In our previous study using 2D electrophoresis-mass spectrometry, we found that RhoGDI2 was a contributor to 5-FU resistance in colon cancer [
4]. Later on, we showed that RhoGDI2 also confers resistance to 5-FU in gastric cancer cells [
9]. Here we reported a new mechanism by which RhoGDI2 induces multidrug resistance. It is that RhoGDI2 up-regulates P-gp expression via Rac1.
Accumulative evidence show that RhoGDI2 confer multi-drug resistance in ovarian cancer, gastroenterologic cancer [
4-
9], breast cancer [
13]. It was shown that RhoGDI2 was over-expressed in chemo-resistant fibrosarcoma cells and paclitaxel-resistant ovarian cancers, respectively [
18,
19]. Hee et al. [
6] reported that RhoGDI2 confers resistance against multiple chemotherapeutic agents (cisplatin, etoposide, and staurosporin) -induced apoptosis in gastric cancer cells. Together with our results, we conclude that high levels of RhoGDI2 expression are associated with chemotherapy resistance in certain types of cancers, like gastroenterologic cancer.
P-gp, the product of the
ABCB1 (
mdr-1) gene, is a full transporter comprised of 12 transmembrane segments divided into TM domains, each linked with an ATP-binding domain [
20]. It was first identified due to its overexpression in drug-resistant tumor cells, where it functions as a broad range drug transporter, thereby conferring resistance to many important chemotherapeutic agents including vinblastine, doxorubicin, and paclitaxel [
20,
21]. Here, we reported that ectopic expression of RhoGDI2 increased P-gp expression and activity in gastric cancer. Furthermore, in patients with gastric cancer, the expression of RhoGDI2 was also positively associated with P-gp expression in cancer cells. Our previous study showed RhoGDI2 reverted the low dose 5-FU-induced G2/M arrest. Taken all these together, RhoGDI2 confers multi-drug resistance in gastric cancer by multiple mechanisms. Rho GTPase plays important roles in regulation of astral microtubules and the interaction of spindle microtubules with chromosomes during mitosis. It is reported inhibition of Rho GTPase causes a mitotic arrest. Here, we show RhoGDI2 up-regulates P-gp transcription via Rho GTPase (Rac1). Hence it is possible the versatile Rho GTPase contributes to the multiple mechanisms underlining RhoGDI2 induced MDR.
Rac1 was recognized as the important co-operator of RhoGDI2 and mediator of its function [
16]. Unlike other member of Rho GTPase, RhoGDI2 preferentially binds to Rac1 and affects its activity [
16]. A recent study showed RhoC was also regulated by RhoGDI2 [
22]. However, numerous studies have shown the conflicting role of RhoGDI2 in the regulation of Rac1 dependenting on the tumour types and/or cellular microenvironment. it is demonstrated that RhoGDI2 inhibited Rac1 activity in MDA-MB-231 human breast cancer cells [
13] and mouse embryonic fibroblasts [
23], whereas others showed RhoGDI2 acted as a positive regulator of Rac1 in T24 and UMUC3 human bladder cancer cells [
14], H9c2 cardiomyoblast cells [
17] and ovarian cancer. Both Cho et al. and our results demonstrated that RhoGDI2 up-regulated Rac1 activity [
7] in gastric cancer cell lines. The mechanisms by which RhoGDI2 regulates Rac1 aren’t fully understood. Boulter et al described an evolutionarily conserved mechanism by which RhoGDI1 controls the homeostasis of Rho proteins in eukaryotic cells [
24]. They showed that depletion of RhoGDI1 led to destabilization and degradation of unbound RhoGTPases resulting in a reduction of GTPase protein levels, while unexpectedly activating the remaining membrane bound fraction [
24]. It isn’t clear whether RhoGDI2 function in a similar manner as RhoGDI1 to protect Rho family GTPases from degradation. Huang et al [
17] reported in cardiomyoblast cell line H9c2 unlike RhoGDI1, RhoGDI2 up-regulates Rac1 mRNA transcripts, rather than Rac1 protein stability. Our and other studies didn’t show alteration of RhoGDI2 expression could results in Rac1 protein level change in cancer cells [
7,
13,
14]. Therefore it is expected RhoGDI2 might act in a different way to regulate Rac1. Extraction Rho GTPases from the membrane by RHOGDIs is the second mechanism to regulated RHO GTPase activity [
16,
25]. In Breast cancer cells MDA-MB-231, knockdown of RhoGDI2 results in Rac1 translocation from the cytosol to cellular membrane compartments, leading to constitutive Rac1 activation and cell growth inhibition [
13]. In contrast in bladder cancer, RhoGDI2 activated Rac activity without affecting Rac membrane/cytosol ratios [
14]. However, point mutants of RhoGDI2 that increase or decrease the affinity of RhoGDI2 for GTPases abolished its ability to activate Rac [
26]. It indicates there is a mechanism distinct from inhibition of membrane association involved in activation of Rac1 by RhoGDI2. Recently, using system biological method, a model was constructed to reveal how RhoGDIs act as positive regulators of Rho GTPase [
27]. It indicates 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. It need further study to confirm whether and which GAPs are involved in our study.
In conclusion, we suggest that there is a new mechanism by which RhoGDI2 contribute to multi-drug resistance in gastric cancer cells. It may effects drug efflux by up-regulation of P-gp expression via Rac1, rather than disturbance the apoptotic pathway as shown by other group.