A contribution of intracellular
dehydration to
insulin resistance has been established in human subjects and in different experimental systems. Here the effect of hyperosmolarity (405 mosmol/l) on
insulin-induced
mitogen-activated
protein (MAP)
kinase phosphatase (MKP)-1 expression was studied in H4IIE rat
hepatoma cells.
Insulin induces robust MKP-1 expression which correlates with a
vanadate-sensitive decay of
extracellular-signal-regulated kinase (Erk-1/Erk-2) activity. Hyperosmolarity delays MKP-1 accumulation by
insulin and this corresponds to impaired MKP-1 synthesis, whereas MKP-1 degradation remains unaffected by hyperosmolarity.
Rapamycin, which inhibits signalling downstream from the
mammalian target of rapamycin (mTOR) and a
peptide inhibiting
protein kinase C (PKC) zeta/lambda abolish
insulin-induced MKP-1
protein but not
mRNA expression, suggesting the involvement of the p70 ribosomal S6
protein kinase (p70S6-kinase) and/or the eukaryotic
initiation factor 4E-binding
proteins (4E-BPs) as well as atypical
PKCs in MKP-1 translation. Hyperosmolarity induces sustained suppression of p70S6-kinase and 4E-BP1 hyperphosphorylation by
insulin, whereas
insulin-induced
tyrosine phosphorylation of the
insulin receptor (IR) beta subunit and the IR substrates IRS1 and IRS2, recruitment of the
phosphoinositide 3-kinase (PI 3-kinase) regulatory subunit p85 to the receptor substrates as well as
PI 3-kinase activation, and Ser-473 phosphorylation of
protein kinase B and Thr-410/403 phosphorylation of PKC zeta/lambda are largely unaffected under hyperosmotic conditions. The hyperosmotic impairment of both, MKP-1 expression and p70S6-kinase hyperphosphorylation by
insulin is insensitive to K(2)CrO(4),
calyculin A and
vanadate, and inhibition of the Erk-1/Erk-2 and p38 pathways. The suppression of MKP-1 may further contribute to
insulin resistance under dehydrating conditions by allowing unbalanced MAP
kinase activation.