PAS kinase drives lipogenesis through SREBP-1 maturation

Elevated hepatic synthesis of fatty acids and triglycerides, driven by hyperactivation of the SREBP-1c transcription factor, has been implicated as a causal feature of metabolic syndrome. SREBP-1c activation requires the proteolytic maturation of the endoplasmic-reticulum-bound precursor to the active, nuclear transcription factor, which is stimulated by feeding and insulin signaling. Here, we show that feeding and insulin stimulate the hepatic expression of PASK. We also demonstrate, using genetic and pharmacological approaches, that PASK is required for the proteolytic maturation of SREBP-1c in cultured cells and in the mouse and rat liver. Inhibition of PASK improves lipid and glucose metabolism in dietary animal models of obesity and dyslipidemia. Administration of a PASK inhibitor decreases hepatic expression of lipogenic SREBP-1c target genes, decreases serum triglycerides, and partially reverses insulin resistance. While the signaling network that controls SREBP-1c activation is complex, we propose that PASK is an important component with therapeutic potential.

Substitution of the LTR of Abelson murine leukemia virus does not alter the cell type of virally induced tumors

The long terminal repeat (LTR) of the pre-B cell tropic Abelson murine leukemia virus (A-MuLV) was replaced with the LTR of the erythrotropic Friend MuLV or with the LTR of the erythropic/fibrotropic Harvey murine sarcoma virus (Ha-MuSV) to generate the viruses F-ABL and H-ABL, respectively. The parental A-MuLV and the recombinant viruses induced pre-B cell lymphomas in susceptible mice with similar frequencies. Recombinant virus-induced tumor DNAs were analysed by nucleic acid hybridization and were shown to contain the appropriate recombinant provirus. F-ABL was 100-1000 fold less efficient than A-MuLV or H-ABL in the in vitro transformation of primary bone marrow cells, as detected by lymphoid colony formation in agarose. To compare the level of transcription initiated from the different viral LTRs, we investigated the ability of the U3 region of these retroviral LTRs to promote transcription in a battery of cell lines using the chloramphenicol acetyl-transferase (CAT) assay, and with some exceptions we found the following hierarchy of activities: Ha-MusSV greater than or equal to M-MuLV greater than A-MuLV greater than F-MuLV, regardless of the cell line transfected. These results indicate that the LTR is not a determinant of the pre-B cell disease specificity of A-MuLV, and suggest that this specificity resides in the v-abl oncogene. Also, our results suggest that a threshold amount of the v-abl protein product is necessary for in vitro transformation, and this level of expression may be different from the level selected during in vivo tumorigenesis.