Caspases are important for apoptosis but are also involved in mammalian cell survival and cell division. Here we report that caspase-3 is a negative regulator of B cell cycling. Mice deficient in caspase-3 (Casp3-/- mice) have increased numbers of splenic B cells that show normal apoptosis but enhanced proliferation in vivo and hyperproliferation after mitogenic stimulation in vitro. Cdkn1a encodes p21 (also called Waf1 or Cip1), a cyclin-dependent kinase (CDK) inhibitor. Although expression of p21 was increased, CDK activities and proliferating cell nuclear antigen (PCNA) were increased in Casp3-/- B cells. Using Casp3-/-Cdkn1a-/- mice, we show that the hyperproliferation of Casp3-/- B cells is abolished when Cdkn1a is also deleted. Our genetic and biochemical data demonstrate that caspase-3 is essential in the regulation of B cell homeostasis.
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