A hedgehog survival pathway in ‘undead’ lipotoxic hepatocytes

Abstract

Background & Aims Ballooned hepatocytes in nonalcoholic steatohepatitis (NASH) generate sonic hedgehog (SHH). This observation is consistent with a cellular phenotype in which the cell death program has been initiated but cannot be executed. Our aim was to determine if ballooned hepatocytes have potentially disabled the cell death execution machinery, and if so, can their functional biology be modeled in vitro. Methods Immunohistochemistry was performed on human NASH specimens. In vitro studies were performed using Huh-7 cells with shRNA targeted knockdown of caspase 9 (shC9 cells) or primary hepatocytes from caspase 3−/− mice. Results Ballooned hepatocytes in NASH display diminished expression of the caspase 9. This phenotype was modeled using shC9 cells; these cells were resistant to lipoapoptosis by palmitate (PA) or lysophosphatidylcholine (LPC) despite lipid droplet formation. During lipid loading by either PA or LPC, shC9 cells activate JNK which via AP-1 induces SHH expression. An autocrine hedgehog survival signaling pathway was further delineated in both shC9 and caspase 3−/− cells during lipotoxic stress. Conclusion Ballooned hepatocytes in NASH downregulate caspase 9, a pivotal caspase executing the mitochondrial pathway of apoptosis. Hepatocytes engineered to reduce caspase 9 expression are resistant to lipoapoptosis, in part, due to a hedgehog autocrine survival signaling pathway.

Lipotoxic lethal and sublethal stress signaling in hepatocytes: relevance to NASH pathogenesis

“…Short of causing cell death, lipid overload leads to ballooned hepatocytes (29), which do not progress to cell death (30). We will also discuss ways in which these metabolically stressed cells induce distinct signaling pathways, such as sonic hedgehog (30,31) and release of proinflammatory EVs (27,32,33), that contribute to lipotoxic damage.…”

“…These cells exist in "undead" state and secrete various factors, including sonic hedgehog, to promote tissue remodeling. On this basis, Kakisaka et al (30) suggested that ballooned hepatocytes have a cellular phenotype similar to undead cells and modeled undead ballooned hepatocytes in vitro by treating caspase 9-deficient liver-derived cell lines with palmitate. Caspase 9-deficient cells were not only protected against palmitate-and LPCinduced lipoapoptosis, but also displayed increased expression of sonic hedgehog upon palmitate and LPC treatment in a JNK-dependent manner.…”

Mitogen-activated protein kinases and Hedgehog-GLI signaling in cancer: A crosstalk providing therapeutic opportunities?

“…Moreover, caspase 9 deprived cells are more resistant to the lipotoxic effect of fatty acids. These results may suggest how in nonalcoholic steatohepatitis, a condition that might predispose to HCC, ballooned hepatocytes that express lower level of caspase 9 compared to neighboring normal cells may escape cell death [140].…”

Reduced lipoapoptosis, hedgehog pathway activation and fibrosis in caspase-2 deficient mice with non-alcoholic steatohepatitis

“…Indeed, apoptosis is believed to be crucial in the pathogenesis of NASH-related cirrhosis. Special attention has been given to effector caspase-3 and caspase-7,12 and more recently, to initiator caspase-8 and caspase-9 13 14. A prevailing concept is that injured hepatocytes behave as undead cells, initiating the apoptotic process but failing to complete it, thereby providing a sustained source of apoptosis-associated molecular signals, including hedgehog, that drive liver inflammation and fibrosis 14–16…”