Transforming G protein-coupled receptors block insulin and ras-induced adipocytic differentiation in 3T3-L1 cells: evidence for a PKC and MAP kinase independent pathway

Nox4 acts as a switch between differentiation and proliferation in preadipocytes

OBJECTIVE

Insulin promotes differentiation of preadipocytes into adipocytes. Insulin also stimulates reactive oxygen species (ROS) production, and the NADPH oxidases Nox1 and Nox4 are important sources of ROS. We determined in human and mouse preadipocytes whether Nox proteins contribute to ROS formation and differentiation in response to insulin.

METHODS AND RESULTS

The expression of Nox1 and Nox4 was increased during insulin-induced differentiation, and insulin increased ROS production. SiRNA against Nox4 but not Nox1 inhibited insulin-induced differentiation and ROS production but promoted proliferation. Nox4 overexpression yielded the opposite effect. As observed by siRNA and overexpression, Nox4 controlled the expression of MAP kinase phosphatase-1 (MKP-1), which reduces insulin-induced ERK1/2 activation. Consequently, downregulation of Nox4 promoted ERK1/2 signaling: Proliferation was increased and through phosphorylation of the inhibitory site serine612, ERK1/2 inhibited the activation of the insulin-receptor substrate-1 (IRS-1) and thereby prevented differentiation in response to insulin. Inhibition of ERK1/2 or overexpression of MPK-1 promoted insulin-induced differentiation. Accordingly, insulin-induced proliferation was enhanced by siRNA against MKP-1, whereas inhibition of ERK1/2 or overexpression of MKP-1 attenuated proliferation.

CONCLUSIONS

Nox4 acts as a switch from insulin-induced proliferation to differentiation by controlling MKP-1 expression, which limits ERK1/2 signaling.

Melanin-concentrating hormone activates signaling pathways in 3T3-L1 adipocytes

Energy homeostasis is regulated by peripheral signals, such as leptin, and by several orexigenic and anorectic neuropeptides. Recently, we reported that the orexigenic neuropeptide melanin-concentrating hormone (MCH) stimulates leptin production by rat adipocytes and that the MCH receptor (MCH-R1) is present on these cells. Here, we show that MCH-R1 is present on murine 3T3-L1 adipocytes. Treatment of 3T3-L1 adipocytes with 1 micromolar MCH for up to 2 h acutely downregulated MCH-R1, indicating a mechanism of ligand-induced receptor downregulation. Potential signaling pathways mediating MCH-R1 action in adipocytes were investigated. Treatment of 3T3-L1 adipocytes with 1 micromolar MCH rapidly induced a threefold and a fivefold increase in p44/42 MAPK and pp70 S6 kinase activities, respectively. In addition, 3T3-L1 adipocytes transiently transfected with a murine leptin-luciferase promoter construct showed a fourfold and a sixfold increase in leptin promoter-reporter gene expression at 1 h and 4 h, respectively, in response to MCH. Activity decreased to basal levels at 8 h. Furthermore, MCH-stimulated leptin promoter-driven luciferase activity was diminished in the presence of the MAP/ERK kinase inhibitor PD-98059 and in the presence of rapamycin, an inhibitor of pp70 S6 kinase activation. These results provide further evidence for a functional MCH signaling pathway in adipocytes.

Galpha(i2) enhances insulin signaling via suppression of protein-tyrosine phosphatase 1B

Suppression of the expression of the heterotrimeric G-protein Galpha(i2) in vivo has been shown to provoke insulin resistance, whereas enhanced insulin signaling is observed when Galpha(i2) is overexpressed in vivo. The basis for Galpha(i2) regulation of insulin signaling was explored in transgenic mice with targeted expression of the GTPase-deficient, constitutively active Q205L Galpha(i2) in fat and skeletal muscle. Phosphorylation of insulin receptor and IRS-1 in response to insulin challenge in vivo was markedly amplified in fat and skeletal muscle expressing Q205L Galpha(i2). The expression and activity of the protein-tyrosine phosphatase 1B (PTP1B), but not protein-tyrosine phosphatases SHP-1, SHP-2, and LAR, were constitutively decreased in tissues expressing the Q205L Galpha(i2), providing a direct linkage between insulin signaling and Galpha(i2). The loss of PTP1B expression may explain, in part, the loss of PTP1B activity in the iQ205L transgenic mice. Activation of Galpha(i2) in mouse adipocytes with lysophosphatidic acid was shown to decrease PTP1B activity, whereas pertussis toxin inactivates Galpha(i2), blocks lysophosphatidic acid-stimulated inhibition of PTP1B activity, and blocks tonic suppression of PTP1B activity by Galpha(i2). Elevation of intracellular cAMP in fat cells is shown to increase PTP1B activity, whereas either depression of cAMP levels or direct activation of Galpha(i2) suppresses PTP1B. These data provide the first molecular basis for the interplay between Galpha(i2) and insulin signaling, i.e. activation of Galpha(i2) can suppress both the expression and activity of PTP1B in insulin-sensitive tissues.

Myeloid leukemia cell growth and differentiation are independent of mitogen-activated protein kinase ERK1/2 activation

The mitogen-activated protein kinase ERK1/2 pathway is essential in the control of cell proliferation and differentiation in most cellular systems. As such, it has been considered a potential target for antineoplastic therapy. For this purpose, we have examined the role of ERK activation in myeloid leukemia cell growth and differentiation. Using a representative set of myeloid leukemia cell lines, we show that cell proliferation was not accompanied by increases on ERK1/2 activation, and mitogenic stimulation did not enhance ERK activity. Moreover, abolition of ERK function by the inhibitor PD98059 or by a dominant inhibitory mutant ERK2 had no significant effects on proliferation. With the aid of various differentiation inducers, we found that within the same cell line, differentiation to a given lineage could occur with and without ERK1/2 activation, depending on the stimulus. Also, a differentiator could have the same effect in the presence or absence of ERK stimulation, depending on the cell line. ERK inhibition did not affect the differentiation elicited by stimuli whose effects were accompanied by ERK activation. Finally, constitutive ERK activity was also ineffective on proliferation and differentiation. Thus, our results indicate that ERK1/2 activation is not an essential requirement for leukemic cell growth and differentiation.