Chapter Eight - Ubiquitin-Mediated Regulation of Cellular Responses to Vascular Endothelial Growth Factors

Sex-specific differences in the intracellular proteome of human endothelial cells from dizygotic twins

Differences between men and women are being continuously identified in many human diseases. The underlying reasons are not yet fully understood. Beside the influence of endogenous hormones and life style, intrinsic sex-specific dimorphisms at the cellular level may also play a role. HUVECs from twin pairs of opposite sex provide an excellent tool to address the question of sex-specific differences at the molecular level. We compared for the first time protein levels of male and female HUVECs from dizygotic twins using a proteomic approach. To investigate differences under basal and stress conditions, cells were either left untreated or wounded and serum starved for different time points. Approximately 10% of all proteins monitored showed significant sexual dimorphisms in their level under the different conditions tested. The majority of the proteins displayed a higher abundance in female cells. The magnitude of the difference in protein levels between male and female cells was rather small. The most prominent differences throughout all conditions were observed for several X-chromosome encoded proteins with higher levels in female (UBA1, HDHD1) or in male cells (G6PD). Proteins involved in basic cellular processes, such as gene expression and translation (e.g. HMGN1, SRP54) displayed sex-specific levels in particular conditions only. SIGNIFICANCE: This study provides novel insights into sexual dimorphic protein levels in HUVECs from twin pairs of the opposite sex. The findings identify proteins with sex-specific differences in their levels under different cell culture conditions. The study also highlights the presence of X-chromosome encoded proteins escaping X-chromosomal inactivation. The results emphasize the need to consider the cellular sex of male and female HUVECs in in vitro experiments.

Receptor Tyrosine Kinase Ubiquitination and De-Ubiquitination in Signal Transduction and Receptor Trafficking

Receptor tyrosine kinases (RTKs) are membrane-based sensors that enable rapid communication between cells and their environment. Evidence is now emerging that interdependent regulatory mechanisms, such as membrane trafficking, ubiquitination, proteolysis and gene expression, have substantial effects on RTK signal transduction and cellular responses. Different RTKs exhibit both basal and ligand-stimulated ubiquitination, linked to trafficking through different intracellular compartments including the secretory pathway, plasma membrane, endosomes and lysosomes. The ubiquitin ligase superfamily comprising the E1, E2 and E3 enzymes are increasingly implicated in this post-translational modification by adding mono- and polyubiquitin tags to RTKs. Conversely, removal of these ubiquitin tags by proteases called de-ubiquitinases (DUBs) enables RTK recycling for another round of ligand sensing and signal transduction. The endocytosis of basal and activated RTKs from the plasma membrane is closely linked to controlled proteolysis after trafficking and delivery to late endosomes and lysosomes. Proteolytic RTK fragments can also have the capacity to move to compartments such as the nucleus and regulate gene expression. Such mechanistic diversity now provides new opportunities for modulating RTK-regulated cellular responses in health and disease states.