HuRrying colon cancer progression

HuR (or HuA) is a member of the Hu/ELAV (embryonic lethal abnormal vision in D. melanogaster) family of RNA-binding proteins.^1–5 Other ELAV family members, such as HuB, HuC and HuD, are expressed primarily in the brain, while the more widely expressed HuR protein is found in spleen, thymus, intestine and reproductive organs.⁶ Increased cytoplasmic expression of HuR has been reported in several epithelial cancers, including those of the breast, ovary and colon.^7–10

In this issue of Cancer Biology & Therapy, Brosens et al show that nuclear HuR protein is present in both normal and neoplastic colonic epithelial tissues from patients with familial adenomatous polyposis (FAP), a genetic syndrome with associated with increased risk of colon adenomas and cancer. Cytoplasmic HuR protein is expressed in only 10% of samples from apparently normal colonic mucosa, but nearly 90% of colon adenocarcinomas, obtained from these patients. A similar trend for cytoplasmic HuR protein expression was found in normal colonic mucosa and sporadic colorectal adenocarcinomas in non-FAP patients. A small, but statistically uncertain, increase in cytoplasmic HuR protein was observed in adenomas from FAP patients, but not in sporadic adenomas. These results suggest the interesting possibility that intracellular trafficking, rather than expression per se, of HuR contributes to the later stages of invasive cancer development in the process of colon carcinogenesis.

The Hu proteins possess three RNA-recognition motifs through which they bind to target mRNA containing AU-rich elements (ARE) and regulate their stability. The AREs contain multiple copies of the sequence AUUUA and are usually located in a 3′untranslated region (UTR) of short-lived mRNAs.¹¹ AREs have been recognized as repressors of gene expression since the presence of these sequences within 3′-UTR accelerates their decay.¹² Recent evidence, however, indicates that certain AREs can function as sites of translation stimulation as well as repression.¹³ This switching function appears to involve recruitment of several factors, including microRNAs.

Nuclear HuR participates in the processing of pre-mRNAs. HuR shuttles between the nucleus and the cytoplasm and is thought to be involved in RNA export from the nucleus. In the cytoplasm, it can stabilize a number of transcripts, as shown in Figure 1. In an effort to understand the HuR trafficking, several protein ligands to HuR have been identified: SETα, SETβ, pp32 and acid protein rich in leucine (APRIL) (reviewed in ref. 12). SETα, SETβ and pp32 had been previously reported as inhibitors of protein phospahtase 2A (PP2A), which can influence a diverse set of cellular functions, including cell cycle progression, DNA replication, transcription, splicing, development and morphogenesis.

Figure 1.

HuR is found in the nucleus of normal and neoplastic cells. HuR can move to the cytoplasm and influence the expression of a variety of proteins, which are discussed in the text and are associated with carcinogenesis of the colon and other tissues. Cytoplasmic HuR acts to increase protein expression, in part, by stabilizing RNAs encoding these proteins. The appearance of cytoplasmic HuR in colonic tissues is predominantly associated with colon adenocarcinomas in both FAP and non-FAP patients. Levels of HuR are much lower in normal colonic mucosa and adenomas from both these patient populations.

Translocation of HuR from the nucleus to the cytoplasm can occur in response to a variety of factors, including DNA damaging agents such as UV-light and actinomycin D, dihydrotestosterone treatment and several proliferative and stress signals.¹⁴ Wang and coworkers have correlated the increased cytoplasmic localization of HuR during late G₁, S and G₂ phases in colorectal carcinoma RKO cells with the stabilization of the ARE-containing mRNAs.¹⁵ Cytoplasmic HuR expression was associated with COX-2 expression in breast and ovarian cancer, colon, stomach, lung, brain and uterine cervical carcinomas.^{6–8,16–19} Thus, HuR may directly promote tumorigenesis through induction of COX-2 and other proteins. Overexpression of HuR has been shown to stabilize many cellular mRNAs, as depicted in Figure 1. These RNAs encode a variety of proteins implicated in tumorigenesis, and include VEGF, endothelial nitric oxide synthase (eNOS), TNF-α, interleukins and the cell cycle regulators p21(WAF1) c-MYC, cyclin A, cyclin B1, c-FOS, nucleophosmin and p53 (reviewed in refs. ^{12, 16, 20} and ²¹). HuR-dependent expression of the p53 tumor suppressor gene is modulated by the AMP-activated protein kinase (AMPK), which has been shown to reduce the cytoplasmic level of HuR.²² It is interesting that loss of p53 on chromosome 17p is also associated with the transition of colon adenomas to carcinomas.²³ These data suggest a possible association between p53 loss and HuR cytoplasmic trafficking.

Under hypoxia-like conditions HuR can enhance the translation of hypoxia-inducible factor 1α (HIF-1α), in part by binding to the HIF-1α5′UTR.²⁴ Since HuR, enhances the stability and translation of mRNAs involved in stress and proliferation, it is gaining significant attention as an important regulator of gene expression during cell division and tumorigenesis. HuR has been shown to stabilize urokinase and urokinase receptor mRNAs and MMP-9.^16,25 Differential effects of cytoplasmic HuR on growth and invasive phenotypes could be due to the loss of certain tumor suppressor genes, such p53 gene or mutations affecting HuR binding to specific RNAs encoding tumor suppressors.

There is significant current interest in monitoring global changes in levels of RNA and proteins in tissues as markers of cancer progression. Analysis of human tissue arrays from healthy and neoplastic tissues obtained from patients with cancers of the breast, colon, lung and ovary showed increased expression of HuR in almost all cancer tissues compared to the normal counterparts.¹⁶ The paper by Brosen et al provides evidence for intracellular trafficking of the HuR protein as another potential marker of colon cancer progression in colon cancers from both FAP and non-FAP patients. The intracellular location of HuR likely then affects the expression of a number of genes, via post-transcriptional mechanisms, that are responsible for the invasive phenotype of CRC. Inhibitors of HuR have been reported that interfere with HuR-RNA binding, HuR trafficking, cytokines expression and T-cell activation.²⁶ These agents could represent a novel class of anticancer drugs.