Insights into LINE-1 reverse transcription guide therapy development.

Subsets of long interspersed nuclear element 1 (LINE-1) retrotransposons can 'retrotranspose' throughout the human genome at a cost to host cell fitness, as observed in some cancers. Pharmacological inhibition of LINE-1 retrotransposition requires a comprehensive understanding of the LINE-1 ORF2p reverse transcriptase. Two recent publications, by Thawani et al. and Baldwin et al., report structures of LINE-1 ORF2p and address long-standing mechanistic gaps regarding LINE-1 retrotransposition. Both studies will be critical to design new specific inhibitors of the LINE-1 ORF2p reverse transcriptase.

Paxillin regulates Rab5-mediated vesicle motility through modulating microtubule acetylation.

Rab GTPase-mediated vesicle trafficking of cell surface proteins, including integrins, through endocytic and recycling pathways is important in controlling cell-extracellular matrix interactions during cell migration. The focal adhesion adaptor protein, paxillin, plays a central role in regulating adhesion dynamics and was previously shown to promote anterograde vesicle trafficking through modulation of microtubule acetylation via its inhibition of the deacetylase HDAC6. The role of paxillin in retrograde trafficking is unknown. Herein, we identified a role for paxillin in the modulation of the Rab5 GTPase, which is necessary for regulating early endosome dynamics and focal adhesion turnover. Using MDA-MB-231 breast cancer cells and paxillin (-/-) fibroblasts, paxillin was shown to impact Rab5-associated vesicle size and distribution, as well as Rab5 GTPase activity, through its modulation of HDAC6. Using a combination of real-time imaging and particle tracking analysis, paxillin was shown to promote Rab5-associated vesicle motility through inhibition of HDAC6-mediated micro-tubule deacetylation, along with the localization of active integrin to focal adhesions.

Paxillin promotes breast tumor collective cell invasion through maintenance of adherens junction integrity.

Distant organ metastasis is linked to poor prognosis during cancer progression. The expression level of the focal adhesion adapter protein paxillin varies among different human cancers, but its role in tumor progression is unclear. Herein we utilize a newly generated PyMT mammary tumor mouse model with conditional paxillin ablation in breast tumor epithelial cells, combined with in vitro three-dimensional (3D) tumor organoids invasion analysis and 2D calcium switch assays, to assess the roles for paxillin in breast tumor cell invasion. Paxillin had little effect on primary tumor initiation and growth but is critical for the formation of distant lung metastasis. In paxillin-depleted 3D tumor organoids, collective cell invasion was substantially perturbed. The 2D cell culture revealed paxillin-dependent stabilization of adherens junctions (AJ). Mechanistically, paxillin is required for AJ assembly through facilitating E-cadherin endocytosis and recycling and HDAC6-mediated microtubule acetylation. Furthermore, Rho GTPase activity analysis and rescue experiments with a RhoA activator or Rac1 inhibitor suggest paxillin is potentially regulating the E-cadherin-dependent junction integrity and contractility through control of the balance of RhoA and Rac1 activities. Together, these data highlight new roles for paxillin in the regulation of cell-cell adhesion and collective tumor cell migration to promote the formation of distance organ metastases.