Homeostasis of the gastrointestinal epithelium is dependent upon a balance between cell proliferation and apoptosis.
Cyclin-dependent kinases (Cdks) are well known for their role in cell proliferation. Previous studies from our group have shown that
polyamine-depletion of intestinal epithelial cells (IEC-6) decreases
cyclin-dependent kinase 2 (Cdk2) activity, increases p53 and p21Cip1
protein levels, induces G1 arrest, and protects cells from
camptothecin (
CPT)-induced apoptosis. Although emerging evidence suggests that members of the Cdk family are involved in the regulation of apoptosis, their roles directing apoptosis of IEC-6 cells are not known. In this study, we report that inhibition of Cdk1, 2, and 9 (with the broad range Cdk inhibitor,
AZD5438) in proliferating IEC-6 cells triggered DNA damage, activated p53 signaling, inhibited proliferation, and induced apoptosis. By contrast, inhibition of Cdk2 (with
NU6140) increased p53
protein and activity, inhibited proliferation, but had no effect on apoptosis. Notably,
AZD5438 sensitized, whereas,
NU6140 rescued proliferating IEC-6 cells from
CPT-induced apoptosis. However, in colon
carcinoma (Caco-2) cells with mutant p53, treatment with either
AZD5438 or
NU6140 blocked proliferation, albeit more robustly with
AZD5438. Both Cdk inhibitors induced apoptosis in Caco-2 cells in a p53-independent manner. In serum starved quiescent IEC-6 cells, both
AZD5438 and
NU6140 decreased TNF-α/
CPT-induced activation of p53 and, consequently, rescued cells from apoptosis, indicating that sustained Cdk activity is required for apoptosis of quiescent cells. Furthermore,
AZD5438 partially reversed the protective effect of
polyamine depletion whereas
NU6140 had no effect. Together, these results demonstrate that Cdks possess opposing roles in the control of apoptosis in quiescent and proliferating cells. In addition, Cdk inhibitors uncouple proliferation from apoptosis in a p53-dependent manner.