First generation vanadium-based PTEN inhibitors: Comparative study in vitro and in vivo and identification of a novel mechanism of action.

PTEN, a tumor suppressor phosphatase, regulates cellular functions by antagonizing the growth promoting PI3K/Akt/mTOR pathway through the dephosphorylation of the second messenger PIP₃. Many preclinical cellular and animal studies have used PTEN inhibitors to highlight specific disease contexts where acute activation of PI3K/Akt/mTOR pathway might offer therapeutic advantages. In the present study we have re-evaluated first-generation PTEN inhibitors, including established bisperoxo-vanadium^(V) complexes (bpVs). In vitro, all compounds tested inhibited PTEN with IC₅₀ values between 0.2-0.8 μM, although their activity diminished under reducing conditions. bpV(phen) and bpV(HΟpic) significantly increased pSer473Akt levels in PTEN wild-type cells while bpV(phen) induced phosphorylation in PTEN null cells upon re-expression of functional PTEN. bpV(ΗΟpic) was less specific since it also triggered PTEN-independent Erk1/2 phosphorylation. In vivo, bpV(phen) administration in Wistar rats enhanced pS6 levels in kidney and liver tissues, but not in several CNS tissues, and led to reduced locomotion and exploratory behaviour in the open field test. The consensus mechanism of action of first generation PTEN inhibitors appears to be oxidative inhibition, however bpV(phen) does not induce oxidation of cellular endogenous PTEN. Instead, our findings suggest that the inhibition of PTEN by bpV(phen) in cells and in vivo may proceed through a mechanism involving non-specific S-nitrosylation of PTEN. Our study highlights the complexity of PTEN inhibition by first-generation compounds and their limitations, such as low specificity, adverse effects and non-specific mechanisms of action, and emphasizes the need for developing more selective and potent PTEN inhibitors with improved efficacy and well-defined mechanisms of actions.