Alpha fetoprotein mediates HBx induced carcinogenesis in the hepatocyte cytoplasm

Abstract

Although tumor-associated fetal protein AFP has demonstrated utility as a clinical tumor marker, the significance of intracellular AFP is still unclear. The aim of this study was to explore the role of cytoplasmic AFP during HBx induced carcinogenesis, which had not previously been recognized; 614 HCC patients were analyzed for correlation of HBV infection with AFP level, and much higher AFP levels were found in HBsAg positive patients. Tumor tissue specimens from 20 HCC patients were used for analysis of AFP and GADD45a. Analysis of HCC specimens showed that upregulation of cytoplasmic AFP is associated with down-regulation of GADD45a in neoplastic tissue. Transfected HBx promotes transcription of AFP by acting on the elements in the AFP gene regulatory region. HBx itself did not directly impact transcription of GADD45a. However, the obstruction of RAR signaling by HBx induced elevation of AFP, which led to down-regulation of GADD45a. Cytoplasmic AFP was able to interact with RAR, disrupting its entrance into the nucleus and binding to the elements in the regulatory region of the GADD45a gene. Knockdown of AFP in siRNA-transfected AFP positive cell lines was synchronously associated with an incremental increase of RAR binding to DNA, as well as upregulation of GADD45a and it was contrary in AFP gene-transfected AFP negative cell lines. These results indicate cytoplasmic AFP is not only a histochemical tumor biomarker for human hepatoma but is also an intracellular signal molecule and potential participant in HBx induced hepatocarcinogenesis.Over the last decade, a multitude of studies have established circulating alpha fetoprotein (AFP) as a growth regulator during ontogenic growth and tumor progression.1-6 While this is a still developing field replete with unanswered questions, compelling progress which has been made in determining the biological role of cytoplasmic AFP as a signal molecule has attracted considerable interest, as aberrantly elevated AFP disturbs the normal signaling network and results in hepatocyte proliferation. As shown in previous studies, regulation of cytoplasmic AFP in development and growth of malignant hepatic tumor cells is achieved through binding to signal molecules involved in growth or apoptosis. These findings have shown cytoplasmic AFP may interfere with the proliferative and apoptotic signaling of the caspase-3 cascade and PI3K signaling, and this leads to aberrant growth of hepatocellular carcinoma (HCC) cells. 7,8 Findings in this study revealed a hitherto undiscovered role for cytoplasmic AFP in carcinogenesis and drug resistance. The regulatory attributes and clinical significance of AFP have been recently summarized and discussed.9 However, to date findings have been insufficient for complete clarification of the intrinsic mechanism underlying the impact of cytoplasmic AFP, and in particular have been insufficient to clarify its clinical relationship to virus induced carcinogenesis.HCC is the fifth most frequent neoplasm worldwide, and accounts for 5.6% of all human canc...

Nonsecreted cytoplasmic alpha-fetoprotein: a newly discovered role in intracellular signaling and regulation. An update and commentary

“…The authors concluded that HBx-induced CyAFP expression promoted the malignant transformation of liver cells via the activation of PI3K/ mTOR signaling. As demonstrated in the above discussions, CyAFP is associated with downregulation of GADD153 through binding to RAR and disruption of its nuclear transpassage; this resulted in the failure of GADD153 gene activation [58]. As shown above, transfected HBx promoted expression of the CyAFP which serves as an active agent in liver carcinogenesis.…”

Hepatoepigenetic Alterations in Viral and Nonviral-Induced Hepatocellular Carcinoma

“…Under the influence of HBx transcriptional activities in the cytoplasm, RAR may also promote hepatocarcinogenesis by interacting with other proteins such as AFP. It was recently shown that interaction of HBx-induced AFP with RAR resulted in perturbed RAR signalling pathway, leading to repression of growth arrest and DNA damage 45 α (GADD45 α ) protein expression [107]. GADD45 α is an 18.4 kDa RNA-binding acidic protein that is expressed in response to DNA damage for repairing and induction of apoptosis by inhibiting G2/M transition of cell cycle.…”

“…GADD45 α is an 18.4 kDa RNA-binding acidic protein that is expressed in response to DNA damage for repairing and induction of apoptosis by inhibiting G2/M transition of cell cycle. Downregulation of GADD45 α in HCC was associated with uncontrolled HBV-infected hepatocytes growth and hepatocarcinogenesis, suggesting its effects in allowing the cells to evade senescence and apoptosis [104, 107]. Although the role of GADD45 α in promoting instability through DNA demethylation has been reported previously, it has not been explored in HBV-induced HCC and therefore warrants investigation [108, 109].…”

Icaritin inhibits the expression of alpha-fetoprotein in hepatitis B virus-infected hepatoma cell lines through post-transcriptional regulation

“…Circulating AFP acts as a growth regulator during oncogenic growth and tumor progression, and is considered a diagnostic and prognostic tumor marker [1–3]. In recent years, remarkable progress has been made in determining the biological role of cytoplasmic AFP as a signal molecule: aberrantly elevated AFP disturbs the normal signaling network and shows a strong association with the high mortality rate of HCC [4–7]. …”

“…In addition, the caspase-3 cascade and the tumor necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) induce apoptosis is virtually abolished in the presence of AFP [5, 9]. Our previous research has shown the potential association of AFP levels with HBV infection and revealed a hitherto undiscovered role for cytoplasmic AFP in mediating HBV-induced hepatocyte carcinogenesis [7]. Given that cytoplasmic AFP has been defined as a growth-promoting molecule, AFP gene silencing would be beneficial for therapy of HCC patients.…”