Transforming growth factor-β adaptor, β2-spectrin, modulates cyclin dependent kinase 4 to reduce development of hepatocellular cancer

Abstract

Transforming growth factor beta (TGF-b) is an important regulator of cell growth, and loss of TGF-b signaling is a hallmark of carcinogenesis. The Smad3/4 adaptor protein b2-spectrin (b2SP) is emerging as a potent regulator of tumorigenesis through its ability to modulate the tumor suppressor function of TGF-b. However, to date the role of the TGFb signaling pathway at specific stages of the development of hepatocellular carcinoma (HCC), particularly in relation to the activation of other oncogenic pathways, remains poorly delineated. Here we identify a mechanism by which b2SP, a crucial Smad3 adaptor, modulates cyclin dependent kinase 4 (CDK4), cell cycle progression, and suppression of HCC. Increased expression of b2SP inhibits phosphorylation of the retinoblastoma gene product (Rb) and markedly reduces CDK4 expression to a far greater extent than other CDKs and cyclins. Furthermore, suppression of CDK4 by b2SP efficiently restores Rb hypophosphorylation and cell cycle arrest in G 1 . We further demonstrate that b2SP interacts with CDK4 and Smad3 in a competitive and TGF-b-dependent manner. In addition, haploinsufficiency of cdk4 in b2sp 1/2 mice results in a dramatic decline in HCC formation compared to that observed in b2sp 1/2 mice. Conclusion: b2SP deficiency leads to CDK4 activation and contributes to dysregulation of the cell cycle, cellular proliferation, oncogene overexpression, and the formation of HCCs. Our data highlight CDK4 as an attractive target for the pharmacologic inhibition of HCC and demonstrate the importance of b2sp 1/2 mice as a model of preclinical efficacy in the treatment of HCC. (HEPATOLOGY 2011;53:1676-1684 T he transforming growth factor b (TGF-b) signaling pathway is involved in multiple cellular processes, including cell growth, differentiation, adhesion, migration, and apoptosis. TGF-b is particularly active as an antimitogenic cytokine, functioning as a profound tumor suppressor by inhibiting cell cycle progression and arresting cells in early G 1 phase. TGFb signaling is mediated by type I and type II transmembrane serine/threonine kinase receptors (TbRI and TbRII) and such intracellular mediators as the Abbreviations: b2SP, b2-spectrin; CDK4, cyclin dependent kinase 4; HCC, hepatocellular cancer; TGF-b, transforming growth factor-b. From the

TGF-β Signaling in Onset and Progression of Hepatocellular Carcinoma

“…One molecular mechanism for how liver cells can escape from the cytostatic TGF-β response was described by the work of Mishra and coworkers [14,15,16,17,18,19,20,21,22,23,24]. Initially, they found that disruption of the adaptor protein ELF, a β-spectrin, leads to disruption of TGF-β/Smad signaling.…”

“…TGF-β triggers phosphorylation and association of ELF with Smad3 and Smad4, followed by nuclear translocation, therewith indicating an unexpected molecular link between a scaffolding protein and the TGF-β signaling pathway [24]. ELF-mediated TGF-β-dependent transcriptional responses are strongly related to its cytostatic response, comprising, for example, induction of p15, p21, MMP-9, p57, β-cadherin, and p16 expression as well as repression of β-catenin, c-myc, hTERT, IGF2, and LDLR transcription [14,18,20,21]. Additionally, PRAJA, a RING-H2 protein, was identified to interact with ELF leading to enhanced degradation via substantial E3-dependent ubiquitinylation.…”

Actin-bundling proteins in cancer progression at a glance

“…Has also been proposed to be a differentiation marker in colonic neoplasia Reduced expression of spectrin associated with poor prognosis in pancreatic cancer and progression in hepatocellular cancer. Spectrin contributes to platinum chemotherapy resistance (Baek et al, 2011;Jiang et al, 2010;Kitisin et al, 2007;Maeda et al, 2011;Simpson and Page, 1992;Sormunen et al, 1994;Sormunen et al, 1999;Thenappan et al, 2009;Tuominen et al, 1996;Younes et al, 1989) Supervillin Stress fibres and focal adhesions of multiple cell types. Implicated in nuclear architecture…”

Transforming growth factor‑β decreases side population cells in hepatocellular carcinoma in�vitro

“…These results are similar to those of previous studies investigating the tumor suppressive effects of TGF-β in liver cancer. Baek et al reported that TGF-β inhibited cell proliferation through cell cycle arrest (22). Senturk et al demonstrated that TGF-β induced downregulation of p21 (Cip1) and p15 (Ink4b) and inhibited cell proliferation through cell cycle arrest in HCC cell lines (23).…”