Inhibin and p27 interact to regulate gonadal tumorigenesis

Tumor suppressors function as antiproliferative signaling proteins, and defects in these genes lead to uncontrolled cell proliferation and cancer. For example, absence of the tumor suppressor p27^Kip1, a cyclin-dependent kinase inhibitor (CKI), results in increased body size, hyperplasia of several organs including the testes, and cancer in mice. Similarly, lack of inhibins, α/β heterodimeric members of the transforming growth factor-β (TGFβ) superfamily, causes testicular and ovarian tumors of the granulosa/Sertoli cell lineage beginning at 4 weeks of age and adrenal tumors in gonadectomized mice. Neither the cell cycle alterations in the absence of inhibin nor the cause of the increased testis size in the p27 knockout mice is known. To study the molecular (cell cycle) changes that result from absence of inhibins, we analyzed the regulation of cell cycle proteins in gonadal tumors derived from inhibin α knockout mice (Inha^-/-). Northern blot analyses demonstrate that cyclin-dependent kinase 4 (Cdk4) and cyclin D2 mRNA levels are elevated, and immunohistochemistry shows that p27 protein levels are decreased in both ovarian and testicular tumors from Inha^-/- mice. These findings suggest that increased Cdk4/cyclin D2 (positive) activity and decreased p27 (negative) activity is causal for gonadal tumor formation. To test this hypothesis, we generated double mutant mice lacking both p27 and inhibin α to determine whether the tumor suppressors p27 and inhibin have additive suppressor activity in the gonads. Like Inha^-/- mice, p27^-/-Inha^-/- mice demonstrate elevated serum activin levels, ovarian and testicular tumors, and a resultant lethal cachexia-like syndrome. However, whereas 95% of the Inha^-/- female mice die by 18 weeks of age, 100% of the p27^-/-Inha^-/- female mice are dead by 8 weeks. Similarly, 95% of the Inha^-/- single mutant males die by 13 weeks while 100% of the p27^-/-Inha^-/- male mice die by 10 weeks. Moreover, tumor foci in p27^-/-Inha^-/- mice can be observed as early as 2 weeks of age in males and as early as 4 weeks in females. These findings demonstrate that absence of both inhibin and p27 in mice causes earlier development of ovarian and testicular tumors and earlier death compared with absence of inhibin alone.