Aging in rat causes hepatic hyperresposiveness to interleukin-1β which is mediated by neutral sphingomyelinase-2

Abstract

The process of aging has recently been shown to substantially affect the ability of cells to respond to inflammatory challenges. We demonstrate that aging leads to hepatic hyperresponsiveness to interleukin 1␤ (IL-1␤), and we examine the factors that could be responsible for this phenomenon. IL-1␤-induced phosphorylation of c-jun N-terminal kinase (JNK) in hepatocytes isolated from aged rats was 3 times more potent than that in hepatocytes from young rats. Moreover, JNK was activated by substantially lower doses of IL-1␤. These age-related changes in JNK phosphorylation correlated with diminished IL-1␤-induced degradation of interleukin-1 receptor-associated kinase-1 (IRAK-1). Expression levels of IL1␤ receptor I, total JNK, IRAK-1, and transforming growth factor-␤-activated kinase-1 (TAK-1) were not affected by aging. However, increased neutral sphingomyelinase activity was observed in hepatocytes from old animals, which we show is caused by induction of the plasma membrane localized neutral sphingomyelinase-2 (NSMase-2). We provide evidence that NSMase-2 is both required and sufficient for the onset of IL-1␤ hyperresponsiveness during aging. Overexpression of NSMase-2 in hepatocytes from young rats leads both to a reduction in IRAK-1 degradation and potentiation of JNK phosphorylation, mimicking that seen in hepatocytes from old animals. More importantly, inhibition of NSMase activity in hepatocytes from aged rats using either scyphostatin or short interfering ribonucleic acid (siRNA) leads to reversion to the "young" phenotype of IL-1␤ response. Conclusion: These results show that the process of aging causes increased basal NSMase-2 activity in hepatocytes, which in turn leads to IRAK-1 stabilization, JNK potentiation, and ultimately IL-1␤ hyperresponsiveness. (HEPATOLOGY 2007;46:1166-1176

Interleukin 1β Regulation of FoxO1 Protein Content and Localization

“…7E). This outcome is in agreement with the role nSMase-2 has in the IL-1␤ signaling cascade, which was described in detail in our earlier studies (8,9,20).…”

“…7E). We found that the overexpression of nSMase-2, which elevates the hepatocyte ceramide content by 30% and augments JNK activation (20), conversely increased the magnitude of IL-1␤-induced stimulation of FoxO1 (Fig. 7E).…”

“…In previous studies we identified nSMase-2 as being IL-1␤-inducible signaling sphingomyelinase, responsible for the transient accumulation of ceramide and the activation of JNK activation following IL-1␤ stimulation (9,19,20). The role of nSMase-2 and ceramide in JNK activation was shown to be intricate.…”

Regulation of insulin-like growth factor binding protein-1 expression during aging

“…JNK phosphorylation in response to IL-1β was found substantially (2-3-fold) higher in hepatocytes isolated from aged rats, as compared to hepatocytes from young animals. Total JNK levels, as well as the levels of the IL-1 receptor type I were not affected by aging and were similar in young and aged hepatocytes, as reported previously [19].…”

“…No differences in total JNK levels were observed after IL-1β treatment ( Fig. 2A), as reported previously [19]. To confirm that phosphorylation results in increased activity, JNK activity was measured in vitro using GST-c-Jun as a substrate (Fig.…”