Plasminogen deficiency leads to impaired remodeling after a toxic injury to the liver

Abstract

Cellular proliferation and tissue remodeling are central to the regenerative response after a toxic injury to the liver. To explore the role of plasminogen in hepatic tissue remodeling and regeneration, we used carbon tetrachloride to induce an acute liver injury in plasminogen-deficient (Plg(o)) mice and nontransgenic littermates (Plg(+)). On day 2 after CCl(4), livers of Plg(+) and Plg(o) mice had a similar diseased pale/lacy appearance, followed by restoration of normal appearance in Plg(+) livers by day 7. In contrast, Plg(o) livers remained diseased for as long as 2.5 months, with a diffuse pale/lacy appearance and persistent damage to centrilobular hepatocytes. The persistent centrilobular lesions were not a consequence of impaired proliferative response in Plg(o) mice. Notably, fibrin deposition was a prominent feature in diseased centrilobular areas in Plg(o) livers for at least 30 days after injury. Nonetheless, the genetically superimposed loss of the Aalpha fibrinogen chain (Plg(o)/Fib(o) mice) did not correct the abnormal phenotype. These data show that plasminogen deficiency impedes the clearance of necrotic tissue from a diseased hepatic microenvironment and the subsequent reconstitution of normal liver architecture in a fashion that is unrelated to circulating fibrinogen.

Figure 1

Persistent injury of Plg^o livers after CCl₄. (Upper) Plg⁺ livers have a diffuse pale/lacy appearance 2 days after CCl₄, followed by restoration of normal appearance by days 7 and 14 (arrows point to residual areas of abnormal appearance). (Lower) In contrast, although the same diseased appearance is seen in Plg^o livers on day 2, absence of plasminogen leads to a persistent diseased appearance at days 7 and 14. Although not depicted in the figure, Plg^o livers on days 30 and 70 resemble those on day 14.

Figure 2

Persistent centrilobular injury of Plg^o livers. Sections of Plg⁺ livers show an injury to centrilobular hepatocytes 2 days after CCl₄, followed by normalization by day 14. In contrast, the centrilobular lesion seen in Plg^o livers on day 2 persists at day 14, with no evidence of ongoing repair. At day 70, the centrilobular space of Plg^o livers contains a few normal-appearing hepatocytes and abundant dystrophic calcification (Insert), which is also observed sporadically in Plg⁺ livers (arrows). (Magnifications: panels, ×200; Insert, ×400.)

Figure 3

BrdUrd labeling after liver injury. BrdUrd-hepatocytes are abundant and distributed uniformly throughout the noninjured lobular region 2 days after CCl₄ in Plg⁺ and Plg^o livers. Thereafter, labeled hepatocytes are infrequently found in Plg⁺ livers. Note that labeled hepatocytes are similarly inconspicuous in Plg^o livers, despite the persistent centrilobular injury at day 7. (Magnification: ×200.)

Figure 4

Fibrin deposition in Plg^o livers. Immunohistochemical staining showing fibrin deposition in diseased areas (pink) of Plg⁺ and Plg^o livers 2 days after CCl₄. In Plg⁺ livers, resolution of injury is accompanied by the timely removal of fibrin. In the absence of plasminogen, fibrin deposits persist up to day 14 after toxic injury; at day 70, minimal residual immunostaining is seen in the area of dystrophic calcification (arrows). (Magnification: ×200.)

Figure 5

Fibrin deficiency does not rescue the lesion of Plg^o livers. Liver sections show a similar centrilobular injury regardless of the genotype 2 days after CCl₄. On day 14, histology of Plg⁺/Fib⁺ and Fib^o livers is normal, whereas the lesion persists in Plg^o and Plg^o/Fib^o livers. (Bottom) Immunohistochemical staining shows the characteristic accumulation of fibrin in diseased Plg^o livers 14 days after CCl₄, whereas no specific staining is noted in diseased Plg^o/Fib^o livers. (Arrowheads, portal vein; arrows, centrilobular region; magnification: ×200.)