HER2neu and the PI3K/AKT pathway

Simplifying the Comprehension Chasm

“We are glorious accidents of an unpredictable process with no drive to complexity, not the expected results of evolutionary principles that yearn to produce a creature capable of understanding the mode of its own necessary construction.” ~ Stephen Jay Gould

There is a lot of discussion about cellular pathways. Almost every month it seems that there is a new drug marketed that inhibits a specific pathway within the cell and shuts down an errant behavior of that cell within the human body.

Pathways, pathway, pathways...

After all what on earth are they all talking about?

Pathways? What are these, like roads? Well not exactly, they are more like chutes and ladders, I think! The chutes take the information down to the interior of the cell, the nucleus while the ladders climb up the scaffolding to reach the inside part of the cells limit, the cell membrane. A built in feedback loop.

Okay, here is the deal. If you don’t click off now, I’ll tell you a little story:

One day a little mouse happens to run by large piece of Munster cheese. You know the kind that is better in taste than Gouda, has the sprinkle of red on its side and is just heavenly? Yeah, that one! Anyway as you might expect his whiskers erected at the site but he couldn’t smell it, because it was in a wooden container with a glass front.

He ran around with joy for a moment until the hurdle ahead became clear. He sniffed and sniffed and ran some more in encircling joy, but the cheese would not yield its scent, it was behind the “glass curtain.”

What to do?

His name was not Colombo for nothing, so he snooped and saw a tiny hole in the side of the wooden side piece. “There is a start,” he exhaled. He called out to his friend, Ralph who climbed the short table in a hurry at the news. And both of them started chewing away at the hole from the sides. The hole got bigger and bigger as the gnawing and chewing got effusive. Finally they could see inside the container.

Looking in, there was a surprise waiting for them; they found their little friend, Mark. He had already somehow gotten inside, the little trickster! He was inside the container but curled into a ball in the corner, apparently asleep. They called his name quietly and he awakened.

“Hey Mark, how’s it going?” Columbo asked.

“Who, What…?” Mark shook the dream away.

“How’d you get in there?”

“By accident, been here for ages, seems to me.” Is all Mark would say.

“Okay, buddy, we have a plan to get to that cheese. You with us?”

“Of course. I have been waiting for this moment, or the other when the owner lifts the wooden case and swats at me."

Mark described the inside that the container apparently had another chamber within where the cheese was placed. The cheese container was composed of a double chamber of glass. Realizing the difficulty, the two outside devised a method to push a small fork lying by the side in through the hole for the mouse inside to use as leverage to lift the inside glass enclosure.

After hours of work in the dark with all kinds of contraptions at their disposal, including flashlights, and spoons and knives, they were able to use the small fork as a fulcrum and lift the glass container, just enough for the inside mouse to enter the hallowed chamber where the cheese sat in perfect uneaten, rectangular state.

Soon the cheese was gone and the container empty and the three mice lay with their underbellies exposed, unable to move.

That is the P13K/AKT pathway - in the mousy world! For the human world, here is a more deliberate but tedious exercise...

The messy receptors and the signaling pathways that litter the inside of a cell! (from Cell Signaling)

The cell surface has receptors that attract growth molecules. When these molecules attach to the receptor site, they trigger an over expression or dimerization (merging with another of its kind) to help propagate the signal. So having gained the growth molecule...

The dimerization of the Receptor Tyrosine Kinases leads to the activation of the PI3Kinase. The active PI3Kinase migrates to and docks with PIP-2 a serine-threonine kinase with its lipid-layer docking-site on the inner side of the cell membrane. The phosphorylation of the PIP-2, leads to an activated PIP-3 which in turn activates AKT and that circulates around the cell cytoplasm, invoking different mechanisms of action:

PI3K / AKT Pathway

a) AKT binds with BAX, which shuts down apoptosis, (the chute mode) “Akt is a major mediator of cell survival through direct inhibition of pro-apoptotic signals”

b) AKT enhances the protein synthesis through RheB impact the mTOR pathway, which activates S6K that binds to the ribosome and produces messenger RNA to produce the protein for cellular proliferation (the ladder mode). (Inhibitors of mTOR include: Rapamycin, Metformin and Starvation)

c) AKT can merge with FOXO a human tumor suppressor protein and restrict it’s function by enabling cytoplasmic sequestration and thereby allow the tumor or cancer to grow at full speed(The chute mode).

Realize the conundrum?

The mentioned modes of operation are limited to only three, but there are many other mechanisms equally as effective in promoting cellular division and proliferation of the cancer progeny. “The Akt cascade is activated by receptor tyrosine kinases, integrins, B and T cell receptors, cytokine receptors, G protein coupled receptors and other stimuli that induce the production of phosphatidylinositol 3,4,5 triphosphates (PtdIns(3,4,5)P3) by phosphoinositide 3-kinase (PI3K).”

http://youtu.be/ewgLd9N3s-4

So the dimerization (Two pieces coming together) of the RTK is the combined effort of Columbo and Ralph, while AKT is Mark, who is given and uses various different mechanisms to lift the interior glass chamber to get to the cheese. The cheese here is the excess luxury food that the mice eat that fattens them and drives them into an obese food coma and ultimately kills them (maybe via excess Insulin Receptor Site-1 (IRS-1 that is an important mediator in the promotion of the PI3K/AKT pathway, but that is another story). No, no, maybe not! Okay, bringing that out in the open and away from the brackets, let me discuss the importance of the IRS-1 both in the Diabetes pathway and cancer promotion. The risk of high Insulin levels (as in Insulin Resistance) trigger the Insulin Receptor Factors  (IRS1 and IGF) that activate a downstream signal transduction through the various other -not mentioned -pathways into the nucleus of the cell to initiate/promote a potential or actual malignant process.  “There is a link, then, Isn’t there?” you cry. “Yes indeed! Observational studies seem to imply a higher incidence of cancer in diabetics for a reason after all.”

This vile behavior of AKT (Mark) creates mechanisms to shut down the suppressor proteins (FOXO, that acts as a kill-switch for abnormal growth) and/or excite the "promoters"(activating S6K) to perpetuate the cellular growth via the messenger RNA, into the bloated cancer that eventually kills.

But what of the HER-2-neu you ask?

HER2neu protein molecule

The Receptor Tyrosine Kinases are the Epidermal Growth Factor Receptors or EGFR/erb and the HER-2-neu is one of the four EGFRs. "HER-2/neu (erbB-2) encodes an 185-kDa orphan receptor tyrosine kinase that is constitutively active as a dimer and displays potent oncogenic activity when overexpressed." The “HER” stands for Human Epidermal Receptor and the “neu” stands for neural factor (It was discovered in a rodent glioblastoma initially). So the PI3K/AKT pathway is subservient to the HER-2-neu Receptors as one of the many triggers. (Besides HER-2-neu there are also several other receptor sites that exist on the cell surface that can provoke the PI3K/AKT pathway).

Considering Her-2-neu is Colombo and Ralph is HER-4 and these are two brothers in a four children family that got together to go after the cheese.

MAP Kinase pathway

The Her-2-neu protein is managed by the ERBB2 proto-oncogene that resides on Chromosome 17 ~ which is the switch! And we have a few antibodies directed mechanisms against the HER2neu: including Herceptin and Pertuzamab others...

Herceptin: that Blue "Y" thingy on top!

And just in case you think there is only a single version of a PI3Kinase, you’d be off target. There are many isoforms of the PI3Kinases and they are expressed in various degrees. “For example, the expression of p110! which is generally thought to be ubiquitous and uniform in various tissues, appears to be markedly enriched in the non-proliferating tumour regions of ovarian cancer in vivo.” ~ Interesting!

Other mechanisms also modulate the PI3K expression especially the p53 gene/protein. Active over expression of p53 suppresses PI3K activity ~ a good thing!

RAF / MEK pathway

“Indeed, p53 overexpression reduces p110" protein levels, whereas suppression of p53 expression increased p110" mRNA and protein levels. These data are in line with earlier reports of negative regulation of PIK3CA expression by p53.”

Just like p53 can act as an inhibitor there are promoters of the signal excitation and transmission: “Recently, three studies have begun to characterize the PIK3CA promoter, indicating that p110" expression can be positively controlled by FOXO3a (forkhead box O3a) and NF-!B (nuclear factor-kappa B) ( NFkB is one of the inflammasones, molecules that is triggered and is intimately involved with the process of inflammation ~ as all chronic disease, including diabetes and cancer have a large underpinnings of the inflammatory component embedded within).

Additionally Estrodiol has an impact on PI3K expression too, “In several cell types, stimulation with estradiol leads to an induction of PI3K activity, ranging from minutes to hours The immediate increase in PI3K activity upon estradiol stimulation is likely due to a direct interaction between ER! and p85! leading to increased PI3K recruitment to the plasma membrane. Yet, in MCF-7 breast cancer

cells, Akt phosphorylation was increased up to 72 h after estradiol stimulation and is accompanied by an increase in p85 protein expression, suggesting that enhanced p85! expression might result in increased PI3K activity.” This brings us the Estrogen Receptor triggers seen in various cancers, especially breast cancer and others like ovarian cancer, colon cancer, uterine cancer and prostate cancer. All Receptors by the way have a controlling genetic switch behind them. For example the ER receptors are controlled by ESR1 and ESR2 genes found on 6^th and 14^th Chromosomes.

The entire process is an elegant dance between need and desire. Need, after the cell has achieved its growth phase it sends back a feedback via loops to shut down further transmission of any proliferative signals. In the desire version, it is the desire of the growth ligand (attachment) that triggers the receptor to over express itself or dimerize (join with its counterpart) into keep signaling, to the nucleus, unchecked. 

By the way most of these receptors are needed during the early formative years to shape our human destiny. It is the over stimulation that leads to trouble in most cases. For example take p53 (aka The Guardian of the Genome) without this the cellular division if left unchecked leads to untold human misery. (Li Fraumeni Syndrome). The p53 monitors any mismatch occurrences within the swapped codes between mother and daughter cells. Any abnormalities in that DNA code is picked up by p53 and the cell is thrown in the trash heap for recycling.

Under un-stimulated conditions the PI3K expression is under a tight transcriptional control via miRNA. Over stimulation leads to the dysfunctional signal propagation and that leads to a deregulated cellular behavior. (I repeat myself sometime)

Okay! Okay! I didn’t say that the metaphor was that foolproof! But hopefully you got the gist. And here is another disclaimer, there are many other crossroads that link AKT to the nuclear proliferating machinery with many other negative and positive feedback loops to accelerate or decelerate the cellular division ~ that we know not of -yet!

Oh and while I am at it, if one considers the PI3K activator as the leveraging fork, then the treatment paradigm might just nicely fit into our little tale of the tails. Stop making the tiny forks and make more chopsticks (PI3K inhibitors) instead. The thought can be quite moving if you swim in it for a while.

(cartoons employed from articles below, Genentech website, the web and Cell Signalling).

REFERENCES:

Carnero A, Blanco-Aparicio C, Renner O, Link W, Leal JF (2008) The PTEN/PI3K/AKT signaling pathway in cancer, therapeutic implications.  Curr Cancer Drug Targets 8(3), 187–98.

Liu P, Cheng H, Roberts TM, Zhao JJ (2009) Targeting the phosphoinositide 3-kinase pathway in cancer. Nat Rev Drug Discov 8(8), 627–44.

Manning BD, Cantley LC (2007) AKT/PKB signaling: navigating downstream. Cell 129(7), 1261–74.

Yang, N. et al. (2008). Transcriptional regulation of PIK3CA oncogene by NF-kappaB in ovarian cancer microenvironment. PLoS ONE 3, e1758.

Geering, B., Cutillas, P.R., Nock, G., Gharbi, S.I. and Vanhaesebroeck, B. (2007). Class IA phosphoinositide 3-kinases are obligate p85-p110 heterodimers. Proc Natl Acad Sci U S A 104, 7809-14.

Okkenhaug, K. and Vanhaesebroeck, B. (2001). New responsibilities for the PI3K regulatory subunit p85 alpha. Sci STKE 2001, PE1.

Lee, Y.R., Park, J., Yu, H.N., Kim, J.S., Youn, H.J. and Jung, S.H. (2005). Up-regulation of PI3K/Akt signaling by 17beta-estradiol through activation of estrogen receptor-alpha, but not estrogen receptor-beta, and stimulates cell growth in breast cancer cells. Biochem Biophys Res Commun 336, 1221-6.

Hui, R.C. et al. (2008). The forkhead transcription factor FOXO3a increases PI3K/Akt activity in drug-resistant leukaemic cells through induction of PIK3CA expression. Mol Cell Biol, epub

Astanehe, A., Arenillas, D., Wasserman, W.W., Leung, P.C., Dunn, S.E., Davies, B.R., Mills, G.B. and Auersperg, N. (2008). Mechanisms underlying p53 regulation of PIK3CA transcription in ovarian surface epithelium and in ovarian cancer. J Cell Sci 121, 664-74.

Bandyopadhyay, G.K., Yu, J.G., Ofrecio, J. and Olefsky, J.M. (2005). Increased p85/55/50 expression and decreased phosphotidylinositol 3-kinase activity in insulin-resistant human skeletal muscle. Diabetes 54, 2351-9.

Castoria, G. et al. (2001). PI3-kinase in concert with Src promotes the S-phase entry of oestradiol- stimulated MCF-7 cells. Embo J 20, 6050-9.

Simoncini, T., Hafezi-Moghadam, A., Brazil, D.P., Ley, K., Chin, W.W. and Liao, J.K. (2000). Interaction of oestrogen receptor with the regulatory subunit of phosphatidylinositol-3-OH kinase. Nature 407, 538-541.

Guo, C., Sah, J.F., Beard, L., Willson, J.K., Markowitz, S.D. and Guda, K. (2008). The noncoding RNA, miR-126, suppresses the growth of neoplastic cells by targeting phosphatidylinositol 3- kinase signaling and is frequently lost in colon cancers. Genes Chromosomes Cancer

http://www.cellsignal.com/reference/pathway/Akt_PKB.html

http://www.cellsignal.com/reference/tutorials/video_akt.html