Nongenomic regulation of lipid synthesis by the
Examination of the association of c-Fos with enzymes of the pathway of phospholipid synthesis by fluorescence resonance energy transfer microscopy
FRET measures molecular proximity of less than 10 nm, a range of distance that is typical of protein-protein interactions. In the micrographs, a pseudo colored cell in the upper row shows the association of co-expressed YFP-tagged c-Fos (c-Fos) with the CFP-tagged enzyme CDS1 (CDS1) at the perinuclear endoplasmic reticulum evidenced by FRET (red) between the fluorophores. The lower row shows a cell co-expressing the CFP-tagged enzyme PIS1 (PIS1) and YFP-tagged c-Fos. Note the absence of positive FRET values (blue) and hence the lack of association between c-Fos and PIS1.
Lipids — phospholipids, glycolipids, cholesterol and so forth — are the most abundant molecular species of every cell membrane. Consequently, it is expected that their synthesis be synchronized with the cell’s diverse functional states.
In cells actively involved in proliferation or in plasma-membrane extension processes that demand massive membrane biogenesis, lipid biosynthesis rates must be higher than those rates in cells that are neither dividing nor actively growing. However, the nature of the regulatory events underlying such processes is poorly understood. We have shown that the protein c-Fos is actively involved in these regulatory events.
c-Fos was first described more than 15 years ago as a member of the AP-1 family of inducible transcription factors (1). c-Fos content is highly regulated in cells: It is at the limit of detection in quiescent cells, whereas its expression is induced rapidly and only transiently when cells are stimulated to re-enter the cell cycle (2).
It has been hypothesized that this AP-1 activity of c-Fos (forming dimers with other members of the AP-1 family of transcription factors) participates in transmitting short-term, growth-promoting cellular signals into longer lasting changes by regulating the expression of cell-growth-related genes (2).
We have established that c-Fos is a moonlighting protein capable of regulating growth not only by its transcription-factor activity but also by its capacity to act as a cytoplasmic activator of the biosynthesis of lipids in normal and pathological cellular processes that demand high rates of membrane biogenesis. Such is the case in light-stimulated retina ganglion cells (3, 4), in growing NIH 3T3 cells (5), in PC12 cells induced to differentiate (6, 7) and in tumors of the central and peripheral nervous system (8, 9). Highlighting the importance of lipid-synthesis activation for these events is the observation that by specifically blocking c-Fos expression, or in c-Fos−∕− mice, proliferation and growth of normal and tumor cells are slowed or halted without substantial changes in their AP-1 content (8).
Lipid synthesis is also regulated by the total amount of active c-Fos present in cells. Quiescent cells contain very low amounts of c-Fos, which is Tyr-phosphorylated, is not membrane bound and shows basal levels of phospholipid synthesis (10). However, inducing cells to grow promotes abundant c-Fos expression, and c-Fos is dephosphorylated by the phosphatase TC-PTP, enabling it to associate to the ER and to activate phospholipid synthesis (11).
Co-immunoprecipitation and fluorescence resonance energy transfer assays showed that lipid-synthesis activation by c-Fos is accomplished through a physical association between the N-terminal domain of c-Fos and the enzymes it modulates (12). However, an increase in the reaction rate is promoted through the basic 20-amino acid domain of c-Fos that spans from amino acid 139 to 159 (12). Only one other protein, Fra-1, was found that contains a c-Fos-homologous basic domain, and interestingly, when expressed, it also activates lipid synthesis in human breast tumors (13).
In light of the importance of c-Fos-activated lipid synthesis for normal and pathological cell growth and proliferation, we are studying which lipid synthesis pathways c-Fos regulates and the enzymes involved. Perhaps we will learn how to limit the unrestricted proliferation and growth of tumor cells.
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Beatriz L. Caputto (firstname.lastname@example.org) is a full professor at the department of biological chemistry (CIQUIBIC-CONICET) at the School of Chemical Sciences of the National University of Córdoba, Argentina.