Biology

Dr. Sorab Dalal
Advanced Centre for Treatment, Research and Education in Cancer, Tata Memorial Centre, Navi Mumbai

In this talk I will focus on two different unpublished stories that have recently developed in the laboratory, both of which illustrate the futility of focusing on translation as an outcome in research.

Plakophilin 3 (PKP3) is a desmosomal plaque protein whose loss leads to an increase in invasion, tumor formation, metastasis and radio-resistance ^1-3. Further experiments demonstrated that the levels of lipocalin 2 (LCN2) were increased upon PKP3 loss and was required for the increase in tumor formation ⁴. We further demonstrated that the increase in LCN2 expression is required for the ability of the PKP3 knockdown clones to be resistant to γ-radiation, 5-fluorouracil (5FU) and doxorubicin, but not to paclitaxel. An analysis of the PKP3 knockdown cells post radiation demonstrates that loss of PKP3 leads to an increase in autophagy and a decrease in reactive oxygen species suggesting that one mechanism by which PKP3 may promote tumor progression is by promoting an increase in resistance to oxidative stress and the resultant DNA damage resulting from oxidative stress and that LCN2 might serve as a potential therapeutic target in radio and chemo resistant tumors.

Previous work from our laboratory has demonstrated that 14-3-3ε and 14-3-3γ are required to inhibit cell cycle progression in response to an incomplete S-phase or the induction of DNA damage ^5,6. This is due to their ability to inhibit the activity of the mitotic phosphatase, cdc25C ^5-7. The loss of either 14-3-3ε or 14-3-3γ results in an increase in centrosome duplication in multiple cell types and loss of both proteins leads to a decrease in tumour formation ⁸, which is probably due to disruption of the 14-3-3 cdc25C complex. In addition to the ability of 14-3-3 proteins to form a complex with phosphorylated ligands such as cdc25C, 14-3-3 proteins also have ATPase activity ⁹. A mutant of 14-3-3γ, D129A, shows an increase in ATPase activity and oligomerization while another mutant E136A shows a decrease in ATPase activity. Our results show that the expression of the ATPase mutants of 14-3-3γ, results in an alteration of centrosome number in both transformed and immortal cell lines. Cells expressing these mutants also exhibit defects in spindle formation and mitosis. These insights have further helped our understanding of how these proteins regulate ligand function and have raised the possibility that disruption of complex formation between 14-3-3 proteins and their ligands could be a potential source of therapeutic intervention.

Bibliography

1       Gosavi, P. et al. E-cadherin and plakoglobin recruit plakophilin3 to the cell border to initiate     desmosome assembly. Cell Mol Life Sci 68, 1439-1454, doi:10.1007/s00018-010-0531-3   (2011).

2       Khapare, N. et al. Plakophilin3 Loss Leads to an Increase in PRL3 Levels Promoting K8 Dephosphorylation, Which Is Required for Transformation and Metastasis. PLoS One 7, e38561, doi:10.1371/journal.pone.0038561 PONE-D-11-23082 [pii] (2012).

3       Kundu, S. T. et al. Plakophilin3 downregulation leads to a decrease in cell adhesion and promotes metastasis. Int J Cancer 123, 2303-2314 (2008).

4       Basu, S. et al. Plakophilin3 loss leads to an increase in lipocalin2 expression, which is required for tumour formation. Exp Cell Res, doi:10.1016/j.yexcr.2018.05.026 (2018).

5       Hosing, A. S., Kundu, S. T. & Dalal, S. N. 14-3-3 Gamma is required to enforce both the incomplete S phase and G2 DNA damage checkpoints. Cell Cycle 7, 3171-3179 (2008).

6       Telles, E., Hosing, A. S., Kundu, S. T., Venkatraman, P. & Dalal, S. N. A novel pocket in 14-3-3e is required to mediate specific complex formation with cdc25C and to inhibit cell cycle progression upon activation of checkpoint pathways. Exp Cell Res 315, 1448-1457 (2009).

7       Dalal, S. N., Yaffe, M. B. & DeCaprio, J. A. 14-3-3 family members act coordinately to regulate mitotic progression. Cell Cycle 3, 672-677 (2004).

8       Mukhopadhyay, A. et al. 14-3-3γ Prevents Centrosome Amplification and Neoplastic Progression. Scientific Reports 6, 26580, doi:10.1038/srep26580 http://www.nature.com/articles/srep26580#supplementary-information (2016).

9       Ramteke, M. P. et al. Identification of a novel ATPase activity in 14-3-3 proteins--evidence from enzyme kinetics, structure guided modeling and mutagenesis studies. FEBS Lett 588, 71-78, doi:10.1016/j.febslet.2013.11.008 (2014).