ReviewDeranged hepatocyte intracellular Ca2+ homeostasis and the progression of non-alcoholic fatty liver disease to hepatocellular carcinoma
Graphical abstract
Introduction
Hepatocellular carcinoma (HCC) involves the accumulation of mutations in hepatocytes, the principle cell type present in the liver. HCC is an important health issue in both Western and Asian countries [[1], [2], [3]]. Nonalcoholic fatty liver disease (NAFLD) is a major risk factor for the development of HCC [2,3]. Changes in the concentration of Ca2+ in the cytoplasmic space, endoplasmic reticulum (ER) and mitochondria are essential for the normal regulation of carbohydrate, lipid and protein metabolism, as well as many other pathways such as the excretion of bile acids [[4], [5], [6]]. Studies conducted over the past 10 years have pointed to significant lipid-induced alterations in Ca2+ signaling in steatotic hepatocytes (reviewed in [[7], [8], [9], [10], [11]]). This, in turn, leads to further lipid accumulation and to the creation of an environment which is thought to promote mutagenesis and the progression of HCC [7,[11], [12], [13], [14], [15], [16]] (Fig. 1). The aim of this review is to summarise the changes in intracellular Ca2+ signaling caused by lipid accumulation in hepatocytes, to describe the proposed links between altered Ca2+ signaling, lipids, and HCC, and to discuss the evidence that lipid-induced changes in intracellular Ca2+ signaling promote the development of HCC.
Mutations in many Ca2+ transporters, channels, and Ca2+ binding proteins have been identified in DNA derived from liver tumors in patients with HCC, and in animal models of HCC [[16], [17], [18]]. Moreover, the expression of a number of Ca2+ signaling and Ca2+ binding proteins is altered in HCC tissue [16,[19], [20], [21]]. While some of these mutations or changes in protein expression may involve DNA from non-hepatocyte cell types present in liver, the observations suggest that, in the transformed hepatocytes which constitute liver tumors, Ca2+ signaling is altered [16]. While these changes in Ca2+ signaling are undoubtedly important in the progression and migration of established HCC, the focus of this review is on lipids and the liver environment which is proposed to set up the initiation and promotion of HCC.
Section snippets
Hepatocellular carcinoma
The term “primary liver cancer” encompasses HCC, intrahepatic cholangiocarcinoma, fibrolamellar hepatocellular cancer and pediatric hepatoblastoma, all of which originate in the liver [22]. HCC is the second leading cause of cancer-related deaths in men, and the sixth in women, world-wide [[1], [2], [3]]. HCC is often not noticed in the early stages, and is difficult to treat in later stages [[1], [2], [3]]. Major risk factors which pre-dispose to the development of HCC are hepatitis B (HBV),
Non-alcoholic fatty liver disease as a risk factor for hepatocellular carcinoma
The term non alcoholic fatty liver disease refers to a spectrum of liver disorders which range from the accumulation of excess lipid in hepatocytes (simple steatosis) to NASH in which lipid accumulation over time leads to hepatocyte injury, inflammation, and peri-cellular fibrosis [16,[31], [32], [33]]. The progression from simple steatosis to NASH is principally mediated by toxic free fatty acids (lipotoxicity) and an increase in reactive oxygen species (ROS) (oxidative stress) [16,[31], [32],
Changes in endoplasmic reticulum and mitochondrial calcium homeostasis in steatotic hepatocytes
Results obtained in a number of laboratories have shown that lipid accumulation in steatotic hepatocytes causes substantial changes in intracellular Ca2+ signaling (summarized, with references, in Table 1). The Ca2+ channels and transporters which have so far been identified as being altered in steatotic hepatocytes, and which have been studied in some detail, are the sarco/endoplasmic reticulum (Ca2++Mg2+)ATP-ase2b (SERCA2b), the type 1 inositol trisphosphate receptor (InsP3R1), and
Changes in store-operated Ca2+ entry in steatotic hepatocytes
In addition to the effects of intracellular lipid accumulation on the ER, SERCA2b, InsP3R1 and on the transfer of Ca2+ between the ER and mitochondria, lipid accumulation in steatotic hepatocytes also inhibits SOCE (Table 1). SOCE is one of the main pathways of Ca2+ entry to hepatocytes, and plays a central role in maintaining adequate concentrations of [Ca2+]ER under conditions of hormonal Ca2+ signaling involving InsP3R-mediated ER Ca2+ efflux and Ca2+ extrusion from the cell via plasma
Lipid-induced inhibition of store-operated Ca2+ homeostasis and deranged intracellular Ca2+ homeostasis exacerbate lipid accumulation in hepatocytes
Several different experimental approaches have provided evidence that altered intracellular Ca2+ homoeostasis causes and/or exacerbates lipid accumulation in hepatocytes, as well as in other cell types. The main initiating change in Ca2+ transporters, channels, and Ca2+ concentrations together with the proposed mechanisms are summarized in Table 2. These involve decreased lipolysis, inhibition of lipid autophagy, inhibition of beta-oxidation, increased lipogenesis and decreased secretion of
Deranged intracellular Ca2+ homeostasis in steatotic hepatocytes creates an environment which could promote the development and progression of hepatocellular carcinoma
As discussed above, chronically elevated lipid in steatotic hepatocytes of NAFLD subjects is a major risk factor for the development of HCC [2,23]. Animal studies, principally with mice and rats, have shown that a high fat diet enhances the development of HCC, although the nature and extent of HCC depends on the agent employed to induce mutagenesis [[92], [93], [94], [95], [96]]. A recent study involving a mouse model of steatohepatitis has provided evidence that incorporation of cholesterol
Hepatocyte Ca2+ channels and transporters as potential therapeutic targets in preventing the development of hepatocellular carcinoma in non alcoholic fatty liver disease
The information presented above indicates that altered hepatocyte intracellular Ca2+ homeostasis leads to intracellular and intra organ environments which could promote the development of HCC. This suggests that normalization of intracellular Ca2+ in the hepatocytes of NAFLD patients may reduce the likelihood of NAFLD proceeding to NASH and HCC. Therefore, pharmaceutical interventions which could normalize hepatocyte intracellular Ca2+ homeostasis in NAFLD are of potential value. Table 4 lists
Conclusions
Lipid-induced alterations in intracellular Ca2+ homeostasis in steatotic hepatocytes play important roles in HCC pathology, including exacerbation of lipid accumulation, generation of ER stress and ROS, activation of Ca2+ calmodulin-sensitive kinases and activation of transcription factors. Temporal changes in Ca2+ transporters, channels and intracellular Ca2+ concentrations during the progression of simple steatosis to NASH and HCC, and the mechanisms involved, are yet to be fully understood.
Declaration of Competing Interest
The authors have no conflict of interest to declare.
Acknowledgements
This research was supported by grants from the Australian Research Council (DP140100259), Flinders Center for Innovation in Cancer, and the Flinders Foundation.
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