BRIT1/MCPH1: a guardian of genome and an enemy of tumors.

The DNA of every cell is constantly exposed to insult mediated by endogenous and environmental factors that induced damage in its structure. To react to these attacks and maintain the integrity of the genome, eukaryotic cells are equipped with sophisticated mechanisms to detect, signal the presence of and repair DNA damage. The cellular response to DNA damage is a critical event for maintaining genomic stability and limiting neoplastic transformation. BRIT1, a newly identified protein, forms specific irradiation-induced nuclear foci. Our recent investigation demonstrates that BRIT1 functions as a proximal factor in the DNA damage checkpoints that control multiple damage sensors and early mediators. BRIT1 is also implicated in cell cycle checkpoints, controlling and regulating other important molecules and thus affecting the timing of mitosis. Depletion of BRIT1 abolishes the DNA damage response and results in centrosomal abnormalities and chromosomal aberrations. Moreover, aberrantly reduced expression of BRIT1 in human carcinomas implicates this protein in cancer initiation and progression. Together, the findings identify BRIT1 as a potential tumor suppressor. Fully elucidating the function of this intriguing protein may lead to new therapeutic approaches for the improved cancer treatment.

Expression of dentin sialophosphoprotein in human prostate cancer and its correlation with tumor aggressiveness.

Recent studies have demonstrated that two SIBLING family members, bone sialoprotein (BSP) and osteopontin (OPN), are overexpressed in human prostate cancer. The expression of these proteins is associated with the acquisition of a metastatic phenotype by cancer cells and a poor prognosis for the patient. Dentin sialophosphoprotein (DSPP) shares several structural and genetic features with OPN and BSP. The presence of DSPP has been recently established in salivary glands, indicating that its expression is not restricted to mineralized tissues. However, its potential expression in human tumors has not been addressed yet. In this study, we sought to evaluate the expression of DSPP in human prostate cancer. Immunohistochemistry was performed on 69 prostate cancer specimens using LFMb-21 anti-DSPP monoclonal antibody. All of the prostate cancer lesions examined expressed detectable levels of DSPP, as compared with no or low level of expression in adjacent normal glands (p < 0.0001). High grade prostatic intraepithelial neoplasia (HGPIN) glands generally displayed DSPP expression levels that were similar to those found in neighboring cancer glands. DSPP expression was significantly associated with the pathological stage (p = 0.0087) and the Gleason score (p = 0.0176) of the tumors. Western Blot was performed on 5 representative prostate tumor extracts and 3 prostatic tumor cell lines (PC3, LNCaP and DU145). All tumor extracts and cell lines analyzed have been found to express DSPP. In addition, in situ hybridization was used to assess the presence of DSPP mRNA. DSPP was detected at the RNA level in both HGPIN and tumoral glands. This study shows for the first time that DSPP is ectopically expressed in human prostate cancer. The expression of this SIBLING protein strongly correlates with conventional histopathological prognostic indicators of prostate cancer progression.

Zoledronic acid up-regulates bone sialoprotein expression in osteoblastic cells through Rho GTPase inhibition.

Clinical practice reveals that osteoporotic women treated with BPs (bisphosphonates) show an increased bone mass density and a reduced risk of fractures. However, the mechanisms leading to these beneficial effects of BPs are still poorly understood. We hypothesized that ZOL (zoledronic acid), a potent third-generation BP, may induce the expression of proteins associated with the bone-forming potential of osteoblastic cells such as BSP (bone sialo-protein). Expression of BSP gene is up-regulated by hormones that promote bone formation and has been associated with de novo bone mineralization. Using real-time reverse transcriptase-PCR and Western-blot analysis, we demonstrated that ZOL increased BSP expression in Saos-2 osteoblast-like cells. Nuclear run-on and mRNA decay assays showed no effect at the transcriptional level but a stabilization of BSP transcripts in ZOL-treated cells. ZOL effect on BSP expression occurred through an interference with the mevalonate pathway since it was reversed by either mevalonate pathway intermediates or a Rho GTPase activator. We showed that ZOL impaired membrane localization of RhoA in Saos-2 cells indicating reduced prenylation of this protein. By the use of small interfering RNAs directed to RhoA and Rac1, we identified both Rho GTPases as negative regulators of BSP expression in Saos-2 cells. Our study demonstrates that ZOL induces BSP expression in osteoblast-like cells through inactivation of Rho GTPases and provides a potential mechanism to explain the favourable effects of ZOL treatment on bone mass and integrity.