Map4k4 suppresses Srebp-1 and adipocyte lipogenesis independent of JNK signaling

Adipose tissue lipogenesis is paradoxically impaired in human obesity, promoting ectopic triglyceride (TG) deposition, lipotoxicity, and insulin resistance. We previously identified mitogen-activated protein kinase kinase kinase kinase 4 (Map4k4), a sterile 20 protein kinase reported to be upstream of c-Jun NH₂-terminal kinase (JNK) signaling, as a novel negative regulator of insulin-stimulated glucose transport in adipocytes. Using full-genome microarray analysis we uncovered a novel role for Map4k4 as a suppressor of lipid synthesis. We further report here the surprising finding that Map4k4 suppresses adipocyte lipogenesis independently of JNK. Thus, while Map4k4 silencing in adipocytes enhances the expression of lipogenic enzymes, concomitant with increased conversion of ¹⁴C-glucose and ¹⁴C-acetate into TGs and fatty acids, JNK1 and JNK2 depletion causes the opposite effects. Furthermore, high expression of Map4k4 fails to activate endogenous JNK, while Map4k4 depletion does not attenuate JNK activation by tumor necrosis factor α. Map4k4 silencing in cultured adipocytes elevates both the total protein expression and cleavage of sterol-regulated element binding protein-1 (Srebp-1) in a rapamycin-sensitive manner, consistent with Map4k4 signaling via mechanistic target of rapamycin complex 1 (mTORC1). We show Map4k4 depletion requires Srebp-1 upregulation to increase lipogenesis and further show that Map4k4 promotes AMP-protein kinase (AMPK) signaling and the phosphorylation of mTORC1 binding partner raptor (Ser792) to inhibit mTORC1. Our results indicate that Map4k4 inhibits adipose lipogenesis by suppression of Srebp-1 in an AMPK- and mTOR-dependent but JNK-independent mechanism.     

Both p38alpha(MAPK) and JNK/SAPK pathways are important for induction of nitric-oxide synthase by interleukin-1beta in rat glomerular mesangial cells

Interleukin 1beta (IL-1beta) induces expression of the inducible nitric-oxide synthase (iNOS) with concomitant release of nitric oxide (NO) from glomerular mesangial cells. These events are preceded by activation of the c-Jun NH(2)-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38(MAPK). Our current study demonstrates that overexpression of the dominant negative form of JNK1 or p54 SAPKbeta/JNK2 significantly reduces the iNOS protein expression and NO production induced by IL-1beta. Similarly, overexpression of the kinase-dead mutant form of p38alpha(MAPK) also inhibits IL-1beta-induced iNOS expression and NO production. In previous studies we demonstrated that IL-1beta can activate MKK4/SEK1, MKK3, and MKK6 in renal mesangial cells; therefore, we examined the role of these MAPK kinases in the modulation of iNOS induced by IL-1beta. Overexpression of the dominant negative form of MKK4/SEK1 decreases IL-1beta-induced iNOS expression and NO production with inhibition of both SAPK/JNK and p38(MAPK) phosphorylation. Overexpression of the kinase-dead mutant form of MKK3 or MKK6 demonstrated that either of these two mutant kinase inhibited IL-1beta-induced p38(MAPK) (but not JNK/SAPK) phosphorylation and iNOS expression. Interestingly overexpression of wild type MKK3/6 was associated with phosphorylation of p38(MAPK); however, in the absence of IL-1beta, iNOS expression was not enhanced. This study suggests that the activation of both SAPK/JNK and p38alpha(MAPK) signaling cascades are necessary for the IL-1beta-induced expression of iNOS and production of NO in renal mesangial cells.     

Salicylic acid reverses phorbol 12-myristate-13-acetate (PMA)- and tumor necrosis factor alpha (TNFalpha)-induced insulin receptor substrate 1 (IRS1) serine 307 phosphorylation and insulin resistance in human embryonic kidney 293 (HEK293) cells

Salicylates, including aspirin, have been shown to improve insulin sensitivity both in human and animal models. Although it has been suggested that salicylates sensitize insulin action by inhibiting IkappaB kinase beta (IKKbeta), the detailed mechanisms remain unclear. Protein kinase C isoforms and tumor necrosis factor alpha (TNFalpha) signaling pathways are well described mediators of insulin resistance; they are implicated in the activation of IKKbeta and the subsequent inhibition of proximal insulin signaling via insulin receptor substrate 1 (IRS1) and Akt. This study investigated the effect of salicylic acid on phorbol 12-myristate 13-acetate (PMA)- and TNFalpha-induced insulin resistance in a human embryonic kidney 293 (HEK293) cell line stably expressing recombinant human IRS1. The results showed that both PMA and TNFalpha inhibited insulin-induced Akt phosphorylation and promoted IRS1 phosphorylation on Ser-307. Salicylic acid pretreatment completely reversed the effects of PMA and TNFalpha on both Akt and IRS1. Whereas PMA activated protein kinase C isoforms and IKKbeta, TNFalpha activated neither. On the other hand, both PMA and TNFalpha activated the c-Jun N-terminal kinase (JNK), which has been reported to directly phosphorylate IRS1 Ser-307. SP600125, a JNK inhibitor, prevented PMA and TNFalpha-induced IRS1 Ser-307 phosphorylation. Finally, salicylic acid inhibited JNK activation induced by both PMA and TNFalpha. Taken together, these observations suggest that salicylic acid can reverse the inhibitory effects of TNFalpha on insulin signaling via an IKKbeta-independent mechanism(s), potentially involving the inhibition of JNK activation. The role of JNK in salicylic acid-mediated insulin sensitization, however, requires further validation because the JNK inhibitor SP600125 appears to have other nonspecific activity in addition to inhibiting JNK activity.