Hippo Signaling in the Liver – A Long and Ever-Expanding Story

The first description of Hippo signaling in mammals a little more than 10 years ago showed a striking phenotype in the liver, linking the role of this signaling pathway to organ size control and carcinogenesis. Even though Hippo signaling has been extensively studied in the liver and other organs over the recent years, many open questions remain in our understanding of its role in hepatic physiology and disease. The functions of Hippo signaling extend well beyond cancer and organ size determination: components of upstream Hippo signaling and the downstream effectors YAP and TAZ are involved in a multitude of cell and non-cell autonomous functions including cell proliferation, survival, development, differentiation, metabolism, and cross-talk with the immune system. Moreover, regulation and biological functions of Hippo signaling are often organ or even cell type specific – making its role even more complex. Here, we give a concise overview of the role of Hippo signaling in the liver with a focus on cell-type specific functions. We outline open questions and future research directions that will help to improve our understanding of this important pathway in liver disease

Recruitment of Pontin/Reptin by E2f1 amplifies E2f transcriptional response during cancer progression

Changes in gene expression during tumorigenesis are often considered the consequence of de novo mutations occurring in the tumour. An alternative possibility is that the transcriptional response of oncogenic transcription factors evolves during tumorigenesis. Here we show that aberrant E2f activity, following inactivation of the Rb gene family in a mouse model of liver cancer, initially activates a robust gene expression programme associated with the cell cycle. Slowly accumulating E2f1 progressively recruits a Pontin/Reptin complex to open the chromatin conformation at E2f target genes and amplifies the E2f transcriptional response. This mechanism enhances the E2f-mediated transactivation of cell cycle genes and initiates the activation of low binding affinity E2f target genes that regulate non-cell-cycle functions, such as the Warburg effect. These data indicate that both the physiological and the oncogenic activities of E2f result in distinct transcriptional responses, which could be exploited to target E2f oncogenic activity for therapy

Recruitment of Pontin/Reptin by E2f1 amplifies E2f transcriptional response during cancer progression

Changes in gene expression during tumorigenesis are often considered the consequence of de novo mutations occurring in the tumour. An alternative possibility is that the transcriptional response of oncogenic transcription factors evolves during tumorigenesis. Here we show that aberrant E2f activity, following inactivation of the Rb gene family in a mouse model of liver cancer, initially activates a robust gene expression programme associated with the cell cycle. Slowly accumulating E2f1 progressively recruits a Pontin/Reptin complex to open the chromatin conformation at E2f target genes and amplifies the E2f transcriptional response. This mechanism enhances the E2f-mediated transactivation of cell cycle genes and initiates the activation of low binding affinity E2f target genes that regulate non-cell-cycle functions, such as the Warburg effect. These data indicate that both the physiological and the oncogenic activities of E2f result in distinct transcriptional responses, which could be exploited to target E2f oncogenic activity for therapy

Age independent survival benefit for patients with hepatocellular carcinoma (HCC) without metastases at diagnosis: a population-based study

OBJECTIVE Hepatocellular carcinoma (HCC) is a major cause of death worldwide and its incidence is expected to increase globally. Aim of this study was to assess whether the implementation of screening policies and the improvement of treatment options translated into a real-world survival benefit in HCC patients. DESIGN 4078 patients diagnosed with HCC between 1998 and 2016 from the Munich Cancer Registry were analysed. Tumour characteristics and outcome were analysed by time period and according to age and presence of metastases at diagnosis. Overall survival (OS) was analysed using Kaplan-Meier method and relative survival (RS) was computed for cancer-specific survival. Cox proportional hazard models were conducted to control for prognostic variables. RESULTS While incidence of HCC remained substantially stable, tumours were diagnosed at increasingly earlier stages, although the median age at diagnosis increased. The 3 years RS in HCC improved from 19.8% in 1998-2002, 22.4% in 2003-2007, 30.6% in 2008-2012 up to 31.0% in 2013-2016. Median OS increased from 6 months in 1998-2002 to 12 months in 2008-2016. However, analysis according to the metastatic status showed that survival improved only in patients without metastases at diagnosis whereas the prognosis of patients with metastatic disease remained unchanged. CONCLUSION These real-world data show that, in contrast to the current assumptions, the incidence of HCC did not increase in a representative German region. Earlier diagnosis, likely related to the implementation of screening programmes, translated into an increasing employment of effective therapeutic options and a clear survival benefit in patients without metastases at diagnosis, irrespective of age

Notch signaling inhibits hepatocellular carcinoma following inactivation of the RB pathway

Mice lacking all three Rb genes in the liver develop tumors resembling specific subgroups of human hepatocellular carcinomas, and Notch activity appears to suppress the growth and progression of these tumors

Organ Size Control Is Dominant over Rb Family Inactivation to Restrict Proliferation In Vivo

In mammals, a cell’s decision to divide is thought to be under the control of the Rb/E2F pathway. We previously found that inactivation of the Rb family of cell cycle inhibitors (Rb, p107, and p130) in quiescent liver progenitors leads to uncontrolled division and cancer initiation. Here, we show that, in contrast, deletion of the entire Rb gene family in mature hepatocytes is not sufficient for their long-term proliferation. The cell cycle block in Rb family mutant hepatocytes is independent of the Arf/p53/p21 checkpoint but can be abrogated upon decreasing liver size. At the molecular level, we identify YAP, a transcriptional regulator involved in organ size control, as a factor required for the sustained expression of cell cycle genes in hepatocytes. These experiments identify a higher level of regulation of the cell cycle in vivo in which signals regulating organ size are dominant regulators of the core cell cycle machinery