Growth factor activation of protein kinase C-dependent and -independent pathways of protein phosphorylation in fibroblasts: relevance to activation of protein kinase C in neuronal tissues

Transduction of the bradykinin response in human fibroblasts: prolonged elevation of diacylglycerol level and its correlation with protein kinase C activation

Stimulation of quiescent human fibroblasts with the peptide mitogen bradykinin (BK) led to a biphasic elevation in cellular 1,2-diacylglycerol (DAG), as estimated by either measurement of total DAG mass or [3H]arachidonate incorporation. A rapid initial transient that peaked 15 s after BK addition was followed by a decline to near basal levels then a second rise to a plateau phase during which DAG levels remained elevated for less than or equal to 45 min. The source of the initial DAG transient appeared to be primarily polyphosphoinositides as these phospholipids were rapidly hydrolyzed after BK addition. This transient correlates well temporally with previous observations of the kinetics of inositol trisphosphate accumulation and intracellular free [Ca2+] observed in the same cells. Cultures preincubated with [3H]myristic acid incorporated label predominantly into the phosphatidylcholine (PC) pool. Subsequent addition of BK under these conditions caused only a relatively slow accumulation of [3H]DAG to a plateau level, without an initial transient. Together with the observation that PC was found to decrease upon BK stimulation, these observations suggest that the late phase of DAG accumulation may involve breakdown of other phospholipids including PC. To investigate the consequences of DAG elevation we examined the phosphorylation of an acidic 80 kDa protein, whose phosphorylation is solely dependent on the activation of protein kinase C (PK-C). The 80 kDa fibroblast protein could be immunoprecipitated by an antibody to bovine brain "myristoylated and alanine-rich C-kinase substrate" (MARCKS) and phosphopeptide maps of brain and fibroblast MARCKS were similar. Stimulation of [32P]-prelabeled fibroblasts with serum, BK, vasopressin, or 12-O-tetradecanoyl phorbol acetate, but not epidermal growth factor or calcium ionophores, resulted in the rapid phosphorylation of MARCKS. With BK or serum this phosphorylation showed an initial transient peak at less than 1 min then rose again to a plateau level that was sustained for less than or equal to 45 min. Removal of BK resulted in a rapid decline in MARCKS phosphorylation. These studies show that the biphasic DAG signal in BK-stimulated human fibroblasts correlates well with the state of activation of PK-C. However, the persistent activation of PK-C does not appear to require continued high levels of Ca2+.

Regional variations in protein phosphorylating activity in rat brain studied in micro-slices labeled with [32P]phosphate

Regional variations in protein phosphorylating activity in the rat brain were studied. Micro-slices (1 mm diameter) were prepared from 19 brain areas, phosphoproteins labeled by incubation with [32P]phosphate, and the tissue analyzed by nonequilibrium two-dimensional electrophoresis and autoradiography. Attention was focused on three phosphorylating systems that showed consistent variation in activity. (1) A system that phosphorylates a substrate of 47 kDa (ppH-47) whose activity was highest in the hippocampus. The next highest activity of this system was observed in the globus pallidus, followed by the periventricular gray matter of the aqueduct, lateral septum, cerebellar cortex, entorhinal cortex, hypothalamus, mammillary nuclei, amygdala, and substantia nigra. Activity was low or undetectable in the cerebral cortex, neostriatum, and the colliculi. (2) A system that phosphorylates a substrate of 50 kDa (ppC-50) whose activity was highest in the caudate nucleus. The activity of this system was roughly inversely correlated with that of the ppH-47 system. (3) The protein kinase C system that phosphorylates an 82- to 87-kDa substrate known as MARCKS. The highest activity of this system was observed in the cerebellar cortex, followed by the hypothalamus, mammillary nuclei, periventricular gray matter of the aqueduct, and the superior colliculus. Activity of this system was relatively low in several regions of the cerebral cortex, the neostriatum, and the inferior colliculus.

Insulin and growth factor effects on c-fos expression in normal and protein kinase C-deficient 3T3-L1 fibroblasts and adipocytes

We investigated the expression of the protooncogene c-fos in 3T3-L1 fibroblasts and adipocytes in response to a variety of growth-promoting agents in normal cells and in cells preincubated with phorbol esters to deplete them of protein kinase C. There was a rapid accumulation of c-fos mRNA in fibroblasts and adipocytes treated with phorbol 12-myristate 13-acetate, platelet-derived growth factor, fibroblast growth factor, fetal calf serum, bombesin, and insulin, especially in the adipocytes. Phorbol 12-myristate 13-acetate pretreatment abolished the increase in c-fos mRNA due to additional phorbol 12-myristate 13-acetate treatment and decreased but did not eliminate the ability of platelet-derived growth factor, fibroblast growth factor, fetal calf serum, bombesin, and insulin to stimulate c-fos mRNA. These data suggested that c-fos mRNA could be induced in serum-deprived 3T3-L1 fibroblasts and adipocytes by at least two separate pathways, one involving protein kinase C and the other independent of protein kinase C. In the very insulin-sensitive 3T3-L1 adipocytes, insulin rapidly and transiently increased c-fos expression (c-fos mRNA appeared by 15 min and disappeared after 60 min) via interaction with its own cellular receptor, rather than by interacting with receptors for one of the insulin-like growth factors. Cycloheximide treatment in combination with insulin or phorbol 12-myristate 13-acetate resulted in superinduction of c-fos mRNA. We conclude that insulin can rapidly stimulate c-fos mRNA accumulation in 3T3-L1 adipocytes and that part of the growth factor-stimulated increase in this mRNA that occurs in protein kinase C-deficient cells may be due to activation of a pathway similar or identical to that activated by insulin.