Compared to the other members of this family (Raf-1 and B-Raf), very little is known about A-Raf. It seems to share many of the properties of the other isoforms, but its biological functions are not as thoroughly researched. All three Raf proteins are involved in the MAPK signaling pathway.
There are several ways A-Raf is different from the other Raf kinases. A-Raf is the only steroid hormone-regulated Raf isoform.[3] In addition, the A-Raf protein has amino acid substitutions in a negatively charged region upstream of the kinase domain (N-region). This could be responsible for its low basal activity.[4]
Like Raf-1 and B-Raf, A-Raf activates MEK proteins which causes the activation of ERK and ultimately leads to cell cycle progression and cell proliferation. All three Raf proteins are located in the cytosol in their inactive state when bound to 14-3-3. In the presence of active Ras, they translocate to the plasma membrane.[5] Among the Ras kinase family, A-Raf has the lowest kinase activity towards MEK proteins in the Raf kinase family.[6] Thus, it is possible that A-Raf has other functions outside the MAPK pathway or that it helps the other Raf kinases activate the MAPK pathway. In addition to phosphorylating MEK, A-Raf also inhibits MST2, a tumor suppressor and proapoptotic kinase not found in the MAPK pathway. By inhibiting MST2, A-Raf can prevent apoptosis from occurring. However, this inhibition is only possible when the splice factor heterogenous nuclear ribonucleoprotein H (hnRNP H) maintains the expression of a full-length A-Raf protein. Tumorous cells often overexpress hnRNP H. When hnRNP H is downregulated, the A-RAF gene is alternatively spliced. This prevents the expression of full-length A-Raf protein.[7] Thus, overexpression of hnRNP H in tumor cells leads to full-length expression of A-Raf which then inhibits apoptosis, allowing cancerous cells that should be destroyed to stay alive.
A-Raf also binds to pyruvate kinase M2 (PKM2), again outside the MAPK pathway. PKM2 is an isozyme of pyruvate kinase that is responsible for the Warburg effect in cancer cells.[8] A-Raf upregulates the activity of PKM2 by promoting a conformational change in PKM2. This causes PKM2 to transition from its low-activity dimeric form to a highly active tetrameric form. In cancer cells, the ratio between dimeric and tetrameric forms of PKM2 determines what happens to glucose carbons. If PKM2 is in the dimeric form, glucose is channeled into synthetic processes such as nucleic acid, amino acid, or phospholipid synthesis. If A-Raf is present, PKM2 is more likely to be in the tetrameric form. This causes more glucose carbons to be converted to pyruvate and lactate, producing energy for the cell. Thus, A-Raf can be linked to energy metabolism regulation and cell transformation, both of which are very important in tumorigenesis.[9]
In addition, researchers have proposed a model of how A-Raf is linked to endocytosis. Upstream of A-Raf, receptor tyrosine kinases (RTKs) are activated, leading to RAS-mediated activation of Raf kinases, including A-Raf. Once activated, A-Raf binds to membranes rich in Phosphatidylinositol 4,5-bisphosphate (PtdIns (4,5)P2 and signals endosomes. This leads to activation of ARF6, a central regulator of endocytic trafficking.[10]
↑Lee, J. E.; Beck, T. W.; Wojnowski, L.; Rapp, U. R. (1996-04-18). "Regulation of A-raf expression". Oncogene. 12 (8): 1669–1677. ISSN0950-9232. PMID8622887.
↑Baljuls, Angela; Mueller, Thomas; Drexler, Hannes C. A.; Hekman, Mirko; Rapp, Ulf R. (2007-09-07). "Unique N-region determines low basal activity and limited inducibility of A-RAF kinase: the role of N-region in the evolutionary divergence of RAF kinase function in vertebrates". The Journal of Biological Chemistry. 282 (36): 26575–26590. doi:10.1074/jbc.M702429200. ISSN0021-9258. PMID17613527.
↑Yin XL, Chen S, Yan J, Hu Y, Gu JX (February 2002). "Identification of interaction between MEK2 and A-Raf-1". Biochim. Biophys. Acta. 1589 (1): 71–6. doi:10.1016/S0167-4889(01)00188-4. PMID11909642.
↑Yin XL, Chen S, Gu JX (February 2002). "Identification of TH1 as an interaction partner of A-Raf kinase". Mol. Cell. Biochem. 231 (1–2): 69–74. doi:10.1023/A:1014437024129. PMID11952167.
Further reading
Belanger BF, Williams WJ, Yin TC (1976). "A flexible renewal process simulator for neural spike trains". IEEE Trans Biomed Eng. 23 (3): 262–6. doi:10.1109/TBME.1976.324641. PMID1262038.
Popescu NC, Mark GE (1989). "Localization of the pKs gene, a raf related sequence on human chromosomes X and 7". Oncogene. 4 (4): 517–9. PMID2717185.
Papin C, Eychène A, Brunet A, Pagès G, Pouysségur J, Calothy G, Barnier JV (1995). "B-Raf protein isoforms interact with and phosphorylate Mek-1 on serine residues 218 and 222". Oncogene. 10 (8): 1647–51. PMID7731720.
Lee JE, Beck TW, Brennscheidt U, DeGennaro LJ, Rapp UR (1994). "The complete sequence and promoter activity of the human A-raf-1 gene (ARAF1)". Genomics. 20 (1): 43–55. doi:10.1006/geno.1994.1125. PMID8020955.
Gardner AM, Vaillancourt RR, Johnson GL (1993). "Activation of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase by G protein and tyrosine kinase oncoproteins". J. Biol. Chem. 268 (24): 17896–901. PMID8394352.
Andersson B, Wentland MA, Ricafrente JY, Liu W, Gibbs RA (1996). "A "double adaptor" method for improved shotgun library construction". Anal. Biochem. 236 (1): 107–13. doi:10.1006/abio.1996.0138. PMID8619474.
Wu X, Noh SJ, Zhou G, Dixon JE, Guan KL (1996). "Selective activation of MEK1 but not MEK2 by A-Raf from epidermal growth factor-stimulated Hela cells". J. Biol. Chem. 271 (6): 3265–71. doi:10.1074/jbc.271.6.3265. PMID8621729.
Boldyreff B, Issinger OG (1997). "A-Raf kinase is a new interacting partner of protein kinase CK2 beta subunit". FEBS Lett. 403 (2): 197–9. doi:10.1016/S0014-5793(97)00010-0. PMID9042965.
King TR, Fang Y, Mahon ES, Anderson DH (2000). "Using a phage display library to identify basic residues in A-Raf required to mediate binding to the Src homology 2 domains of the p85 subunit of phosphatidylinositol 3'-kinase". J. Biol. Chem. 275 (46): 36450–6. doi:10.1074/jbc.M004720200. PMID10967104.
Fang Y, Johnson LM, Mahon ES, Anderson DH (2002). "Two phosphorylation-independent sites on the p85 SH2 domains bind A-Raf kinase". Biochem. Biophys. Res. Commun. 290 (4): 1267–74. doi:10.1006/bbrc.2002.6347. PMID11812000.
Yin XL, Chen S, Yan J, Hu Y, Gu JX (2002). "Identification of interaction between MEK2 and A-Raf-1". Biochim. Biophys. Acta. 1589 (1): 71–6. doi:10.1016/S0167-4889(01)00188-4. PMID11909642.
Yin XL, Chen S, Gu JX (2002). "Identification of TH1 as an interaction partner of A-Raf kinase". Mol. Cell. Biochem. 231 (1–2): 69–74. doi:10.1023/A:1014437024129. PMID11952167.
Yuryev A, Wennogle LP (2003). "Novel raf kinase protein-protein interactions found by an exhaustive yeast two-hybrid analysis". Genomics. 81 (2): 112–25. doi:10.1016/S0888-7543(02)00008-3. PMID12620389.
Liu W, Shen X, Yang Y, Yin X, Xie J, Yan J, Jiang J, Liu W, Wang H, Sun M, Zheng Y, Gu J (2004). "Trihydrophobin 1 is a new negative regulator of A-Raf kinase". J. Biol. Chem. 279 (11): 10167–75. doi:10.1074/jbc.M307994200. PMID14684750.
