Regulation of sterol regulatory-element binding protein 1 gene expression in liver: role of insulin and protein kinase B/cAkt

Insulin stimulates the transcription of the sterol regulatory-element bindi ng protein (SREBP) 1/ADD1 gene in liver. Hepatocytes in primary culture wer e used to delineate the insulin signalling pathway for induction of SREBP1 gene expression. The inhibitors of phosphoinositide 3-kinase (PI 3-kinase), wortmannin and LY 294002, abolished the insulin-dependent increase in SREB P1 mRNA, whereas the inhibitor of the mitogen-activated protein kinase casc ade, PD 98059, was without effect. To investigate the role of protein kinas e B (PKB)/cAkt downstream of PI 3-kinase, hepatocytes were transduced with an adenovirus encoding a PKB oestrogen receptor fusion protein. The PKB act ivity of this recombinant protein was rapidly activated in hepatocytes chal lenged with 4-hydroxytamoxifen (OHT), as was endogenous PKB in hepatocytes challenged with insulin. The addition of OHT to transduced hepatocytes resu lted in accumulation of SREBP1 mRNA, with a time-course and magnitude simil ar to the effect of insulin in non-transduced cells. The level of SREBP1 mR NA was not increased by OHT in hepatocytes expressing a mutant form of the recombinant protein whose PKB activity was not activated by OHT. Thus acute activation of PKB is sufficient to induce SREBP1 mRNA accumulation in prim ary hepatocytes, and might be the major signalling event by which insulin i nduces SREBP1 gene expression in the liver.