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Lipin-1 is a multifunctional metabolic regulator, involving in triacylglycerol and bioactive glycerolipids synthesis as an enzyme, transcriptional regulation as a coactivator, and adipogenesis. In obesity, adipose lipin-1 expression is decreased. Although lipin-1 is implicated in the pathogenesis of obesity, the mechanism is still not clear. Since TNF-alpha is deeply involved in the pathogenesis of obesity, insulin resistance, and diabetes, here we investigated the role of TNF-alpha on lipin-1 expression in adipocytes. Quantitative PCR studies showed that TNF-alpha suppressed both lipin-1A and -1B isoform expression in time- and dose-dependent manners in mature 3T3-L1 adpocytes. A Jak2 inhibitor, AG490, reversed the suppressive effect of TNF-alpha on both lipin-1A and -1B. In contrast, NF-kappaB, MAPKs, ceramide, and beta-catenin pathway tested were not involved in the mechanism. These results suggest that TNF-alpha could be involved in obesity-induced lipin-1 suppression in adipocytes and Jak2 may play an important role in the mechanism.

Diacylglycerol kinase (DGK) is one of lipid-regulating enzymes, catalyzes phosphorylation of diacylglycerol to phosphatidic acid. Because skeletal muscle, a major insulin-target organ for glucose disposal, expresses DGK, we investigated in the present study a role of DGK on glucose transport in skeletal muscle cells. PCR study showed that C2C12 myotubes expressed DGKalpha, delta, epsilon, zeta, or theta isoform mRNA. R59022, a specific inhibitor of DGK, significantly increased glucose transport, p38 and MKK3/6 activation in C2C12 myotubes. The R59022-induced glucose transport was blocked by SB203580, a specific p38 inhibitor. In contrast, R59022 failed to stimulate both possible known mechanisms to enhance glucose transport, an IRS1-PI3K-Akt pathway, muscle contraction signaling or GLUT1 and 4 expression. All these results suggest that DGK may play a role in glucose transport in the skeletal muscle cells through modulating a MKK3/6-p38 signaling pathway.

The present study was performed to examine a role of adipose differentiation-related protein (ADRP) in the process of liver steatosis. Immunohistochemical findings indicated that ADRP expression is increased in the hepatocytes in patients with fatty liver when compared with normal liver. ADRP expression is localized in the surface of lipid droplets in the hepatocytes. Increased expression of ADRP mRNA and protein was similarly observed in fatty liver in ob/ob mice and the liver steatosis induced by high fat diet in mice. The up-regulation of ADRP mRNA and protein in the liver by high fat diet was identified in the surface of lipid droplets in a time-dependent manner. Recent studies demonstrated that up-regulation of PPARgamma in the hepatocytes is deeply involved in liver steatosis. To clarify whether ADRP expression is increased by PPARgamma activation in hepatocytes, we examined the effect of a PPARgamma ligand, troglitazone, on ADRP mRNA expression in HepG2 cells. ADRP mRNA expression was increased by troglitazone in dose- and time-dependent manners. All these results suggest that ADRP is up-regulated in liver steatosis in human and mice, and that high fat diet increases expression of ADRP through PPARgamma activation, followed by induction of liver steatosis.

In the present study, we examined a role of mitogen-activated protein kinases (MAPKs), extracellular signal-related kinase (ERK), c-Jun N-terminal protein kinase, and p38 MAPK in troglitazone-induced inhibition of cell growth in human pancreatic cancer cells. Among the three kinases, troglitazone specifically inhibited the phosphorylation of ERK1/2 in a dose- and time-dependent manner. Troglitazone also down-regulated the protein expression of mitogen-activated protein kinase kinase (MEK)1/2, an upstream molecule that regulates ERK phosphorylation. Treatment of human pancreatic cancer cells with specific MEK inhibitor, PD98059 or U0126, inhibited ERK1/2 phosphorylation and cell growth. These results suggest for the first time that the inhibition of the MEK1/2-ERK1/2 signaling pathway may be implicated in the growth inhibitory effect by troglitazone in human pancreatic cancer cells.

In this study, we investigated the involvement of Akt and members of the mitogen-activated protein kinase (MAPK) superfamily, including ERK, JNK, and p38 MAPK, in gemcitabine-induced cytotoxicity in human pancreatic cancer cells. We found that gemcitabine induces apoptosis in PK-1 and PCI-43 human pancreatic cancer cell lines. Gemcitabine specifically activated p38 MAPK in a dose- and time-dependent manner. A selective p38 MAPK inhibitor, SB203580, significantly inhibited gemcitabine-induced apoptosis in both cell lines, suggesting that phosphorylation of p38 MAPK may play a key role in gemcitabine-induced apoptosis in pancreatic cancer cells. A selective JNK inhibitor, SP600125, failed to inhibit gemcitabine-induced apoptosis in both cell lines. MKK3/6, an upstream activator of p38 MAPK, was phosphorylated by gemcitabine, indicating that the MKK3/6-p38 MAPK signaling pathway is indeed involved in gemcitabine-induced apoptosis. Furthermore, gemcitabine-induced cleavage of the caspase substrate poly(ADP-ribose) polymerase was inhibited by pretreatment with SB203580, suggesting that activation of p38 MAPK by gemcitabine induces apoptosis through caspase signaling. These results together suggest that gemcitabine-induced apoptosis in human pancreatic cancer cells is mediated by the MKK3/6-p38 MAPK-caspase signaling pathway. Further, these results lead us to suggest that p38 MAPK should be investigated as a novel molecular target for human pancreatic cancer therapies.

Farnesyltransferase inhibitors (FTIs) represent a novel class of anticancer drugs and are now in clinical trial. We have previously shown that farnesylamine, synthetic isoprenoid-linked with "amine" which acts as a potent FTI, induces apoptosis in human pancreatic cancer cells through the ras signaling cascade. Since the effect of FTI is usually "cytostatic" rather than "cytotoxic", we speculated another apoptotic machinery of farnesylamine in addition to the effect of FTI. Farnesylamine induced sustained activation of c-jun N-terminal kinase (JNK), which was not caused by other FTI, FTI-277. Blockage of JNK activity by dominant-negative mutant abrogated the DNA laddering and significantly reduced "cytotoxic" effect of farnesylamine. Strikingly similar effect on JNK activation and apoptosis was induced by structurally related long-chain fatty amine (LFA), oleylamine, but not by farnesol, an isoprenoid analogue of farnesylamine without "amine." Taken together, apoptosis induction through JNK activation by farnesylamine based on the LFA structure rather than an effect of FTI., (Copyright 2001 Academic Press.)