A novel pro-apoptotic function of RACK1: suppression of Src activity in the intrinsic and Akt pathways

Abstract

Earlier we showed that RACK1 regulates growth of human colon cells by suppressing Src activity at G 1 and mitotic checkpoints. Here, we show that RACK1 also induces apoptosis of the cells, partly by inhibiting Src. In the intrinsic pathway, RACK1 inhibits expression of anti-apoptotic Bcl-2 and Bcl-X L , induces expression of proapoptotic Bim, targets Bim and Bax to the mitochondria, induces oligomerization of Bax (which requires Bim and inhibition of Src), depolarizes mitochondria membranes, releases cytochrome c, and activates caspases-9 and -3 and death substrates. Bax and Bim are required for RACK1-mediated mitochondrial cell death. RACK1-induced oligomerization of Bax is required for staurosporine-mediated cell death. RACK1 also induces apoptosis by blocking Src activation of the Akt cell survival pathway. This leads to activation of the transcription factor FOXO3, a potent inducer of apoptosis and G 1 arrest. Collectively, our results show that RACK1, partly by inhibiting Src, promotes mitochondrial cell death and blocks Akt-mediated cell survival. Thus, RACK1 inhibits growth and induces death of colon cells. Exploitation of these dual functions could lead to novel colon cancer therapies that mimic RACK1 function.

Rack1 promotes epithelial cell–cell adhesion by regulating E-cadherin endocytosis

“…We chose HT-29 cells for these studies because we showed that Src-specific activity is elevated in the cells and can be suppressed by overexpression of Rack1 or by incubating the cells with cell-permeable peptides that enhance Rack1's interaction with Src (Mamidipudi et al, 2007). We also showed that Rack1 regulates the growth of these cells by suppressing Src activity at G 1 and mitotic checkpoints, and in the intrinsic apoptotic and Akt cell survival pathways (Mamidipudi et al, 2007;Mamidipudi and Cartwright, 2009). Others showed that increased Src activity in HT-29 cells disrupts E-cadherin adhesion as a result of aberrant tyrosine phosphorylation of the E-cadherin/catenin complex (Skoudy et al, 1996).…”

“…Previously we identified Rack1 as a novel substrate and inhibitor of Src (Chang et al, 1998(Chang et al, , 2001(Chang et al, , 2002. We showed that Rack1 regulates the growth of cells by inhibiting Src activity at G 1 and mitotic cell-cycle checkpoints and cell survival pathways (Mamidipudi et al, 2004(Mamidipudi et al, , 2007Mamidipudi and Cartwright, 2009). Rack1 also functions in protein kinase-C, insulin growth factor receptor and integrin signaling pathways (Schechtman and Mochly-Rosen, 2001;McCahill et al, 2002;Cox et al, 2003;Kiely et al, 2005;Doan and Huttenlocher, 2007;Vomastek et al, 2007).…”

The Association of Receptor of Activated Protein Kinase C 1(RACK1) with Infectious Bursal Disease Virus Viral Protein VP5 and Voltage-dependent Anion Channel 2 (VDAC2) Inhibits Apoptosis and Enhances Viral Replication

“…Therefore, our data support the role of RACK1 as an antiapoptotic protein in cells. Several groups reported that RACK1 can promote apoptosis (28,29). Controversially, RACK1 has been reported to suppress apoptosis in multiple types of cells, such as PC-12 cells (30), W7.2 T cells (47,48), and HeLa cells (49).…”

Evidence for a pro-apoptotic function of RACK1 in human breast cancer

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We have read with great interest the recent article by Mamidipudi and Cartwright (2009), in which the authors showed that RACK1 induced apoptosis of human colon cells by inhibiting the expression of antiapoptotic pathways and Src activity, thus leading to death of colon cancer cells.We have recently conducted an analysis of RACK1 messenger RNA (mRNA) expression in 127 human breast cancer and 33 non-cancerous breast tissue specimens. The study was carried out using reverse transcriptase-polymerase chain reaction following RNA extraction from frozen samples.

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“…We have read with great interest the recent article by Mamidipudi and Cartwright (2009), in which the authors showed that RACK1 induced apoptosis of human colon cells by inhibiting the expression of antiapoptotic pathways and Src activity, thus leading to death of colon cancer cells.…”