver.
Adult hepatic progenitor cells are activated during regeneration when hepatocytes and bile duct epithelium are damaged or unable to proliferate. On the basis of its role as a tumor suppressor and in the potential malignant transformation of stem cells in hepatocellular carcinoma, we investigated the role of key transforming growth factor beta (TGF-beta) signaling components, including the Smad3 adaptor protein beta2-Spectrin (beta2SP), in liver regeneration. We demonstrate a streaming hepatocyte-specific dedifferentiation process in regenerating adult human liver less than 6 weeks following living donor transplantation. We then demonstrate a spatial and temporal expansion of TGF-beta signaling components, especially beta2SP, from the periportal to the pericentral zone as regeneration nears termination via immunohistochemical analysis. This expansion is associated with an expanded remaining pool of octamer 3/4 (Oct3/4)-positive progenitor cells localized to the portal tract in adult human liver from more than 6 weeks posttransplant. Furthermore, disruption of TGF-beta signaling as in the beta2SP (beta2SP+/-) knockout mouse demonstrated a striking 2 to 4-fold (P < 0.05) expanded population of Oct3/4-positive cells with activated Wnt signaling occupying an alpha-fetoprotein (AFP)+/cytokeratin-19 (CK-19)-positive progenitor cell niche following two-thirds partial hepatectomy.


CONCLUSIONS
TGF-beta signaling, particularly beta2SP, plays a critical role in hepatocyte proliferation and transitional phenotype and its loss is associated with activation of hepatic progenitor cells secondary to delayed mitogenesis and activated Wnt signaling.
Cancer Genetics, Digestive Diseases, and Developmental Molecular Biology, Department of Surgery, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, DC, USA.
Arun
A
Thenappan
Ying
Y
Li
Krit
K
Kitisin
Asif
A
Rashid
Kirti
K
Shetty
Lynt
L
Johnson
Lopa
L
Mishra
D000818:Animals; D002352:Carrier Proteins / Q000502:physiology*; D006498:Hepatectomy; D006801:Humans; D008115:Liver Regeneration; D051379:Mice; D008840:Microfilament Proteins / Q000502:physiology*; D050814:Octamer Transcription Factor-3 / Q000032:analysis; D015398:Signal Transduction / Q000502:physiology*; D013049:Spectrin / Q000502:physiology; D013234:Stem Cells / Q000502:physiology*; D016212:Transforming Growth Factor beta / Q000502:physiology*; D051153:Wnt Proteins / Q000502:physiology
D002352:Carrier Proteins; D008840:Microfilament Proteins; D050814:Octamer Transcription Factor-3; D013049:Spectrin; D016212:Transforming Growth Factor beta; D051153:Wnt Proteins; C494174:Pou5f1 protein, mouse