Control of microtubule stability by the RASSF1A tumor suppressor

Abstract

The RAS association domain family 1A (RASSF1A) gene is silenced by DNA methylation in over 50% of all solid tumors of different histological types. However, the biochemical function of the RASSF1A protein is unknown. We show that RASSF1A colocalizes with microtubules in interphase and decorates spindles and centrosomes during mitosis. RASSF1A has a strong cytoprotective activity against the microtubule-destabilizing drug nocodazole, and against cold-treatment in vivo. Conversely, loss of RASSF1 in RASSF1 À/À mouse embryonic fibroblasts renders the cells more sensitive to nocodazole-induced depolymerization of microtubules. The domain required for both microtubule association and stabilization was mapped to a 169 amino-acid fragment that contains the RAS association domain. Overexpression of RASSF1A induces mitotic arrest at metaphase with aberrant mitotic cells reminiscent of such produced by the microtubule-stabilizing drug paclitaxel (taxol), including monopolar spindles, or complete lack of a mitotic spindle. Altered microtubule stability in cells lacking RASSF1A is likely to affect spindle assembly and chromosome attachment, processes that need to be carefully controlled to protect cells from genomic instability and transformation. In addition, knowledge of the microtubule-targeting function of RASSF1 may aid in the development of new anticancer drugs.

Functional importance of RASSF1A microtubule localization and polymorphisms

“…Inactivation of RASSF1A and deletions in the APC1 gene are common in human colorectal cancers and may contribute to chromosomal instability in cancer cells. The effect of RASSF1A on chromosomal segregation is well documented and functions to maintain the polymerized tubular structure of tubulin within mitosis even in the presence of the microtubule depolymerization agent, such as nocodazole (Liu et al, 2003;Dallol et al, 2007).…”

“…RASSF1A microtubule localization and polymorphisms M El-Kalla et al loss of RASSF1A using siRNA knockdown (Dallol et al, 2007) or in cells from Rassf1a À/À mice (Liu et al, 2003;van der Weyden et al, 2005). In this study, we specifically addressed whether the loss of microtubule association (as observed for the dMT mutant of RASSF1A) may interfere with the stability of tubulin by modulation of tubulin acetylation.…”

“…In the presence of RASSF1A, tubulin polymerization is stable and tubulins are acetylated . In the absence of RASSF1A (as found in the mouse embryonic fibroblasts from Rassf1a À/À mice; Liu et al, 2003;Rong et al, 2004), cells more sensitive to nocodazole-induced depolymerization of microtubule resulting in tubulin de-acetylation and destabilization. Destabilized tubulin results in inadequate formation of mitotic spindles, possible chromosome missegregation, inheritable aneuploidy and cancer.…”

“…These reports vary considerably identifying regions important for microtubule localization, such as amino acids 120-185 , amino acids 163-257 containing 28% basic charges (Rong et al, 2004), amino acids 289-340 (Liu et al, 2003) and amino acids 125-138 and 167-186 (Liu et al, 2005b). Once RASSF1A loses its microtubule localization it seems to be associated within the nucleus.…”

Hypermethylation of tumor-suppressor gene CpG islands in small-cell carcinoma of the urinary bladder

“…25,26 At this point, more is known about the mechanism of loss of RASSF1A function than about its actual function, although it is known to interact with and stabilize microtubules. [28][29][30][31] Its loss in model systems is known to cause hypersensitivity to microtubuletargeting agents 32 and therefore, its loss of function by methylation or deletion may herald chemosensitivity to microtubule-targeted drugs. In support of DNA methylation in small-cell bladder carcinoma PH Abbosh et al this theory, novel chemotherapy regimens, which include microtubule-targeting agents for TCC, are currently being explored and have shown promising results.…”