The Saccharomyces cerevisiae RhoGAP Rgd1 is phosphorylated by the Aurora B like kinase Ipl1

Jadomycins: A potential chemotherapy for multi-drug resistant metastatic breast cancer

Breast cancer causes the most cancer fatalities in women worldwide. Approximately one-third of breast cancers metastasize, or spread from primary tumors to other tissues, and have a 70% 5-year mortality rate. Current breast cancer treatments like doxorubicin and paclitaxel become ineffective when breast cancer cells develop multi-drug resistance and overexpress ATP-binding cassette transporters, as the transporters cause a substantial efflux of the chemotherapies. Jadomycins, a group of molecules isolated from Streptomyces venezuelae ISP5230, are shown to be cytotoxic against a variety of cancers, especially breast cancer. Furthermore, jadomycins retain their cytotoxic properties in multi-drug resistant breast cancer cells, as they are not expelled through ATP-binding cassette transporters. Here, we describe the research that supports the potential use of jadomycins as a novel chemotherapy in the treatment of multi-drug resistant, metastatic breast cancer. We present the supportive findings, as well as the mechanisms of action investigated thus far. These include copper-mediated reactive oxygen species generation, aurora B kinase inhibition, and topoisomerase IIα and IIβ inhibition. We also suggest future directions of jadomycin research, which will help to determine if jadomycins can be used as a breast cancer chemotherapy in clinical practice.

Role of RhoGAP Rgd1 in Pkc1 signaling-related actin repolarization under heat shock stress in Saccharomyces cerevisiae

BACKGROUND

A serine/threonine kinase Pkc1 is the sole protein kinase C in the budding yeast Saccharomyces cerevisiae, and plays an important role in the regulation of polarized growth and stress responses such as those due to heat shock. Exposure of cells to high temperature transiently arrests polarized growth and leads to depolarization of the actin cytoskeleton, followed by actin repolarization during adaptation to heat shock stress. Actin repolarization is ensured by the activation of Pkc1 signaling; however, the molecular mechanisms underlying this phenomenon remain poorly understood.

METHODS

Using an overexpression construct of a constitutively active mutant of Pkc1 (Pkc1^R398P), we explored the Pkc1 target molecules involved in actin repolarization.

RESULTS

PKC1^R398P overexpression as well as heat shock stress increased the phosphorylation levels of Rho GTPase-activating protein (RhoGAP) Rgd1. Rgd1 was found to contribute to Pkc1-signaling-related actin repolarization during adaptation to heat shock stress in a GAP activity-independent manner, with Ser148 in Rgd1 playing a crucial role. Furthermore, Rgd1 was involved in the maintenance of phosphorylation status of the mitogen-activated protein (MAP) kinase Mpk1, a downstream effector of Pkc1, under heat shock stress.

CONCLUSIONS

Rgd1 is a target of Pkc1 signaling under conditions of heat shock stress, and required for the normal process of actin repolarization during adaptation to heat shock stress.

GENERAL SIGNIFICANCE

Our results provide insights into the molecular mechanism underlying Pkc1-mediated modulation of actin repolarization under heat shock stress.

Positive regulation of Rho GTPase activity by RhoGDIs as a result of their direct interaction with GAPs

Background

Rho GTPases function as molecular switches in many different signaling pathways and control a wide range of cellular processes. Rho GDP-dissociation inhibitors (RhoGDIs) regulate Rho GTPase signaling and can function as both negative and positive regulators. The role of RhoGDIs as negative regulators of Rho GTPase signaling has been extensively investigated; however, little is known about how RhoGDIs act as positive regulators. Furthermore, it is unclear how this opposing role of GDIs influences the Rho GTPase cycle. We constructed ordinary differential equation models of the Rho GTPase cycle in which RhoGDIs inhibit the regulatory activities of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) by interacting with them directly as well as by sequestering the Rho GTPases. Using this model, we analyzed the role of RhoGDIs in Rho GTPase signaling.

Results

The model constructed in this study showed that the functions of GEFs and GAPs are integrated into Rho GTPase signaling through the interactions of these regulators with GDIs, and that the negative role of GDIs is to suppress the overall Rho activity by inhibiting GEFs. Furthermore, the positive role of GDIs is to sustain Rho activation by inhibiting GAPs under certain conditions. The interconversion between transient and sustained Rho activation occurs mainly through changes in the affinities of GDIs to GAPs and the concentrations of GAPs.

Conclusions

RhoGDIs positively regulate Rho GTPase signaling primarily by interacting with GAPs and may participate in the switching between transient and sustained signals of the Rho GTPases. These findings enhance our understanding of the physiological roles of RhoGDIs and Rho GTPase signaling.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0143-5) contains supplementary material, which is available to authorized users.