Karaciğer kanserinde başkalaştırıcı büyüme etmeni-beta?ya bağlı yaşlanma yanıtının moleküler mekanizmaları

Hepatocellular carcinoma (HCC) is the fifth most common cancer in theworld. HCC is associated with several etiological factors including infections withhepatitis B and C viruses, heavy alcohol consumption and chronic aflatoxin B1exposure. Due to its multi-step disease hallmark characterized with geneticheterogeneity, liver cancer has very limited therapeutic options. In light of manyprevious findings, cellular senescence acts as a barrier against immortalization andprohibits the proliferation of premalignant cells in various tumors including HCCs.However, implications of this anti-tumor mechanism in hepatic tissues are not wellknown.TGF-ß is a multifunctional cytokine implicated in diverse cellular processesincluding senescence arrest as well as liver physiology and pathophysiology.Although TGF-ß-induced senescence has been described in different cell types, thisissue has never been addressed for hepatic cells. According to our recent data, TGF-ß1 expression pattern in various HCC malignancies closely correlated with reportedfrequencies of SABG activities in these corresponding disease stages. Therefore, wehypothesized that TGF-ß signaling might play key role in hepatocellular senescence.Well-differentiated (WD) five cell lines characterized with epithelial-likemorphology displayed TGF-ß-induced growth inhibition associated with SABGactivity, with lack of evidence of apoptosis induction. Even a brief exposure to TGF-ß was sufficient to trigger a massive senescence response. Senescence arrest in WDcell lines was linked to c-myc down-regulation and a reciprocal increase in p21Cip1and p15Ink4b protein levels. In addition, TGF-ß-induced senescence was correlatedwith Nox4 induction, intracellular accumulation of reactive oxygen species (ROS)and sustained 53BP1 foci formation as a mark of DNA-damage response. Moreover,intratumoral injection of TGF-ß in human HCC tumors, generated subcutaneously inimmunodeficient mice, induced expanded SABG that was associated with a stronganti-tumor response activity.On the other hand, poorly differentiated (PD) HCC cell lines withmesenchymal-like characteristics appeared to be resistant to TGF?ß-inducedsenescence. However, PD cell lines had intact TGF-ß signaling from cell membraneto nucleus. Resistance of PD cell lines was partially due to zeb2 overexpression,homozygous p15Ink4b deletion and lack of pRb expression. Besides, PD cells did notdisplay Nox4 upregulation and also lacked ROS accumulation upon TGF-ßstimulation.In addition, we demonstrated that sustained exposure to TGF-ß establishedresistant Huh7 subclone. The resistance was partially attributed to deregulated Smadsignaling, permanent epithelial-mesenchymal transition-like transformation.Surprisingly enough, removal of TGF-ß from culture medium of continuously treatedHuh7 subclone did not resolve the resistance phenotype in the rescued subclone.Epigenetic regulations mainly histone modifications are considered as candidatemechanisms responsible for irreversible TGF-ß-resistance and maintenance ofmesenchymal-like phenotype. Taken together, our results establish a close linkbetween senescence arrest and anti-tumor activity of TGF-ß signaling pathway inWD cell lines by delineating the mechanisms underlying TGF-ß-induced growtharrest. Moreover, we propose partial explanation for the resistance to TGF-ß-mediated growth arrest in PD cell lines and thoroughly signify the potentialmechanisms of acquired resistance to TGF-ß in continuously treated cultures. Furtherstudies to enlighten our knowledge about implications of TGF-ß signaling in lessdifferentiated HCCs are necessary. As a conclusion, we identify TGF-ß signaling asa potent therapeutic option for well-differentiated early HCCs.