Regulation of CTP:phosphocholine cytidylyltransferase activity and subcellular location by phosphorylation in Chinese hamster ovary cells. The effect of phospholipase C treatment

The phosphorylation state of cytidylyltransferase in Chinese hamster ovary (CHO) cells was correlated with its subcellular distribution and activity in vivo. Western blot analysis of soluble and particulate fractions from control and phospholipase C-treated cells revealed slower migrating forms of cytidylyltransferase present only in the soluble fraction of control cells. These were abolished by incubating the soluble fraction at 37 degrees C in the presence of 5 mM Mg2+ but persisted if 135 mM NaF was present in the incubation. CHO cells were labeled with 32Pi, and cytidylyltransferase was immunoprecipitated from soluble and particulate fractions from control and phospholipase C-treated cells. The slower migrating forms of cytidylyltransferase, present in the soluble fraction of control cells, were phosphorylated at multiple sites. Although an equivalent amount of cytidylyltransferase was immunoprecipitated from the particulate fraction of phospholipase C-treated cells, it contained little 32P. Pretreatment of the CHO cells with okadaic acid, an inhibitor of type 1 and 2A phosphatases, prevented the stimulation of cytidylyltransferase in vivo by phospholipase C. These results demonstrate that dephosphorylation of soluble cytidylyltransferase is required for the phospholipase C-mediated translocation of cytidylyltransferase in CHO cells.

The role of protein kinase C in the stimulation of phosphatidylcholine synthesis by phospholipase C

The role of protein kinase C in the stimulation of phosphatidylcholine (PC) synthesis by phospholipase C was investigated. Phospholipase C treatment of Chinese hamster ovary cells (CHO) generates diacylglycerol, which is an activator of protein kinase C. The protein kinase C activator, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) stimulated choline incorporation into two CHO cell lines, a wild-type cell line, WTB, and a mutant cell line, DTG 1-5-4. DTG 1-5-4 is a mutant defective in receptor-mediated endocytosis. A 3-h phospholipase C treatment resulted in the activation and translocation of CTP:phosphocholine cytidylyltransferase in both cell lines. TPA treatment, however, resulted in only a slight (20%) translocation of cytidylyltransferase in WTB; no detectable translocation of cytidylyltransferase was observed in DTG 1-5-4. A decrease in the phosphocholine pools was observed in response to TPA treatment in both cell lines, which indicated that the cytidylyltransferase step was being activated. Phospholipase C stimulated choline incorporation into PC even when protein kinase C had been down-regulated in both cell lines. It was concluded that phospholipase C does not activate PC synthesis by activating protein kinase C.

Regulation of eukaryotic phospholipid metabolism

Phospholipids have diverse and critical roles in cellular metabolism and function. Questions about the mechanisms of regulation of phospholipid synthesis are being investigated with a variety of systems and approaches. For example, the yeast Saccharomyces cerevisiae is an organism in which both biochemical and genetic analyses are used. Biochemical approaches have yielded considerable information on the regulatory properties of enzymes of phospholipid biosynthesis. Studies of the activity of purified phosphatidylserine synthase have suggested how that enzyme is influenced by membrane phospholipids in the cell. The enzyme that regulates mammalian phosphatidylcholine biosynthesis, CTP:phosphocholine cytidylyltransferase, is also influenced by phospholipids. In addition, the activity of this enzyme often correlates with its translocation to membranes. The location of such enzymes in the cell is of particular interest in light of the possibility that the enzymatic reactions may be efficiently coupled in vivo. Techniques to render cultured cells permeable to phosphorylated molecules indicated that the enzymes of phosphatidylcholine biosynthesis may exist in an organized compartment so that the precursors of phosphatidylcholine are efficiently channeled through the pathway. To ask how phospholipids are transported in the cell, a combined biochemical and genetic approach has been used. These studies have revealed that the phosphatidylinositol/phosphatidylcholine transfer protein, considered to mediate intracellular phospholipid transfer, is a critical component of the secretory pathway for proteins. These results have allowed formulation of a number of new questions on the regulation of phospholipid metabolism and its relationship to general membrane processes.