Expression of p16Ink4a Compensates for p18Ink4c Loss in Cyclin-Dependent Kinase 4/6–Dependent Tumors and Tissues

Matthew R Ramsey,Norman E Sharpless,Weili Lin,Daniel R Carrasco,Yue Xiong,Janakiraman Krishnamurthy,Chad Torrice,Keith L Ligon,Xin-Hai Pei

doi:10.1158/0008-5472.can-06-3437

Abstract

Cell cycle progression from G(1) to S phase depends on phosphorylation of pRb by complexes containing a cyclin (D type or E type) and cyclin-dependent kinase (e.g., cdk2, cdk4, or cdk6). Ink4 proteins function to oppose the action of cdk4/6-cyclin D complexes by inhibiting cdk4/6. We employed genetic and pharmacologic approaches to study the interplay among Ink4 proteins and cdk4/6 activity in vivo. Mouse embryo fibroblasts (MEF) lacking p16(Ink4a) and p18(Ink4c) showed similar growth kinetics as wild-type MEFs despite increased cdk4 activity. In vivo, germline deficiency of p16(Ink4a) and p18(Ink4c) resulted in increased proliferation in the intermediate pituitary and pancreatic islets of adult mice, and survival of p16(Ink4a-/-);p18(Ink4c-/-) mice was significantly reduced due to aggressive pituitary tumors. Compensation among the Ink4 proteins was observed both in vivo in p18(Ink4c-/-) mice and in MEFs from p16(Ink4a-/-), p18(Ink4c-/-), or p16(Ink4a-/-);p18(Ink4c-/-) mice. Treatment with PD 0332991, a specific cdk4/6 kinase inhibitor, abrogated proliferation in those compartments where Ink4 deficiency was associated with enhanced proliferation (i.e., islets, pituitary, and B lymphocytes) but had no effect on proliferation in other tissues such as the small bowel. These data suggest that p16(Ink4a) and p18(Ink4c) coordinately regulate the in vivo catalytic activity of cdk4/6 in specific compartments of adult mice.

Highlights

Control of the retinoblastoma family proteins through phosphorylation by cyclin-dependent kinases is essential for cell cycle regulation
Cdk2 can bind to cyclin E1, cyclin E2, or cyclin A, whereas cdk4 and cdk6 have been found to pair with the D-type cyclins
We show that in murine embryo fibroblasts (MEF), the loss of p18Ink4c results in compensatory increases in p16Ink4a, whereas loss of p16Ink4a is associated with increased expression of p15Ink4b

Summary

Introduction

Control of the retinoblastoma family proteins (pRb, p107, and p130) through phosphorylation by cyclin-dependent kinases (cdk) is essential for cell cycle regulation. The Ink family of cdk inhibitors consists of four members [p16Ink4a [2], p15Ink4b [3], p18Ink4c [4], and p19Ink4d [5]], and all are specific inhibitors of cdk4/6-cyclin D complexes. Both p16Ink4a and p15Ink4b consist of four ankyrin repeats, with f80% sequence similarity to each other, whereas p18Ink4c and p19Ink4d have a fifth ankryin repeat Using genetic and pharmacologic approaches, we show that specific in vivo compartments in the adult mouse are exquisitely dependent on cdk4/6 kinase activity for proliferation These data suggest that p16Ink4a and p18Ink4c function to regulate physiologic and aberrant proliferation in specific compartments of adult mice. I2007 American Association for Cancer Research. doi:10.1158/0008-5472.CAN-06-3437

Materials and Methods

Results

Discussion