Tumors of the pancreas pose a critical problem in eliminating mortality due to cancer in that incidence and morbidity rates for this disease are nearly equal (NCI SEER database, http://seer.cancer.gov/statfacts/html/pancreas.html
). Even with recent technological advances in genomic analysis, the overall relative 5-year survival rate of pancreatic tumors from 1996 to 2004 was 5.1%, and trend analysis of the period from 2003 to 2005 revealed no significant changes in mortality rate (NCI SEER database, http://seer.cancer.gov/statfacts/html/pancreas.html
). Given that the current prognosis for patients with these tumors is dismal, it is vital that we search for novel therapeutics targeting this disease.
In 1997, Lsm1 was identified through subtractive hybridization cloning in pancreatic cancer cells (1
) and was shown to be over-expressed in 87% of pancreatic cancers. Subsequently, its over-expression has been described in 40% of prostate cancers (2
), a subset (15–20%) of breast cancers that are amplified at the 8p11-12 region (3
) and most recently in lung cancers and mesotheliomas (5
). The direct involvement of Lsm1 in carcinogenesis in these tissues has been demonstrated through analyses of Lsm1's effects on growth and anchorage dependence (2
), contact inhibition (2
), autocrine activity (7
) and tumor establishment and metastases (2
). The increase in Lsm1 levels in these tumors is moderate (about 2- to 5-fold) (7
), suggesting that subtle changes in the levels of Lsm1 can affect the growth properties of mammalian cells. Thus, it is important to elucidate the processes affected by LSM1
over-expression in order to provide new targets for therapeutic development against pancreatic and other Lsm1-over-expressing cancers.
Lsm1 over-expression could affect cellular metabolism in several manners. For example, Lsm1 over-expression has been suggested to destabilize certain tumor suppressor transcripts, allowing for carcinogenesis (2
). This model is based on the fact that Lsm1 in yeast and humans assembles with the Lsm2-Lsm7 proteins to form a heteroheptameric Lsm1-7 complex that binds mRNAs, components of the decapping machinery, and promotes mRNA decapping and degradation (10–14
). Alternatively, Lsm1 over-expression might inhibit the function of the related Lsm2-8 complex, wherein the Lsm1 protein is replaced by the Lsm8 protein. The Lsm2-8 complex binds the 3′-end of the U6 snRNA protecting it from degradation and thereby allowing normal rates of pre-mRNA splicing (15–17
). Consistent with Lsm1 over-expression affecting the nuclear Lsm2-8 complex, over-expression of LSM1
in budding yeast increased the cytoplasmic localization of Lsm7p (18
). Hence, over-expression of LSM1
may actually reduce U6 levels and selectively influence splicing, allowing for carcinogenesis.
To understand how Lsm1 over-expression influences cell processes, we took advantage of the conservation of Lsm1 function in both budding yeast and humans to determine how Lsm1 over-expression affects RNA metabolism in yeast. We found that over-expression of LSM1 in the yeast Saccharomyces cerevisiae leads to defects in pre-mRNA splicing, which is caused by decreased levels of the U6 snRNA. The splicing defect causes yeast strains over-expressing Lsm1 to be hypersensitive to loss of other components required for maintaining levels of U6 snRNA. Moreover, yeast strains over-expressing Lsm1 are more susceptible to mutations inhibiting cytoplasmic deadenylation, which is normally a prerequisite for mRNA decay. These results suggest that inhibition of splicing and/or deadenylation may be effective therapies for LSM1-over-expressing tumors.