Cell cycle biology of fibrolamellar hepatocellular carcinoma

Context: Fibrolamellar hepatocellular carcinoma (FLHCC) has a better prognosis than conventional hepatocellular carcinoma. Nevertheless, FLHCC has a propensity to recur with limited responsiveness to chemotherapy. Objective: The purpose of this study was to provide insight into the cell cycle biology of FLHCC, as it relates to FLHCC's relatively indolent nature and lack of chemoresponsiveness. Design: In seven cases of FLHCC, we assessed: 1. immunoexpression of protein analytes indicating cell cycle progression including Ki-67 (G1, S, G2 and M phases) and S-phase kinase-associated protein (Skp) 2 along with the mitotic index (MI); 2.immunoreactivity for cyclindependent kinase inhibitors of cell cycle progression from G1 to S phase, p27Kip1 and p16INK4. Results: The mean percentage of Ki-67 nuclear positivity in neoplastic hepatocytes ranged from 1.0% to 29.7%. Nuclear Skp2 immunoexpression was not observed in any of the cases. The mitotic index was very low (0-1 mitotic figure/10 high-power fields). All cases showed moderate to strong nuclear p16INK4 positivity (diffuse in five and focal in two). Contrastively, the adjacent non-neoplastic hepatocytes expressed only mild (2 cases) to no (3 cases) p16INK4. Conclusion: Our analysis has revealed that cell cycle arrest in FLHCC occurs in G0G1 phase and is associated with overexpression of the cell cycle regulator, p16INK4 in tumoral cell nuclei compared with non-neoplastic hepatocytes. In conjunction with our previous immunohistochemical demonstration of a constitutively activated nuclear factor (NF)-kappaB pathway and stemness characteristics of FLHCC with limited differentiation, this cell cycle arrest elucidates the biology of FLHCC's indolent nature and relative chemoresistance.