PTEN tumour suppressor is linked to the cell cycle control through the retinoblastoma protein

The tumour suppressor PTEN, also named MMAC1 or TEP1, is associated with a number of malignancies in human populations. This protein has a dual protei n phosphatase activity, being also capable to dephosphorylate phosphatidyli nositol 3,4,5 triphosphate. We have studied the mechanism of growth suppres sion attributable to PTEN, We observed that PTEN overexpression inhibits ce ll growth in a variety of normal and transformed, human and murine cells. B romodeoxyuridine (BrdU) incorporation and TUNEL labelling experiments in tr ansiently transfected cells demonstrate that this inhibition is due to a ce ll cycle arrest rather than induction of apoptosis, Given that PTEN is unab le to cause cell growth arrest in retinoblastoma (Rb)deficient cell lines, we have explored the possible requirement for pRb in the PTEN-induced inhib ition of cell proliferation. We found that the co-expression of SV40 antige n, but not a mutant form (which binds exclusively to p53), and cyclin D1/cd k4 are able to overcome the PTEN-mediated growth suppression. In addition, the reintroduction of a functional pRb, but not its relatives p107 or p130, in Rb-deficient cells restores the sensitivity to PTEN-induced arrest. Fin ally, the hyperphosphorylation of transfected pRb is inhibited by PTEN co-e xpression and restored by PI-3K co-expression, Accordingly, PTEN gene is mo stly expressed, in parallel to Akt, in mid-late G(1) phase during cell cycl e progression prior to pRb hyperphosphorylation. Finally, we have studied t he signal transduction pathways modulated by PTEN expression. We found that PTEN-induced growth arrest can be rescued by the co-expression of active P I-3K and downstream effecters such as Akt or PDK1, and also certain small G TPases such as Rad and Cdc42, but not by active Ha-ras, paf or RhoA, Collec tively, our data link the tumour suppressor activities of PTEN to the machi nery controlling cell cycle through the modulation of signalling molecules whose final target is the functional inactivation of the retinoblastoma gen e product.