Trying to explain in a patient- and Peggybrain-friendly way how molecular pathways which go awry and lead to cancer, I kept reading about enzymes and antagonists. With these various genes with their cloning, overexpressions, mutations, and amplifications, and their antagonizing one another into action or inaction, now I antagonized! Go slow on this, marvel at the body’s complexity and remember there is no magic bullet to end cancer. Sorry to all, especially to the newly diagnosed, but this is true.
Molecular Pathways—Or a Network?
The complexity of the dynamic molecular pathways that are essential to our very beings cannot be understated. Researchers are beginning to understand these signaling systems.and no wonder. In a dynamic dance, push cells to divide, to move and to die off, all to support the human organism. When those actions become aberrant, tiny changes can be life-threatening.
“Pathway” is used to explain these interactions in the molecular processing, but it evokes a linear image, direct and orderly. Each chemical reaction may seem to be a stepping stone on that path of cell growth. Missing a step or shifting into another pathway may impact the information sent to the nucleus of the cell. Missteps in this process can lead to unwanted growth, or the path being interrupted completely. But a molecular pathway is anything but simple and predictable.
Pathways may better be described as a string of knots to be loosened or tightened—or both. Each knot is a point where molecular changes may be triggered by chance interactions from outside that string. Those many pathways with their overlapping functions are wadded together, in an intricate spider web. These tangled paths add efficiency as they can create “work-arounds” as needed, supporting the required needs of the system. All such pathways lead, directly or indirectly to the nucleus of cells, and to some function of the cell or larger system.
As those actions cascade down that string, from one knot to the next, they are influenced by other actions and reactions, and can trigger other pathway cascades. The aberrant or misdirected impulses can trigger unintended growth signals, or fail to stop the appropriate death of unnecessary cells (apoptosis=cell death). If something goes wrong, the exquisite and swift balancing act can shift to support a cancer cell. Once that cell has been created, it may evade the inhibiting signals, subvert other processes, create its own support structure and to move to other parts of the body. This may lead to metastases or spread of a cancer to a new site.
One large and complex pathway which can give rise to sporadic tumors and to genetic syndromes is that of the PI3K (phosphoinositide 3-kinase) pathway. Most often it is referred to as the PI3K/AKT/mTOR pathway, a reminder of its wide span of action. It is an especially involved pathway, as per the long name! Studied since the 1980s, the PI3K pathway plays a key role in essential cellular functions. It is fundamentally involved in development of the embryo, and is one of the most commonly activated signaling pathways in cancer.
Mutations can be found in the inherited gene (germline mutations) or sporadically (somatically) as a part of normal growth, aging or environmental causes. Germline mutations make some people more likely to develop a certain cancer, while other people get a similar cancer by sheer chance. By studying germline mutations, researchers gain insight into the sporadic mutation versions of many cancers. Since the PI3K pathway is so fundamental in growth, there is great need to target of this pathway to find relief from the cancer-inducing signals.
Relationship to Receptor Tyrosine Kinases
The PI3K pathway is linked to the large class of Receptor Tyrosine Kinases, (RTKs), and its activation can lead to a wide variety of cancers. The type of those alterations–whether mutations (changes) or amplifications (duplications)—gives rise to different cancers. For example, a mutation of PIK3CA on this pathway is found in 27% of breast cancers, and 17% of urinary tract cancers. Amplifications of that same gene is found in different rates in several lung cancers. Related PIK3CA is found in 53% of squamous cell cancer and just 12% of adenocarcinomas, while the mutation of PIK3CB is expressed in 80% of bladder cancers, and only 5% of breast cancers.
Activation of Pathway
As PI3K becomes activated, whether from PTEN or other growth factors, it subsequently will activate AKT (Protein Kinase B) and then mTOR (mammalian Target of Rapamycin. All play a role in cell proliferation and apoptosis (natural cell death), so any over activation can lead to excessive growth or loss of natural inhibitors. Once cells no longer function under the normal restrictions, they recruit additional growth factors, override immune responses, and proliferate.
Tumor Suppressor PTEN and PI3K
A tumor suppressor PTEN (phosphastase and tensin homolog) can be found on this pathway. This protein is encoded by a gene which is frequently mutated in many cancers. Loss of this tumor suppression activity happens in about 70% of prostate cancers. Coupled with the other alterations in PI3K and its downstream AKT (protein kinase B), the loss of this suppressor can lead to the development, not only of many cancers, but other disorders. Germline (or inherited) mutations in PTEN play a role in, some non-malignant tumors and related syndromes, and possibly some autism spectrum disorders.
Should the PTEN gene mutate and its tumor suppression be limited, changes are triggered along the PI3K pathway. Those mutations can occur in many of the steps along the pathway to the nucleus of the cell. One misstep–an amplification or a mutation–can lead to more such missteps. With those variations, the resulting tumors will have varying incidence of that mutation. An amplification of one element will be found more frequently in certain lung cancers, and rarely in a prostate cancer. Bladder cancer may show overexpression of a related element in 89% of the time, while never exhibit another type of mutation.
All of the elements in this PI3K pathway can contribute to cell proliferation, to cell survival and motility (ability to move) and to angiogenesis (blood vessel development). Agents to target this missteps along the path have been developed, Some act to inhibit in the PI3K subpath, others in the AKT subpath, and several in the mTOR(mammalian target of Rapamycin). These agents are prescribed for cancers as varied as the steps along the pathway.
Therapeutic Agents in Use and Development
The mTOR family of inhibitors includes Temsirolimus (Torisel) and Everolimus (Afinitor), approved for some renal cell, breast and pancreatic cancers. Many others are in development and in trials for a mix of blood and soft tissue tumors.
Upstream from mTOR is the PI3K pathway, so both can be targeted. Currently under study is an inhibitor of the AKT pathway, Perifosine, for the treatment of colorectal cancer and multiple myeloma, in combination with other drugs. Similar drugs are under investigation as they may overcome resistance developed to other drugs.
Genetic Analysis and Treatment Approaches
Multiple genetic alterations these pathways can be found in the tumors or blood of cancer patients. Those alterations may trigger more changes in the primary tumor as it grows. That first kidney tumor can continue to change, following the initial mutation in the first few cancer cells. Billions of cells mutate, evade the immune system response, and respond to the new molecular actions. Thus, new and different cell types may be created. A primary may exhibit certain characteristics, and its metastatic tumors may be quite different. Some tumors may respond to a treatment and nearby tumors will not, as each may have developed in response to different molecular interactions in the same pathway.
It is vital to have a thorough analysis of the tumor’s from several places in the tumor, as well as from any metastases. Only with this can therapeutic agents be chosen to counter the cancer/those cancers. Pathologists may find several different types of cells in one tumor, and in the same tumor find still other unique cells from another tumor sample. The impact of molecular analysis will certainly change treatment, but it must begin with a very sophisticated and thorough gathering of the cellular material deemed to be cancer.