Left to right: Gary S. Goldberg, PhD, associate professor, molecular biology;
Yongquan Shen, postdoctoral fellow; Raaj Khusial, PhD, postdoctoral fellow, all from UMDNJ-School of Osteopathic Medicine; Xun Li, graduate student, UMDNJ-Graduate School of Biomedical Sciences, Stratford

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hile there have been tremendous advances in cancer research and treatment, the mortality caused by this disease is still a dilemma. Cancer is the second leading cause of death in the U.S. In 2008, more than 1.5 million new cancer cases are expected to be diagnosed and more than half a million people are expected to die from cancer in this country. About 90% of these deaths are due to metastasis, which is the ability of cancer cells to migrate from their tissue of origin and colonize elsewhere in the body. Thus, understanding the mechanisms underlying cancer metastasis is of paramount importance.

Cancer cells must be able to migrate in order to metastasize. Cell migration is a multistep process that results from a number of orchestrated events guided by environmental factors, signal transduction and cytoskeleton rearrangement. These events are vital for embryonic development, cell differentiation, wound repairs and immune responses. Moreover, these very pathways for cell migration are often utilized by cancer cells to facilitate tumor cell migration and metastasis.

As a member of the Goldberg laboratory, I am elucidating biological mechanisms that underlie tumor cell growth and migration. Our goal is to develop strategies that can be used to better detect and treat cancer. This work is illuminating fundamental processes that lead to malignant cell growth and metastasis.

How do cancer cells migrate?
Cancer cells metastasize when they gain the ability to move and invade other tissues. For cancer cells to metastasize, they must break away from the primary tumor and colonize elsewhere in the body. Even though very few cancer cells wander from primary tumors and successfully colonize elsewhere, metastasis still constitutes a major cause of morbidity and mortality in cancer patients. There are many cellular events that enable cancer cells to metastasize. One such important event is the breach in intercellular communication.

Intercellular communication is a vital process that governs development, tissue repair, immune responses and homeostasis. Communication between cells can be achieved either directly by cell-cell contact or indirectly by the release of signaling factors that are transmitted between cells. When intercellular communication fails, a number of diseases, such as cancer, diabetes and developmental abnormalities can result. We are investigating how tyrosine kinases affect intercellular communication in order to promote tumor cell migration and metastasis.

Src is a membrane-linked non-receptor tyrosine kinase that is required for regulating many normal cellular processes such as adhesion, differentiation, cytoskeleton reorganization, cell-cycle progression, and migration. Increased Src kinase expression and activity have been implicated in a variety of human cancers including tumors of the colon, liver, lung, breast, and pancreas. In addition, increased Src kinase activity promotes the anchorage independent cell growth and migration that are required for tumor cell invasion and metastasis. However, Src does not work alone. Src utilizes other proteins to promote tumor progression.

Src utilizes Cas to block gap junctional communication
Gap junctions form intercellular channels that connect the cytoplasm of neighboring cells. These channels are composed of proteins called connexins. Connexin 43 (Cx43) is a prominent connexin that is affected in cancers. Mitogens and activated oncogenes generally decrease gap junctional communication, and restoration of Cx43 expression can normalize the growth and morphology of cancer cells. Thus, Cx43 has been identified as a tumor suppressor. Moreover, Src can directly phosphorylate Cx43 to reduce gap junctional communication. However, phosphorylation of Cx43 may not be sufficient to decrease gap junctional communication between cells. Src utilizes other players to block gap junctional communication.

In addition to Cx43 (and other substrates), Src phosphorylates Cas, an integral member of the focal adhesion complex that is required for the regulation of the actin cytoskeleton. Indeed, in collaboration with Todd Miller at Stony Brook University, the Goldberg lab has described how Src phosphorylates Cas to promote tumor cell migration. We have also described functional relationships between Cas and Cx43 that aid in understanding how gap junctional communication is related to tumor cell growth. For example, in collaboration with the Moreno lab at the University of Utah, we found that Cas associates with Cx43 at intercellular junctions, and that Src requires Cas to suppress gap junctional communication mediated by Cx43 (Fig1a). These studies help explain how gap junctional communication can be suppressed between malignant and metastatic tumor cells. In addition, these studies also revealed a novel role of the Cas focal adhesion linker protein in the gap junction complex.

Src may utilize Slit2/Robo1 to promote tumor cell migration
In collaboration with the Ichikawa group at the National Cancer Center in Tokyo, the Goldberg lab has performed a global and comprehensive analysis to identify genes that Src affects to promote transformed cell migration. These studies have found that Src induces the expression of a diffusible factor called Slit2 in malignant cancer cells. Slit2 binds to a transmembrane receptor called Robo1 that is important for neuronal growth guidance and regulation of cell migration. Indeed, Slit2/Robo1 signaling can promote tumor cell migration and angiogenesis required for tumor progression. Studies have implicated Robo1 in hepatocellular carcinoma and breast cancer, two of the most prevalent cancers worldwide. This is a perfect example of how cancer cells can hijack normal signaling pathways to induce tumor cell migration. My research is geared to elucidate mechanisms that Src utilizes to exploit Robo1 signaling in order to promote tumor cell migration (Fig 1b).

Figure 1: Src utilizes Cas to suppress gap junctional communication and promote tumor cell migration. (a) Src phosphorylates Cas and Cx43, which associate with each other at the plasma membrane. Src and Cas act together to suppress gap junctional communication between transformed cells. (b) Src also utilizes Cas to induce Slit2 expression and augment Robo1 signaling to promote tumor cell migration.

Src is a clinically relevant kinase that has been documented in more than 50% of tumors derived from the colon, liver, lung, breast, and pancreas. Thus, inhibiting Src activity seems like an obvious approach for cancer treatment. Many inhibitors targeting Src kinase are available. However, only a few have reached clinical trials. In general, kinase inhibitors can cause adverse side effects. In addition, most current chemotherapy targets proliferating cells. However, in addition to cancer cells, these reagents also target normal proliferating cells, including those of the immune system and digestive tract. This non-specificity results in drastic side effects in patients. I am elucidating how Src affects specific pathways to induce tumor cell migration. My goal is to develop innovative ways to more specifically target malignant and metastatic cancer cells, while avoiding harm to other normal cells in the body.

Raaj P. Khusial, PhD, is a postdoctoral fellow in Gary Goldberg’s laboratory at UMDNJ-School of Osteopathic Medicine. He received his BS and PhD from Stony Brook University. He is interested in how Src tyrosine kinase affects intercellular communication and utilizes other signaling pathways in order to promote tumor cell migration.