The diacylglycerol and protein kinase C pathways are not involved in insulin signalling in primary rat hepatocytes

Maintaining hepatocyte differentiation in vitro through co-culture with hepatic stellate cells

Primary hepatocytes lose their differentiated functions rapidly when in culture. Our aim was to maintain the differentiated status of hepatocytes in vitro by means of vital hepatic stellate cells (HSCs), their soluble and particulate factors and lipid extracts. Hepatocytes were placed into collagen-coated culture dishes in the presence of HSCs at different ages of pre-culture, with or without direct cell to cell contacts, at different cell ratios and in monoculture with cellular HSC components in place of vital cells. Changes in morphology and enhancement of phosphoenolpyruvate carboxykinase (PCK) activity by glucagon were used to determine the differentiated status of hepatocytes in 2d-short-term culture. HSCs proved able to maintain the differentiated function of hepatocytes in co-culture either by direct cell contacts or via factors derived from HSC-conditioned medium. In comparison, however, without cellular contact to hepatocytes five to ten times as many HSCs were necessary to increase the PCK activity to the same degree as in the presence of intercellular contacts. Whereas stimulation in the presence of HSC/hepatocyte contacts was independent of HSC culture age only quiescent, resting HSCs (precultured for 1-2 d) were able to stimulate hepatocytes significantly via soluble factors. Culturing of hepatocytes with a lipid extract or a particulate fraction from HSCs clearly displayed a very strong beneficial effect on enzyme activity and morphology. HSCs maintain hepatocyte function and structure through preferentially cell-bound signalling and transfer of lipids.

The ε-Isoform of PKC Mediates the Hypertonic Activation of Cation Channels in Confluent Monolayers of Rat Hepatocytes

We were interested whether PKC alpha, delta, epsilon or zeta is the isoform actually employed in the activation of hypertonicity-induced cation channels (HICCs) in primary cultures of rat hepatocytes. Quantitative SDS-page and Western-blot experiments revealed that PKC alpha, delta and epsilon were stimulated by Indolactam V (as a DAG substitute for activation of c and nPKCs) but that only PKC delta and epsilon did respond to hypertonic stress. Furthermore, chelation of intracellular Ca(++) by BAPTA-AM did not alter HICC activation in cable-analysis experiments whereas Indolactam V as well as V8 (an Indolactam derivative specific for PKC delta and epsilon) activated HICC currents under isotonic conditions. Finally, by use of Rottlerin (as an inhibitor exhibiting a slight preference for PKC delta over epsilon) PKC epsilon could be identified as the most likely isoform responsible for the activation of the HICC.

Specific protein kinase C isoforms as transducers and modulators of insulin signaling

Recent studies implicate specific PKC isoforms in the insulin-signaling cascade. Insulin activates PKCs alpha, betaII, delta and zeta in several cell types. In addition, as will be documented in this review, certain members of the PKC family may also be activated and act upstream of PI3 and MAP kinases. Each of these isoforms has been shown one way or another either to mimic or to modify insulin-stimulated effects in one or all of the insulin-responsive tissues. Moreover, each of the isoforms has been shown to be activated by insulin stimulation or conditions important for effective insulin stimulation. Studies attempting to demonstrate a definitive role for any of the isoforms have been performed on different cells, ranging from appropriate model systems for skeletal muscle, liver and fat, such as primary cultures, and cell lines and even in vivo studies, including transgenic mice with selective deletion of specific PKC isoforms. In addition, studies have been done on certain expression systems such as CHO or HEK293 cells, which are far removed from the tissues themselves and serve mainly as vessels for potential protein-protein interactions. Thus, a clear picture for many of the isoforms remains elusive in spite of over two decades of intensive research. The recent intrusion of transgenic and precise molecular biology technologies into the research armamentarium has opened a wide range of additional possibilities for direct involvement of individual isoforms in the insulin signaling cascade. As we hope to discuss within the context of this review, whereas many of the long sought-after answers to specific questions are not yet clear, major advances have been made in our understanding of precise roles for individual PKC isoforms in mediation of insulin effects. In this review, in which we shall focus our attention on isoforms in the conventional and novel categories, a clear case will be made to show that these isoforms are not only expressed but are importantly involved in regulation of insulin metabolic effects.

Glucose and fatty acid metabolism in McA-RH7777 hepatoma cells vs. rat primary hepatocytes: responsiveness to nutrient availability

The overabundance of dietary fats and simple carbohydrates contributes significantly to obesity and metabolic disorders associated with obesity. The liver balances glucose and lipid distribution, and disruption of this balance plays a key role in these metabolic syndromes. We investigated (1) how hepatocytes balance glucose and fatty acid metabolism when one or both nutrients are supplied in abundance and (2) whether rat hepatoma cells (McA-RH7777) reflect nutrient partitioning in a similar manner as compared with primary hepatocytes. Increasing media palmitate concentration increased fatty acid uptake, triglyceride synthesis and beta-oxidation. However, hepatoma cells had a 2-fold higher fatty acid uptake and a 2-fold lower fatty acid oxidation as compared with primary hepatocytes. McA-RH7777 cells did not synthesize significant amounts of glycogen and preferentially metabolized the glucose into lipids or into oxidation. In primary hepatocytes, the glucose was mostly spared from oxidation and instead partitioned into both de novo glycogen and lipid synthesis. Overall, lipid production was rapidly induced in response to either glucose or fatty acid excess and this may be one of the earliest indicators of metabolic syndrome development associated with nutrient excess.