Todorčević et al. BMC Genomics 2010, 11:39 http://www.biomedcentral.com/1471-2164/11/39 increased number and size of LDs meat the limited ER capacity. Although ER is mainly considered to be a pro- tein-folding factory, all proposed models of LD forma- tion emphasize the engagement of the ER compartment in the process [61]. UPR utilises two broad strategies to relieve stress in the ER: increased clearance of misfolded proteins from the ER and reduction of new protein influx into the ER. We found ample evidence for the activation of the 26S proteasome and autophagy-lysosomal proteolytic path- ways, which degrade misfolded proteins. Components of the membrane protein complex, derlin 1 and p97-inter- acting cofactor p47, which mediate transport from the ER lumen into the cytosol were induced at day 30, while other proteolytic pathways were up-regulated even ear- lier. UPR-induced proteolysis may be employed to initi- ate the breakdown of superfluous proteins and organelles that must be replaced by more highly specia- lised cellular components during the adipocyte develop- ment. Without a doubt, considerable remodelling of the cellular architecture occurs during terminal differentia- tion of Atlantic salmon adipocytes. UPR also activates genes involved in protein folding. Hsp40s and other J-domain-containing proteins act as co-chaperones for Hsp70s helping to restore homeosta- sis in the ER and in addition may have pro-degradation roles. Down-regulation of translational machinery is commonly observed during UPR [35], and in our study, it involved a dramatic reduction in expression of riboso- mal proteins. Alteration of the ribosomal composition is most cer- tainly involved in the multiple regulation of the ER- stress response. The curious up-regulation of the 40S subunit 18S rRNA through most of the terminal differ- entiation against the majority of down-regulated riboso- mal proteins is bound to have important consequences given the high level of 18S constitutive expression. This could signify a switch to a less efficient protein transla- tion as increase in 18S negatively affects the ribosomal subunit ratio that favours protein production [62]. On the other side, reduced expression of 18S at the very end of differentiation could improve the effectiveness of protein synthesis in adipocytes that acquire secretory phenotype. In conclusion, fully matured adipocytes seem to reprogram the pattern of gene expression to sustain a certain level of protein production, congruent with the now recognised endocrine functions of WAT. Secreted adipsin and visfatin have multiple immune functions as most other adipokines [63]. Overall, expres- sion changes of a large number of immune genes were characteristic of Atlantic salmon aSVF culture. Dissimi- lar temporary profiles suggest different regulatory and physiological roles of these genes. Additionaly, high initial levels and subsequent decrease of the chemokine Page 12 of 17 receptors, TNFa and a panel of TNF-related genes could be explained by the changes in relative abundance of non-adipogenic cells that possess pronounced immune properties. In mammals, TNFa has high anti- adipogenic activity [64], which was recently shown to be conserved in fish [65]. Therefore, down-regulation of the TNF axis at days 7-9 was probably important for the onset of adipocyte differentiation. However, a number of immune genes showed either stable increase since day 4 (complement component C5) or biphasic regulation with the second activation after hormone induction (LECT2). Highest expression levels of many genes with diverse roles in different inflammatory pathways were observed during terminal adipocyte differentiation. Numerous independent studies have provided evidence that a set of inflammatory and stress-response genes is activated in obesity in mammals (reviewed in [66]). Some of these genes likely have important roles in the coordination of homeostasis in WAT, similar to the established roles of TNFa in modulating proliferative abilities in preadipocytes or metabolic activities in mature adipocytes. An intriguing example is a group of virus responsive genes including TLR3, a receptor of double stranded RNA and a group of galectins, previously not reported to have roles in any aspect of adipocyte biology. The lat- ter includes galectin 9, which showed highly specific responses to viruses in previous studies from our group [67,68]. However, a large portion of immune genes, including complement component C5, JAK1, the key actor of interferon signaling and FLAP, the regulator of eicosanoid metabolism, decreased expression by the time of AP1 establishment. Finally, our results do not permit for major conclusions as to whether immune genes are important for adipogenesis or change expres- sion as a consequence of differentiation and/or stress responses. However, it is clear that Atlantic salmon WAT possesses high potential for immune activity. It is tempting to explain the activation of immune genes in differentiating adipocytes as a side effect of adaptation to the oxidative and ER stress, since stress and immune responses share common regulatory path- ways. Ozcan et al. [56] showed that ER-stress-induced JNK-AP1 axis is the central link between TAG overload in liver and diabetes. Most interestingly, this is in agree- ment with our study, which shows that the same players are activated in fish adipocytes in response to increased lipid deposition. Continuous administration of lipids led to the maximal observed TAG loading at the end of the studied period, when the collective and highest expres- sion of the AP1 complex components, including all c- Jun and JunB members was also observed. This addi- tionally coincided with the induction of POH1, a regula- tory subunit of the 26S proteasome that has a specific