Two decades ago, the initial discovery of the protein prohibitin’s (PHB) function as an anti-proliferative has sparked major research initiatives geared at untangling the molecular mechanisms involved in prohibitin’s cell proliferation and tumor suppressing activities. Later research found this activity was largely controlled by the gene encoding prohibitin, leaving some question as to what the actual function of the protein was. To date, the best-characterized function of the prohibitins is as chaperones involved in the stabilization of mitochondrial proteins. However, PHB is also localized to the plasma membrane in certain cell types recent data suggest that it may function as a surface-binding site, where membrane-associated PHB can be targeted to modulate disease states.
Given the earlier supposed role of prohibitin in tumor suppression and controlling cell proliferation, a great deal of research has been done to characterize the relationship between the protein PHB and cancer. Though most research shows that the PHB gene likely functions as the tumor suppressor, researchers have noted that levels of the PHB protein are elevated in cervical, esophageal, stomach, breast, lung, bladder, thyroid, ovarian, and prostate cancer cells. Unfortunately, the role of PHB in cancer cell remains controversial. One clue to this relationship may lay in the oncogene PAR1, the protease activated receptor 1. PAR1 is highly expressed in a variety of cell types, including endothelial cells, platelets, monocytes, neurons and cancer cells, and consequently plays important roles in thrombosis, angiogenesis, inflammation and metastasis, and has long been suspected to be involved in the invasive and metastatic process of breast cancer via activation of Mitogen-Activated Protein (MAP) Kinases, whose hyper-expression may be a key process in the metastatic potential of breast cancer. Similarly, PAR1 has been found to be mainly expressed in primary cells and cancer cells, and both PHB and PAR1 have also been implicated in proliferation and metastasis of carcinoma cells.
Unfortunately, despite strong associations of both the protein PHB and PAR1 signaling with cancer cell growth, some aspects of the internalization and degradation of PAR1 remain unclear, especially regarding the relationship between PHB and PAR1 internalization and degradation. Previous reports found that the precise regulatory mechanisms underlying PAR1 signal termination—including internalization and degradation—are critical in the PAR1response present in many physiological and pathological processes, but exactly how PHB may be involved remains a mystery. This gap is made larger by a lack of clear evidence characterizing the different relationship between cancerous cells and either PHB or PAR1. Explaining these relationship in more detail would likely boost efforts to control or even reverse cancerous cell growth.
Noting the great potential in unravelling the relationship between PHB and PAR1, in a previous study, Zhang Yun and his research team at the Kunming Institute of Zoology (KIZ), Chinese Academy of Sciences (CAS) found that found that PHB localized on the platelet membrane and participated in PAR1-mediated human platelet aggregation, implying that PHB may regulate the signaling of PAR1, serving as a previously unknown cofactor of the PAR1-related signaling pathway. Unfortunately, PHB's exact function in PAR1 internalization and degradation was still not entirely unclear. Building on this premise, Zhang’s team selected two nuclear cell lines to model this phenomena. Due to the previous research that implicated PHB and PAR1 in breast cells, they used normal endothelial cells (HUVECs) and breast cancer cells (MDA-MB-231 cells) to observe how PHB affected PAR1. They found that PHB functioned quite differently in the two different cell lines. In the endothelia cells, PHB participated in PAR1-activated internalization, Erk1/2 phosphorylation and PAR1 degradation induced by PAR1-AP. In the breast cancer calls, the regulation of this process was quite different, where PHB did not seem to participate or regulate PAR1 activated internalization or Erk1/2 phosphorylation at all. Instead, the increased expression of PHB in these cells—a phenomena previously noted in many lines of research—actually inhibited PAR1 degradation.
While Zhang’s observations defined a previously unknown role for PHB in the regulation of PAR1 activated internalization and degradation, there may be greater significance in the observed discrepancy in how PHB functions. The reason for different activity in the different cell lines may lay in where PHB is actually being expressed within the cells. Previous studies found that the subcellular localization of PHB affects cell fate, and if accurate, then PHB’s role in the formation and growth of tumors may be explainable by its subcellular localization. In the current study, Zhang’s team found something odd—while PHB was previously found in both normal endothelial cells and cancerous cells, during Zhang’s experiment PHB was not expressed on the surface of the highly invasive MDA-MB-231 breast cancer cells but it was expressed on the endothelial cell’s surfaces, as well as on the surface of lowly invasive MCF-7 breast cancer cells This difference may imply that persistent PAR1 signaling due to the absence of membrane PHB and decreased PAR1 degradation caused by the up-regulation of intracellular PHB in cancer cells (such as MDA-MB-231 cells) could render cells highly invasive.
Though further testing is likely needed to confirm some of Zhang’s observation, if accurate, Zhang’s work may help explain earlier observed association of PAR1, MAPK signaling and metastatic breast cancer. More importantly, these findings may then serve as the foundation for future uses of PHB as a promising target for developing more effective cancer treatments or even in refining cancer diagnostics and prognoses, greatly aiding researchers and clinicians in their efforts to deal with rising rates of cancer and a lack of viable treatment options.
The complete study was recently published in Biochimica et Biophysica Acta (2014): 1393-1401, available online at: http://www.sciencedirect.com/science/article/pii/S0167488914001232
