Keratin impact on PKCδ/ASMase regulation of hepatocyte lipid raft size: Implication in FasR-associated apoptosis

Abstract

Keratins are epithelial cell intermediate filament (IF) proteins that are expressed as pairs in a cell-differentiation-regulated manner. Hepatocytes express the keratin 8 and 18 pair (denoted K8/K18) of IFs, and a loss of K8 or K18, as in K8-null mice, leads to degradation of the keratin partner. We have previously reported that a K8/K18 loss in hepatocytes leads to altered cell surface lipid raft distribution and more efficient Fas receptor (FasR, also known as TNFRSF6)-mediated apoptosis. We demonstrate here that the absence of K8 or transgenic expression of the K8 G62C mutant in mouse hepatocytes reduces lipid raft size. Mechanistically, we find that the lipid raft size is dependent on acid sphingomyelinase (ASMase, also known as SMPD1) enzyme activity, which is reduced in absence of K8/K18. Notably, the reduction of ASMase activity appears to be caused by a less efficient redistribution of surface membrane PKCδ toward lysosomes. Moreover, we delineate the lipid raft volume range that is required for an optimal FasR-mediated apoptosis. Hence, K8/ K18-dependent PKCδ-and ASMase-mediated modulation of lipid raft size can explain the more prominent FasR-mediated signaling resulting from K8/K18 loss. The fine-tuning of ASMase-mediated regulation of lipid rafts might provide a therapeutic target for deathreceptor-related liver diseases.

Keratin 8/18 regulation of insulin receptor signaling and trafficking in hepatocytes through a concerted phosphoinositide‐dependent Akt and Rab5 modulation

“…This partition uncovers the essential role that lipid rafts play in enabling efficient PI3K/Akt signaling (47). Moreover, we have shown recently that loss of K8/K18 IFs in hepatocytes affects lipid raft size and organization (33, 36). On this basis, we compared the impact of lipid raft integrity on IR‐stimulated signaling in WT vs .…”

“…Lipid rafts are structurally defined as dynamic membrane microdomains that are enriched in cholesterol, sphingolipids, and PIPs (57). Dynamically, small rafts coalesce into domains of various sizes via the accumulation of ceramide derived from the hydrolysis of sphingomyelin, the substrate of the acid sphingomyelinase (ASMase) (36). Functionally, accumulating evidence reveals that lipid rafts play important roles in signal transduction by providing platforms at the surface membrane (57).…”

“…In light of previous work by us and others, it seems that 3 somehow‐related schemes should be considered. First, despite the fact that PIPs, such as PIP 2 (PI3K substrate), are parts of the lipid raft platform (57), our previous work in the context of FasR signaling has revealed K8/K18 IF regulation of lipid raft size in hepatocytes via modulation of the PKC/ASMase pathway that is involved in the generation of ceramide, a key regulator of raft size (36). Concerning the upstream link between K8/K18 IFs and PKC, our previous report on the regulation of integrin/PKC‐mediated adhesion/ migration in hepatic cells revealed plectin and RACK1 involvement as intermediate scaffolds (40).…”

“…We hypothesize that a loss of K8/K18 IFs perturbs the lipid raft/PIP 2 dynamics at the surface membrane, which, in turn, affects IR downstream signaling and endosomal trafficking end points. To address this, we used cultured wild‐type (WT) and K8‐null mouse hepatocytes and a combination of biochemical and cell imaging methods, as previously described (36, 37). Overall, our results provide the first demonstration, to our knowledge, of a K8/K18 IF impact on IR signaling and trafficking events mediated by the concerted action of PIP 2 ‐dependent IRS1/PI3K/Akt signaling and Rab5/EEA1/PI3P trafficking in hepatocytes.…”

The role of lipid raft translocation of prohibitin in regulation of Akt and Raf‐protected apoptosis of HaCaT cells upon ultraviolet B irradiation

“…Lipid rafts play a role in regulation of UVB‐induced apoptosis of skin cells . By changing major components upon stimuli, lipid rafts regulate apoptotic signaling pathways by recruiting as well as inducing aggregation, and activation of lipid raft‐associated proteins, including but not limited to membrane receptors (such as Fas, CD5, and CD20), kinases (such as Akt, JNK, Src, and PKC), and calcium channels . Our recent report indicated that PHB translocates onto lipid rafts upon UVB irradiation .…”