Inhibition of Smad Antiproliferative Function by CDK Phosphorylation

Abstract

Rb family members were the only demonstrated substrates of CDK4 until it was shown recently that Smad3, which plays a key role in mediating TGF-β antiproliferative responses, is phosphorylated by both CDK4 and CDK2 in vivo and in vitro. CDK phosphorylation of Smad3 inhibits its transcriptional activity and antiproliferative function. The Rb pathway is disrupted in almost all human cancers. Most cancers contain high levels of CDK activity due to frequent inactivation of the p16 tumor suppressor or overexpression of cyclin D1. Therefore, disruption of the Rb pathway not only inactivates Rb, but also likely diminishes Smad activity, which may contribute to tumorigenesis and resistance to the TGF-β growth-inhibitory effects in cancers. Although genetic mutation of Smad3 has not been reported, CDK phosphorylation of Smad3 may provide an epigenetic mechanism for inhibition of the tumor suppressive function of Smad3 during the early stages of tumorigenesis.Cell cycle control is closely associated with cancer. 1-3 Periodic expression of various cyclins activates different cyclin-dependent kinases (CDKs) at specific stages of the cell cycle. 4 The CDKs that function during the first gap phase (G 1 ) include the homologous CDK4 and CDK6 as well as CDK2. 4 CDK4 and CDK6 are complexed with D type cyclins in early-mid G 1 phase. CDK2 is complexed with E-and A-type cyclins during late G 1 and S phase, respectively. G 1 CDKs are negatively controlled by two classes of CDK inhibitors. 4 The first class includes four members of the INK4 family (p16, p15, p18 and p19), which specifically inhibit CDK4 and CDK6 activities. 4 The second class consists of three members of the Cip/Kip family (p21, p27, and p57), which inhibit cyclin E or cyclin A associated CDK2 activity, and at high doses, also inhibit cyclin D-CDK4/6 activities. 4 Both CDK4/6 and CDK2 target the Rb tumor suppressor. [1][2][3][4][5] In the hypophosphorylated state, Rb binds to and inhibits E2F activity. 3-6 G 1 CDK phosphorylation of Rb leads to the release of free active E2F, which can then activate the transcription of a number of genes that are essential for G 1 /S transition. [3][4][5][6] In addition to Rb, several other proteins, including certain components of the cell cycle machinery, can be phosphorylated by CDK2. 4 In contrast, the Rb family members have been the only demonstrated substrates for CDK4 for the past eleven years. Our recent study indicates that Smad3, a key mediator for TGF-β antiproliferative responses, is a target for inhibitory phosphorylation by both CDK4 and CDK2, revealing a distinct regulation in the complex cell cycle regulatory network. 7 TGF-β potently inhibits cell proliferation by causing cell cycle arrest at the G 1 phase and is an important tumor suppressor during the early stages of tumorigenesis. 8,9 TGF-β signals through the type I and type II transmembrane serine/threonine kinase receptors. [8][9][10][11] It binds and brings together the type I and type II receptors. In the resulting complex, the constitutively active ...

Cdk2 Deficiency Decreases ras/CDK4-Dependent Malignant Progression, but Not myc-Induced Tumorigenesis

“…Consistent with this hypothesis Liu et al (32,33) have shown that CDK4 and CDK2 phosphorylation of Smad3 inhibits its antiproliferative effects. Whether reduction of p15…”

“…Ink4b transcription can be down-regulated via phosphorylation and inactivation of Smad3 by CDK4 (32,33), but decreased p15 Ink4b was observed in both K5CDK4 and K5CDK4/ CDK2 À/À , suggesting that its down-regulation is not responsible for the CDK4-induced malignant progression.…”

Cyclin D1: polymorphism, aberrant splicing and cancer risk

“…Disruption of this 'p16-cyclin D1-RB' axis is known to occur in the majority of human tumors (Palmero and Peters, 1996;Sherr and McCormick, 2002), thus underscoring the importance of this pathway in growth regulation. Several additional substrates of CDK4/cyclin D1 have been identified, including Smad3 (Liu and Matsuura, 2005), CDT1 , and the RB-related proteins p107 Leng et al, 2002) and p130 (Canhoto et al, 2000). Each putative substrate harbors an established role in cell cycle control and therefore may influence tumorigenesis.…”

Tumor suppressor p16 INK4A is necessary for survival of cervical carcinoma cell lines

“…An unbiased screen for CDK4 and CDK6 substrates yielded 68 potential phosphorylation targets, in addition to pRB, p107, and p130 (34). Several of these putative substrates, including FOXM1 and SMAD3, have been implicated in senescence signaling (34)(35)(36). Thus, the tumor-suppressive activities of CDK4/6 in pRB/p107/p130-defective cells may be mediated through one or several of these, or involve as yet unidentified pathways (Fig.…”