Tumor-Suppressive micro RNAs versus Oncogenic kinase PAK1

During 2008-2010, a few interesting papers emerged, in which microRNAs are
shown to control genes encoding the oncogenic (Ser/Thr-) kinase PAK1 and
a few other oncogenic (Tyr-) kinases such as ACK, ErbB1 and ErbB2. Among
them, in a therapeutic point of view, microRNA7 /miRNA7 appears to be potentially
the most interesting.

According to Rakesh Kumar's group now at George Washinton University and Xandra
Breakefield's group at MGH as well as a few other groups (1-3), miRNA7 inactivates
at least four oncogenic genes encoding PAK1, ACK, ErbB1 (EGF receptor)
and ErbB2, and suppresses the PAK1-dependent growth of breast cancer, glimoma
and schwannoma (NF2 tumor). A decade ago, we have shown that these
four oncogenic kinases are essential for the growth of RAS cancers (such
as pancreatic and colon cancers) as well as NF tumors. In principle, either
anti-PAK1 drugs or miRNA7 would be useful for systemic treatment or (so-called)
gene therapy of these PAK1-dependent tumors. However, expression of microRNAs
in all cancer cells in each patient would be almost impossible at the present
stage of "gene therapy" technology. Thus, in reality, even microRNA therapy
has to depend on either chemical compounds or natural products which activate
miRNA7 gene for human cancer therapy.

So far propolis extracts, which are rich in either CAPE (caffeic acid phenethyl
ester) or ARC (artepillin C), have been the most promising clinically as
the systemic therapeutics of the PAK1-dependent cancers and NF (neurofibromatosis)
tumors. Among these propolis products, Bio 30 (an NZ propolis) and Brazilian
green propolis (P-300) are world-widely used without any adverse effect.
Interestingly both propolis extracts block the oncogenic kinase PAK1 selectively.

Back to miRNA7, I wonder what could activate this gene. According to Rakesh
Kumar's paper (1), a homeobox protein called HoxD10 is essential for the
full expression of miRNA7. However, HoxD10 is not a cell-permeable small peptide,
and you cannot treat cancer with this protein. Instead, we have to find a chemical
compound or natural product which is able to activate this gene effectively for
cancer therapy.

Well, Lilly Bourguignon's group at UCSF found a clue (4). Hyauronic acid
(HA)-CD44 interaction plays a critical role in the growth of many tumors
including breast cancer and schwannoma. In fact dysfunction of the NF2 gene
product (merlin=PAK1 inhibitor) is known to promote the HA-CD44 interaction.
In both breast cancer and schwannoma, PAK1 is abnormally activated, and
anti-PAK1 drugs such as PP1/PP2, which inactivates SRC family kinases, are
able to suppress the growth of these tumors at least in part. Her group
found that the HA-CD44 interaction activates another micro RNA called miRNA10b,
which is oncogenic, instead of anti-oncogenic, through SRC family kinases.
Interestingly, when this oncogenic HA-CD44-SRC signaling pathway is block
by PP2, HoxD10 gene is activated, while miRNA10b is inactivated. In other
words, PP1/PP2 could be among the HoxD10 gene activators that we are looking
for, and eventually activate the anti-oncogenic miRNA7.

However, in our own previous study, PP1/PP2 alone is not sufficient for
inactivating PAK1 and suppressing the growth of these cancers completely.
We need another Tyr-kinase inhibitor called AG 879 or GL-2003, which blocks
the ETK-PAK1 interaction essential for the full PAK1 activation, along with
PP1/PP2. Thus, I got a feeling that the combination of PP1/PP2 and AG 879/GL-
2003, or propolis extracts such as Bio 30 and P300 could activate fully
miRNA7 gene through HoxD10.

In the future, we might need a new transgenic gene called miRNA7-GFP which
is expressed in a tiny transparent nematode called C. elegans. This gene
is a fusion of miRNA-7 promoter and GFP cDNA. When this promoter is activated,
GFP is expressed and the whole worm glow brightly. So if anti-PAK1 prodcuts
such as CAPE and ARC activate miRNA7, the worm would glow. In our previous
work, we used the CL2070 worm which carries Hsp16.2-GFP fusion gene in which
the promoter of Hsp16.2 gene is linked to GFP cDNA. When this worm is pretreated
with either CAPE or ARC overnight, this worm glows rapidly, shortly after
heat shock, proving that Hsp16.2 gene is inactivated by PAK1. If miRNA7
gene is also inactivated by PAK1, the miRNA7-GFP worm should glow with these
anti-PAK1 products...

Such a GFP worm system would offer a very inexpensive and raid in vivo screening
for anti-cancer drugs such as anti-PAK1 products (=AMPK activators) which
eventually extend the life span by activating the tumor suppressing transcription
factor " FOXO" (like "elixirs") as well.

Interestingly, Razelle Kurzrock's group at MD Anderson CC (5) reported in
2008 that at least miRNA22 gene is activated by curcumin, which like CAPE
and ARC inactivates PAK1, in (PAK1-dependent) human pancreatic cancer cells
(5). Over-expression of miRNA22 suppresses the growth of human cancer cell lines (6).
Thus, it would be worth testing if miRNA7 is also activated by these anti-PAK1 products...


References:

1. Reddy, S. et al (MD Anderson CC). MicroRNA-7, a homeobox D10 target, inhibits
PAK1 and regulates its functions. Cancer Res. 2008: 68, 8195-200.

2. Kefas, B. et al (Univ. of Virginia). microRNA-7 Inhibits the EGF Receptor etc
and Is Down-regulated in Glioblastoma. Cancer Res. 2008, 68, 3566-72.

3. Saydam, O. et al (MGH). miRNA-7 attenuation in schwannoma tumors stimulates
growth by upregulating three oncogenic signaling pathways. Cancer Res.
2010, in press.

4. Bourguignon, L. et al (UCSF). HA-CD44 interaction promotes c-Src-mediated
twist signaling, microRNA-10b expression, etc. J Biol Chem. 2010: 285, 36721-35.

5. Sun, M, et al (MD Anderson CC). Curcumin (diferuloylmethane) alters the
expression profiles of microRNAs in human pancreatic cancer cells.
Mol Cancer Ther. 2008, 7:464-73.

6. Xiong, J. et al (Beijing Univ.). Tumor-suppressive microRNA-22 inhibits the
transcription of E-box-containing c-Myc target genes by silencing c-Myc
binding protein. Oncogene 2010, 29, 4980-8.