Summary Background In many differentiated cells microtubules are organized into polarized noncentrosomal arrays, yet few mechanisms that control these arrays have been identified. For example, mechanisms that maintain microtubule polarity in the face of constant remodeling by dynamic instability are not known. Drosophila neurons contain uniform polarity minus-end-out microtubules in dendrites, which are often highly branched. As undirected microtubule growth through dendrite branch points jeopardizes uniform microtubule polarity, we have used this system to understand how cells can maintain dynamic arrays of polarized microtubules. Results We find that growing microtubules navigate dendrite branch points by turning the same way, towards the cell body, 98% of the time, and that growing microtubules track along stable microtubules towards their plus ends. Using RNAi and genetic approaches, we show that kinesin-2, and the +TIPS EB1 and APC, are required for uniform dendrite microtubule polarity. Moreover, the protein-protein interactions and localization of Apc2-GFP and Apc-RFP to branch points suggests these proteins work together at dendrite branches. The functional importance of this polarity mechanism is demonstrated by the failure of neurons with reduced kinesin-2 to regenerate an axon from a dendrite. Conclusions We conclude that microtubule growth is directed at dendrite branch points, and that kinesin-2, APC and EB1 are likely to play a role in this process. We propose that is recruited to growing microtubules by +TIPS, and that the motor protein steers growing microtubules at branch points. This represents a newly discovered mechanism to maintain polarized arrays of microtubules.

Chromate compounds are known human lung carcinogens. Water solubility is an important factor in the carcinogenicity of these compounds with the most potent carcinogenic compounds being water-insoluble or 'particulate'. Previously we have shown that particulate chromates dissolve extracellularly releasing chromium (Cr) and lead (Pb) ions and only the Cr ions induce genotoxicity. Pb ions have been considered to have epigenetic effects and it is thought that these may enhance the carcinogenic activity of lead chromate, perhaps by stimulating Cr-damaged cells to divide. However, this possibility has not been directly tested. Accordingly, we investigated the ability of Pb ions to stimulate human lung cells and possibly force lead chromate-damaged cells to grow. We found that at concentrations of lead chromate that induced damage, human lung cells exhibited cell cycle arrest and growth inhibition that were very similar to those observed for sodium chromate. Moreover, we found that soluble Pb ions were not growth stimulatory to human lung cells and in fact induced progressive mitotic arrest. These data indicate that lead chromate-generated Cr ions cause growth inhibition and cell cycle arrest and that Pb does not induce epigenetic effects that stimulate chromate-damaged cells to grow.

Particulate hexavalent chromium [Cr(VI)] compounds are well-established human carcinogens. Cr (VI)-induced tumors are characterized by chromosomal instability (CIN); however, the mechanisms of this effect are unknown. We investigated the hypothesis that homologous recombination (HR) repair of DNA double strand breaks protect cells from Cr(VI)-induced CIN by focusing on the XRCC3 and RAD51C genes, which play an important role in cellular resistance to DNA double strand breaks. We used Chinese hamster cells defective in each HR gene (irs3 for RAD51C and irs1SF for XRCC3) and compared with their wildtype parental and cDNA-complemented controls. We found that the intracellular Cr ion levels varied among the cell lines after particulate chromate treatment. Importantly, accounting for differences in Cr ion levels, we discovered that XRCC3 and RAD51C cells treated with lead chromate had increased cytotoxicity and chromosomal aberrations, relative to wild-type and cDNA-complimented cells. We also observed the emergence of high levels of chromatid exchanges in the two mutant cell lines. For example, 1 ug/cm 2 lead chromate induced 20 and 32 exchanges in XRCC3-and RAD51C-deficient cells, respectively, whereas no exchanges were detected in the wildtype and cDNA-complemented cells. These observations suggest that HR protects cells from Cr(VI)-induced CIN, consistent with the ability of particulate Cr(VI) to induce double strand breaks.

Particulate hexavalent chromium ((Cr(VI)) compounds are human lung carcinogens. These compounds induce DNA damage, chromosome aberrations, and concentration-dependent cell death in human and Chinese hamster ovary (CHO) cells. The relationship between Cr(VI)-induced DNA damage and chromosome aberrations is poorly understood. Accordingly, this study focused on examining the role of Ku80, a gene involved in nonhomologous end-joining repair, in particulate chromate-induced cytotoxicity and chromosome damage in CHO cells. Three different cell lines were used: CHO-K1 (parental), xrs-6 (Ku80 deficient), and 2E (xrs-6 complemented with Ku80 gene). Levels of cell death were higher in xrs-6 cells when compared to wild type, suggesting that Ku80 was important for protecting cells from lead chromate. However, Ku80 played no role in protecting cells from particulate Cr(VI)-induced chromosome instability (CIN) as gene complementation with Ku80 (2E cells) studies and uptake experiments showed similar frequency and amounts of chromosome damage between the cell lines and that any observed difference based on administered concentration was actually due to differences in Cr(VI) uptake. The spectrum of chromosome damage was also unaffected by Ku80 deficiency. These data indicate that Ku80 protects cells from cytotoxicity but is not involved in protecting cells from particulate chromate-induced CIN.