Molecular mechanisms of PI3K isoform dependence in carcinogenesis
Author(s)
Advisor
Çizmecioğlu, OnurDate
2020-12Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
PI3K pathway is important for cellular proliferation, survival and metabolism. PTENloss and activating mutations of PIK3CA are most frequently seen PI3K related
alterations in various cancer types. Activating mutations in PIK3CA could render
tumors p110α dependent. Also, upon PTEN-loss, p110β isoform of ClassIA PI3Ks
becomes prominent. Nevertheless, the mode of action of p110 prevalence is still not
clear. Here, we aimed to understand the mechanism of the isoform dependence switch
in PTEN-null cancer types. Firstly, we found that PTEN status had an impact on
isoform prevalence in both MEFs and PC3s. p110α dependence decreased in PTENdepleted MEFs and reciprocally there was a decrease in dependence to p110β in
PTEN-reexpressed PC3 cells. On the other hand, upon modulation of PTEN
expression, there was no complete switch-over in dependence from one ClassIA PI3K
isoform to the other. Interestingly, when p110β overexpression was performed in
PTEN depleted MEFs, cells became less dependent on p110α and more dependent on
p110β for cellular viability. However, p110β overexpression in combination with
PTEN status change did not again induce a complete isoform switch within ClassIA
PI3Ks. To reveal additional modules involved in PI3K isoform prevalence, GSE21543
dataset was analyzed and mRNA levels of AK4, SQLE, CREB3L4 genes were found
to be significantly upregulated in constitutively activated p110β in murine models of
prostate cancer. Among these genes, SQLE,a rate limiting enzyme in cholesterol
synthesis, is highly amplified in various tumor samples according to patient datasets.
Breast and prostate cancers have relatively higher amplification rates of SQLE
compared to other cancer types. The simultaneous incidence of PTEN mutation and
SQLE amplification rate is also frequently observed in breast and prostate cancers.
mRNA levels of cholesterol synthesis genes were upregulated in GSE21543 dataset
when the PI3K pathway was constitutively activated by p110β myristoylation. Also,
the expression levels of the cholesterol synthesis genes were decreased upon PI3K
repression with PTEN re-expression in our qPCR results. Constitutively activated
p110β MEFs and PTEN-null prostate, breast cancer cell lines were found to be
sensitive to inhibitors of rate-limiting enzymes of cholesterol synthesis pathway.
Besides in-line with mRNA levels, SQLE protein levels were decreased in PTEN reexpressed prostate cancer cell lines. On the other hand, SQLE protein levels were
stabilized upon PTEN expression with protease inhibitor treatment which indicates
that PTEN and PI3K activity may affect SQLE transcriptionally as well as posttranslationally. We also demonstrated that tamoxifen therapy responders have lower
survival rate and higher SQLE expression according to patient data. In line with this
data, in cellular models of tamoxifen resistant breast cancer, we have seen elevated
levels of SQLE. All in all, our data emphasizes the critical importance of cholesterol
synthesis pathway as a metabolic effector of the PI3K pathway and we can speculate
that p110β dependence in PTEN-null cancer types might arise as a result of its
excessive activation.