Role of PKD1 in the control of liver endoplasmic reticulum stress and insulin signaling responses during nonalcoholic fatty liver disease progression

Abstract

Protein kinase D1 (PKD1) is a ubiquitous Ser/Thr kinase belonging to the CAMK family. It is increasingly implicated in the regulation of fundamental biological processes such as apoptosis, cell proliferation, trafficking and oxidative stress. It has been previously reported that PKD1 plays a role in different tissues including immune cells, cardiac myocytes and pancreas. However, its role in liver metabolism remains unclear. To this end, a mouse model that lacks PKD1 in hepatocytes was generated by using the Cre-loxP system (PKD1ΔHep and PKD1fl/fl as control mice). Primary hepatocytes isolated from PKD1ΔHep mice showed higher levels of ER stress markers and signature of a blockade of autophagic flux in parallel with an increase in PKD2 phosphorylation. Interestingly, in palmitic-acid stimulated hepatocytes lacking PKD1 further enhanced PKD2 phosphorylation was detected above basal in addition to an increased and sustained molecular signature of ER stress, blockade of autophagy flux and lipoapoptosis. Importantly, PKD1-deficient primary hepatocytes showed a reduced sensitivity upon insulin stimulation. Since obesity induced by high fat diet (HFD) has been shown to induce ER stress response and insulin resistance, mice from both strains were fed a HFD for 20 weeks. After this period, PKD1ΔHep mice exhibited higher body weight gain compared to PKD1fl/fl mice. Moreover, pyruvate and insulin tolerance tests revealed that HFD-fed PKD1ΔHep mice presented increased insulin resistance than control mice. These results were also confirmed by decreased phosphorylation levels of IR and AKT. Likewise, the histological analysis of liver sections showed a more elevated NAS score than comprises steatosis, inflammation and ballooning in the PKD1ΔHep group, suggesting that PKD1 deficiency worsens the histological course of non-alcoholic fatty liver disease (NAFLD). In addition, electronic microscopy images revealed that livers from PKD1ΔHep mice presented a pronounced dilation of the ER lumen compared to PKD1fl/fl control mice.