3,5,3′triiodo‐L‐thyronine induces SREBP‐1 expression by non‐genomic actions in human HEP G2 cells

Abstract

Liver is an important target for thyroid hormone actions. T(3) exerts its effects by two mechanisms: (i) Genomic actions consisting of T(3) link to nuclear receptors that bind responsive elements in the promoter of target genes, (ii) non-genomic actions including integrin αvb3 receptor-mediated MAPK/ERK and PI3K/Akt/mTOR-C1 activation. SREBP-1a, SREBP-1c, and SREBP-2 are transcription factors involved in the regulation of lipogenic genes. We show in Hep G2 cells that T(3) determined a dose- and time-dependent increase in the level of the precursor form of SREBP-1 without affecting SREBP-1 mRNA abundance. T(3) also induced phosphorylation of ERK1/2, Akt and of mTOR-C1 target S6K-P70, and the cytosol-to-membrane translocation of PKC-α. Modulation of SREBP-1 protein level by T(3) was dependent on MAPK/ERK, PI3K/Akt/mTOR-C1 pathway activation since the MEK inhibitor PD98059 or the PI3K inhibitor LY294002 abolished the stimulatory effect of T(3) . Conversely, the effect of T(3) on SREBP-1 level was enhanced by using rapamycin, mTOR-C1 inhibitor. These data suggest a negative control of mTOR-C1 target S6K-P70 on PI3K/Akt pathway. The effect of T(3) on SREBP-1 content increased also by using PKC inhibitors. These inhibitors increased the action of T(3) on Akt phosphorylation suggesting that conventional PKCs may work as negative regulators of the T(3) -dependent SREBP-1 increase. T(3) effects were partially abrogated by tetrac, an inhibitor of the T(3) -αvβ3 receptor interaction and partially evoked by T(3) analog T(3) -agarose. These findings support a model in which T(3) activates intracellular signaling pathways which may be involved in the increment of SREBP-1 level through an IRES-mediated translation mechanism.

Protein Kinase Cα Regulates the Expression of Complement Receptor Ig in Human Monocyte–Derived Macrophages

“…Although there have been no reports on the promoter regions of VSIG4, in silico data and nucleotide sequence analysis of the upstream elements of the transcription start site of VSIG4 have identified regulatory trans-acting factor binding sites for factors such as glucocorticoid receptor, c-myb, and SREBP-1 (Genecard database). The expression of SREBP-1 has recently been reported to be negatively regulated by classical PKC isoforms such as PKCa in Hep G2 cells in response to 3,5,39-triiodothyronine (35). It would be of interest to determine, in future studies, whether PKCa negatively regulates CRIg expression by downregulating SREBP-1 expression, resulting in reduced CRIg transcription.…”

Thyroid: biological actions of ‘nonclassical’ thyroid hormones

“…A second clue indicating that T 2 has contrasting effects on hepatic lipogenesis involving SREBP-1c was found in vitro. T 3 increases an active precursor of SREBP-1c in HepG2 cells without modulating SREBP-1c transcription (Gnoni et al 2012). T 2 , however, blocks proteolytic cleavage and thus activation of SREBP-1c (Rochira et al 2013) independent of transcription.…”

Non-classic thyroid hormone signalling involved in hepatic lipid metabolism

“…Lin et al (1996b) demonstrated that thyroid hormone can potentiate interferon gamma-induced antiviral states in a PKCdependent manner. In Hep G cells, T 3 causes a timedependent PKC-a cytosol-to-membrane translocation after 5 min and up to 10 min of treatment, and this activation attenuates the stimulatory effect of T 3 on SREBP1 expression (Gnoni et al 2012). Similarly, thyroid hormone application to cultured myoblasts promoted PKC translocation from the cytosol to the membrane in a PLC-dependent manner (D'Arezzo et al 2004).…”

“…Although the genomic effect of T 3 is the down-regulation of hepatic mouse SREBP1 expression (Hashimoto et al 2006b) and the upregulation of hepatic human SREBP1 (Kawai et al 2004), two other pathways classically associated with insulin signalling have also been associated with T 3 regulation of human SREBP1. Recently, T 3 -induced SREBP1 expression in the human hepatocarcinoma cell line (Hep G) was attenuated by Akt or ERK inhibition (Gnoni et al 2012). However, although the T 3 effect on ERK activation persisted with the use of agarose-conjugated T 3 , Akt activation by T 3 was abolished, suggesting that T 3 interacts with a membrane surface receptor, such as integrin avb3, to activate ERK (Gnoni et al 2012).…”

“…Recently, T 3 -induced SREBP1 expression in the human hepatocarcinoma cell line (Hep G) was attenuated by Akt or ERK inhibition (Gnoni et al 2012). However, although the T 3 effect on ERK activation persisted with the use of agarose-conjugated T 3 , Akt activation by T 3 was abolished, suggesting that T 3 interacts with a membrane surface receptor, such as integrin avb3, to activate ERK (Gnoni et al 2012). Therefore, this report describes two independent nonclassic pathways of thyroid hormones regulating SREBP1 in response to T 3 using the integrin avb3 for the MAPK/ERK pathway and, most likely, the TR-PI3K complex for the PI3K/Akt pathway.…”