Aging reduces liver resiliency by dysregulating Hedgehog signaling

Older age is a major risk factor for damage to many tissues, including liver. Aging undermines resiliency (i.e., the ability to recover from injury) and impairs liver regeneration. The mechanisms whereby aging reduces resiliency are poorly understood. Hedgehog is a signaling pathway with critical mitogenic and morphogenic functions during development. Recent studies indicate that Hedgehog regulates metabolic homeostasis in adult liver. The present study evaluates the hypothesis that Hedgehog signaling becomes dysregulated in hepatocytes during aging, resulting in decreased resiliency and therefore, impaired regeneration and enhanced vulnerability to damage. Methods: Partial hepatectomy (PH) was performed on young and old wild type mice and Smoothened (Smo)-floxed mice treated with AAV8-TBG luciferase (control) or AAV8-TBG-Cre vectors to conditionally delete Smo and disrupt Hedgehog signaling specifically in hepatocytes. Changes in signaling were correlated with changes in regenerative responses and compared among groups. Results: Old livers had fewer hepatocytes proliferating after PH. RNA sequencing identified Hedgehog as a top down-regulated pathway in old hepatocytes before and after the regenerative challenge. Deleting Smo in healthy young hepatocytes before PH prevented Hedgehog pathway activation after PH and inhibited regeneration. GO analysis demonstrated that both old and Smo-deleted young hepatocytes had activation of pathways involved in innate immune responses and suppression of several signaling pathways that control liver growth and metabolism including insulin-like growth factor, Wnt and NOTCH. Hedgehog inhibition promoted telomere shortening and mitochondrial dysfunction in hepatocytes, consequences of aging that promote inflammation and impair tissue growth and metabolic homeostasis. Conclusion: Hedgehog signaling is dysregulated in old hepatocytes. This accelerates aging, resulting in decreased resiliency and therefore, impaired liver regeneration and enhanced vulnerability to damage. Overall design: Global transcriptome profiling of was performed by RNA-seq using freshly isolated hepatocytes from young and old wild type mice or Smo-floxed mice before and at 48 h after PH (n=3wild type mice/group/time point), n=1 Smo-flox-Luci mice/time point and n=2 Smo-flox-Cre mice/time point).