Proadipogenic effect of leptin on rat preadipocytes in vitro: activation of MAPK and STAT3 signaling pathways

Abstract

Because leptin has recently been shown to induce proliferation and/or differentiation of different cell types through different pathways, the aim of the present study was to investigate, in vitro, the influence of leptin on adipogenesis in rat preadipocytes. A prerequisite to this study was to identify leptin receptors (Ob-Ra and Ob-Rb) in preadipocytes from femoral subcutaneous fat. We observed that expressions of Ob-Ra and Ob-Rb increase during adipogenesis. Furthermore, leptin induces an increase of p42/p44 mitogen-activated protein kinase phosphorylated isoforms in both confluent and differentiated preadipocytes and of STAT3 phosphorylation only in confluent preadipocytes. Moreover, exposure to leptin promoted activator protein-1 complex DNA binding activity in confluent preadipocytes. Finally, exposure of primary cultured preadipocytes from the subcutaneous area to leptin (10 nM) resulted in an increased proliferation ([3H]thymidine incorporation and cell counting) and differentiation (glycerol-3-phosphate dehydrogenase activity and mRNA levels of lipoprotein lipase, peroxisome proliferator-activated receptor-γ2, and c-fos). Altogether, these results indicate that, in vitro at least, leptin through its functional receptors exerts a proadipogenic action in subcutaneous preadipocytes.

Leptin Increases Lipoprotein Lipase Secretion by Macrophages: Involvement of Oxidative Stress and Protein Kinase C

“…Consistent with this hypothesis, the present study demonstrates that leptin stimulates the secretion of macrophage LPL in vitro. In accordance with one previous study using preadipocytes (40), we found that leptin enhances LPL gene expression in macrophages. Although we did not perform run-on assays, our data showing that induction of LPL mRNA levels by leptin parallels the increase in LPL synthesis and secretion suggest that macrophage LPL regulation in response to leptin takes place at the transcriptional level.…”

“…Evidence that the conventional PKC selective inhibitor, GF109203X, abrogates the effect of leptin on macrophage LPL expression further stresses the key role of the conventional PKC isoenzymes in the regulation of macrophage LPL by leptin. Previous studies have demonstrated that PKC is involved in AP-1 activation (46) and that leptin stimulates c-fos expression (40). On the basis of these results, it is tempting to postulate that the activation of PKC by leptin that we documented in the present study could result in the binding of its downstream target, c-fos, to the AP-1 sequence of the LPL gene promoter, thereby leading to an increased transcription rate of this gene.…”

“…This possibility is supported by our results showing that GF109203X inhibits the stimulatory effect of leptin on the binding of nuclear proteins to the AP-1 regulatory sequence of the LPL gene. Besides PKC and AP-1, leptin activates in mononuclear cells other signaling pathways, including PI3K and MAPK (47, 48), and increases the expression of peroxisome proliferator-activated receptors (PPARs) (40). Evidence that insulin, a well-known activator of PI3K and MAPK, does not enhance human macrophage LPL (F.M., G.R., unpublished observations) and that PI3K and MAPK inhibitors do not inhibit the stimulatory effect of leptin on macrophage LPL secretion do not argue for a role of these kinases in leptin-induced macrophage LPL expression.…”

Upregulation of proinflammatory and proangiogenic cytokines by leptin in human hepatic stellate cells

“…To correlate the biological activities of leptin with the expression of leptin receptors, we analyzed HSCs isolated from four different donors using antibodies recognizing all ObR isoforms. An approximately 190-kd band representing ObRb, as reported in other cell types or in stable HSC lines, [16][17][18] was found in all isolates (Fig. 3A), together with faster migrating bands that are likely due to ObRa and/or other "short" isoforms.…”

“…The Jak/Stat signaling pathway plays a critical role in mediating the effects of leptin, and Stat3 phosphorylation has been associated with ObRb activation in different cells, including HSCs. 17,18 To further establish the functionality of ObRb, we analyzed the effects of leptin on Stat3 activation (Fig. 3C).…”