We previously demonstrated that type 2C protein phosphatases (PP2C) Ptc2 and Ptc3 are required for DNA checkpoint inactivation after DNA double-strand break repair or adaptation in Saccharomyces cerevisiae. Here, we show the conservation of this pathway in mammalian cells. In response to DNA damage, ataxia telangiectasia mutated (ATM) phosphorylates the Chk2 tumour suppressor kinase at threonine 68 (Thr68), allowing Chk2 kinase dimerization and activation by autophosphorylations in the T-loop. The oncogenic protein Wip1, a PP2C phosphatase, binds Chk2 and dephosphorylates phosphoThr68. Consequently, Wip1 opposes Chk2 activation by ATM after ionizing irradiation of cells. In HCT15 colorectal cancer cells corrected for functional Chk2 activity, Wip1 overexpression suppressed the contribution of Chk2 to the G2/M DNA damage checkpoint. These results indicate that Wip1 is one of the phosphatases regulating the activity of Chk2 in response to DNA damage.

The mechanisms underlying age-related polyuria were investigated in 10- and 30-mo-old female WAG/Rij rats. Urinary volume and osmolality were 3.9 +/- 0.3 ml/24 h and 2,511 +/- 54 mosmol/kgH(2)O in adult rats and 12.8 +/- 0.8 ml/24 h and 1,042 +/- 44 mosmol/kgH(2)O in senescent animals. Vasopressin V(2) receptor mRNA did not significantly differ between 10 and 30 mo, and [(3)H]vasopressin binding sites in membrane papilla were reduced by 30%. The cAMP content of the papilla was unchanged with age, whereas papillary osmolality was significantly lowered in senescent animals. The expression of aquaporin-1 (AQP1) and -4 was mostly unaltered from 10 to 30 mo. In contrast, aquaporin-2 (AQP2) and -3 (AQP3) expression was downregulated by 80 and 50%, respectively, and AQP2 was markedly redistributed into the intracellular compartment, in inner medulla of senescent animals, but not in renal cortex. These results indicate that age-related polyuria is associated with a downregulation of AQP2 and AQP3 expression in the medullary collecting duct, which is independent of vasopressin-mediated cAMP accumulation.