Different mammalian cell lines treated with a high concentration of serum exhibit a circadian expression of genes whose transcription also oscillates in living animals (Balsalobre A. et al, 1998). Thus, cell cultures constitute an excellent model to study circadian regulation of cellular metabolisms. We looked at the incorporation of 32P into phospholipids of NIH 3T3 fibroblast at different phase. Cells were grown in Dulbecco?s modified Eagle medium (DMEM) + 5 % fetal calf serum confluence. At time 0, the medium was changed to 50% horse serum (GIBCO)-rich medium for 2 h, replaced with serum-free DMEM and maintained under this situation for several days. A 30 min-labeling pulse with 32P was given at the different phases raining from 0:30 to 60 h after the serum shock. Interestingly, there was a rhythm of 32P ? phospholipids labeling with levels peaking 5 h after the serum shock and the lowest levels of leveling 29 h after treatment. This rhythm was observed at least for two cycles with a exhibits a daily oscillation with high levels during the first 30 min, which decreased rapidly by 1.5 h and reaches the highest expression 26-29 h later. When per1 expression was knocked down by an specific antisense oligonucleotide, the rhythmicity observed in the 32P-phospholipid labeling was lost. Our findings support the idea that cultured fibroblast oscillates in the biosynthesis of their phospholipids which is driven by an endogenous clock involving per1 expression.
