An investigation into the role of microvesicles in mutant p53 invasive gain-of-function (2015)

p53 is a transcription factor with tumour suppressive attributes which is known to be mutated in over half of human cancers. As well as compromising the ability of p53 to function as a transcription factor, mutations in p53 often result in a gain-of-function phenotype which is characterised by increased ability of cancer cells to migrate and invade. This is mediated by the ability of mutant p53 to increase recycling of α5β1 integrin and receptor tyrosine kinases (RTK) from endosomes to the plasma membrane; a process which is dependent on the Rab11 effector, Rab Coupling Protein (RCP) and the phosphatidic acid generating enzyme, diacylglycerol kinase-α (DGKα). Despite accumulating evidence linking RCP/DGKα-dependent receptor recycling to invasive migration, the mechanisms by which mutant p53 controls endosomal trafficking were still unclear when the current study was instigated. 

Initial experiments indicated that the mutant p53 gain-of-function phenotype was not cell autonomous, and could be passed to p53 null cells by incubating them with conditioned medium from mutant p53 (R273H)-expressing cells. Furthermore, fractionation approaches indicated that the mutant p53 phenotype was transmitted between cells by a microvesicle vector. Upon treatment with microvesicles collected from mutant p53 expressing cells, p53 null cells displayed increased α5β1 integrin and RTK recycling and the consequent invasive/migratory behaviour that was dependent on these RCP and DGKα-regulated trafficking events. 

Despite a requirement for RCP in the response of p53 null cells to microvesicles, this Rab11 effector was not required for the production of pro-invasive microvesicles. Rather, mutant p53-expressing cells relied on Rab35 (but not Rab27a or Rab27b) for the production and/or release of microvesicles that were capable of transferring mutant p53’s gain-of-function phenotype. 

An in-depth RNA sequencing analysis indicated that microvesicles from mutant p53 cells influenced the endocytic trafficking and migratory characteristics of p53 null cells without detectably altering mRNA expression in these recipient cells. This indicated the possibility that microvesicles from mutant p53-expressing cells may act directly on the endomembrane system of recipient cells. Immunoprecipitation experiments indicated that there was a physical interaction between Rab35 and podocalyxin (PODXL), a highly-charged sialomucin which is known to directly influence membrane organisation. Additionally, PODXL was detectable in microvesicular preparations by mass spectrometry. Microvesicles purified from mutant p53-expressing cells in which PODXL had been knocked down using siRNA, had significantly reduced capacity to promote integrin/RTK recycling and mutant p53-like migratory behaviour in p53 null cells, indicating that PODXL, as well as Rab35, is a key factor responsible for transmitting mutant p53’s gain-of-function phenotype between cells. In addition to being incapable of influencing the migration of other cells, Rab35 knockdown cells themselves migrated with the characteristics of p53 null cells. Interestingly, microvesicles from mutant p53-expressing cells restored mutant p53-like migratory behaviour in these Rab35 knockdown cells. These data indicate that Rab35 and PODXL-dependent production of phenotype altering microvesicles not only influences the migration of neighbouring cells in a paracrine fashion, but may constitute an autocrine link between mutant p53 and integrin trafficking in the mutant p53 cells themselves. Finally, I have found that p53 null cells may be educated by microvesicles from mutant p53-expressing cells to themselves release cell migration-altering microvesicles, providing further evidence supporting the existence of microvesicle-based autocrine/paracrine mechanisms that may act to propagate mutant p53’s invasive gain-of-function within both homogeneous and heterogeneous populations of tumour cells.