BI 398: Cancer Biology
Spring 2022
Reading Log #3
Please answer the following questions pertaining to the article by Frankowski et al.
(“Metarrestin, a perinucleolar compartment inhibitor, effectively suppresses metastasis”) and
submit them as an attachment in Blackboard prior to the start of class on Wednesday, April 20.
Please do your best to answer all questions IN YOUR OWN WORDS, not taking wording
directly from the paper.
1. What is the perinucleolar compartment (PNC)? What protein marker did the authors
use to identify this compartment? Why did the authors choose to focus on PNCs as a
target for inhibiting metastasis? The PNC is a subnuclear structure that is enriched in
metastatic cells compared to normal cells. They used the protein PTB as a marker for
PNCs. They hoped that targeting this aspect of metastatic cells would enable discovery
of compounds that would specifically affect the complex process of metastasis and not
more general features of all cancer cells, given that metastasis is responsible for the
vast majority of cancer deaths.
2. The authors’ experimental approach is expressed in this sentence: “Using PNC
reduction as a surrogate metastatic phenotypic marker, we developed a lead compound
upon optimization of hits from a high-content screen.” In your own words, explain what
this sentence means. The authors exposed metastatic cancer cells to a chemical library
and screened the cells by fluorescence microscopy to identify those that had reduced
numbers of PNCs (based on the fluorescence of GFP-tagged PTB). They then created
chemical variants based on the initial compound identified and chose most effective one
as their lead compound (metarrestin).
3. What were the three stages of the screen that identified MLS000556915? 1)
fluorescence-based screening for reduction of PNCs; 2) eliminate compounds that cause
apoptosis, DNA damage, general toxicity or cell cycle arrest; 3) evaluate for the ability to
inhibit invasion.
4. Pick one panel of these three: Fig. 1C, 1D, or 1E; describe how the experiment was
performed and what you can conclude from the data.
1C: Treated various cell lines with metarrestin and screened for reduction of PNCs (using
the GFP-PTB assay) compared to DMSO controls (b/c the metarrestin was dissolved in
DMSO). Can conclude that all of the cell lines showed significant reductions compared to
the controls (P<0.05 in some statistical test not mentioned).
1D: Subjected metastatic prostate and pancreatic cancer cells to an invasion assay (cells
allowed to migrate through Matrigel); compared treatment w/ various concentrations of
metarrestin to DMSO control and saw significant inhibition of invasion w/ increasing
concentrations of metarrestin.
BI 398: Cancer Biology
Spring 2022
1E: Growth assay (% confluence in tissue culture dish) for normal fibroblasts vs.
metastatic cancer cells in the presence of DMSO vs. metarrestin; only the cancer cells were
significantly inhibited in growth, indicating specificity.
5. Refer to specific panels in Figure 2 to support the conclusion that metarrestin inhibits
growth of metastases but not primary tumors.
Fig. 2F shows there was no significant difference in weight of primary tumors in animals
treated with two concentrations of metarrestin vs. DMSO, but it did decrease metastatic
deposits (Fig. 2C&D).
6. What are xenograft mice? What was the point of doing the xenograft experiments
shown in Figure 4?
Xenograft mice are those where human tumor cells are implanted in the animal. They did
these experiments to extend their results to additional cancer models (as the previous
experiments were in genetically engineered cancer models.
7. What is the connection between nucleoli and ribosomes? What is the overall conclusion
from the data shown in Fig. 5?
The nucleolus is structured around the genes that encode rRNA molecules. So transcription
of these genes occurs there, and ribosomal subunits (rRNAs and ribosomal proteins) occurs
in nucleoli. Fig. 5 shows that metarrestin causes morphological changes in nucleoli (based
on electron microscopy studies) and also interferes with synthesis of ribosomal subunits
(based on localization of the ribosomal protein RPL29).
8. Ribosomal RNAs are transcribed by RNA Polymerase I (Pol I) as longer RNAs containing
external transcribed spacers (ETS) and are then cleaved into separate rRNAs (see rRNA
gene structure in Fig. 6E). RPA194 is the major protein subunit of Pol I, and siRNA is a
means for inhibiting expression of particular genes by triggering degradation of their
RNAs. Knowing all this, comment on the significance of the data shown in Fig. 6B and
Fig. 6J.
Fig. 6B shows that transcription of the ETS portion of the rRNA genes in the presence of
metarrestin (as shown by qPCR). Fig. 5J shows that knockdown of the DNA PolI subunit
RPA194 causes disruption of the structure of the nucleoli, similar to what metarrestin does
(as shown in Fig. 5). This allowed the authors to link the effect of metarrestin directly to
ribosome biosynthesis.
9. The authors describe experiments that showed that metarrrestin specifically binds to
eEF1A. What is eEF1A? Together, what main point do the data in Fig. 7 and Fig. 8 make?
eEF1A is a eukaryotic translation elongation factor (therefore involved in protein
synthesis). Fig. 7 shows that metarrestin binds to eEF1A in a specific manner. Other
experiments in this figure show that overexpression of eEF1A can overcome the effects of
metarrestin. Fig. 8 goes on to characterize the effects of reduction of eEF1A expression
BI 398: Cancer Biology
Spring 2022
(using siRNA), and shows the these effects mirror those of metarrestin treatment. Together
these data make a strong case that metarrestin’s effects on metastatic cells is mediated
through eEF1A.
10. What evidence suggests that eEF1A is significant to the process of metastasis? (See
Discussion.) They mention that eEF1A is overexpressed in a number of cancers and that
its overexpression is typically associated with a poor prognosis, which essentially
means that tumors that overexpress it are highly likely to metastasize.