Blocking cancer cells from carrying one of the natural mechanisms employed for recycling intracellular components and proteins blocks tumor growth and metastasis and even leads to the regression of already established tumors, scientists claim.
Researchers at the Albert Einstein College of Medicine have found that the process of chaperone-mediated autophagy (CMA) is upregulated in a wide variety of cancers, irrespective of the level of another recycling process, macroautopagy. Their in vitro and in vivo studies showed that knocking out a protein critical to the binding of targeted substrates to lysosomal membranes disrupted cancer cell homeostasis and led to tumor cell death. When used as an approach to treating existing lung tumors in mice the approach led to rapid tumor progression.
Ana Maria Cuervo, M.D., and colleagues, describe their work in Science Translational Medicine. Their paper is titled “Chaperone-Mediated Autophagy Is Required for Tumor Growth.”
The two most well characterized pathways for autophagy in mammalian cells are macroautophagy and CMA, the authors explain. Macroautophagy delivers proteins and organelles to lysosomes for degradation in a vesicle (autophagosome), whereas in CMA, protein substrates are selectively identified and targeted to lysosomes through the interaction of a cytosolic chaperone protein, hsc70.
Macroautophagy has been shown to act as a tumor suppressor in part by contributing to the maintenance of genome stability, and inhibition of the process promotes tumorigenesis in animal models. However, even in these cells autophagic pathways must still be active to maintain homeostasis, suggesting that cancer cells may be able to switch between different pathways. The researchers therefore devised a series of experiments to see whether CMA was one of the mechanisms that compensates for low macroautophagy in cancer cells.
Their studies indicated that CMA was consistently upregulated in from cells from a range of different primary human tumors, regardless of the level of macroautophagy. Two critical CMA proteins, the chaperone hsc70, and LAMP-2A (a key protein involved in target substrate binding to lysosomes), were in particular upregulated in cancer cells. LAMP-2A is a limiting component of CMA and was in fact found to be up-regulated in 32 of 41 different types of human tumor sample analyzed.
Significantly, the investigators also showed that using an shRNA to block LAMP-2A production and hence block CMA—initially in two different human lung cancer cell lines that display very different macroauthophagic activity—led to significant reductions in the proliferative capacity of tumor cells and higher rates of apoptosis and nonapoptotic cell death under basal culture conditions. The mechanisms behind this appeared to relate to changes in glycolytic and oxidative metabolism, resulting from elevated levels of p53.
Testing the effects of blocking CMA on the tumorigenic capacity of human lung cancer cells transplanted into experimental mice showed that in comparison with control cancer cells, the LAMP-2A knockout cells grew markedly slower, and collected much more cellular debris around their periphery. These effects were evident in the absence of any differences in vascular supply or immune responses between xenograft tumors derived from control cancer cells and LAMP-2A knockout cells.
The team also investigated whether the slower growth of tumors from lung cancer cells with reduced CMA activity might affect metastasis. Labeling studies in mice implanted with either control cancer cells or LAMP-2A knockout cells showed that CMA blockade resulted in up to75% fewer cancer cells circulating at any one time. Histological analyses also confirmed that animals transplanted with LAMP-2A knockout cells developed far fewer spontaneous metastases. In contrast, genetic blockade of macroautophagy reduced the number of circulating cells but didn’t significantly reduce the numbers of lung metastases.
Critically, injections of LAMP-2A shRNA directly into pre-existing and well-established lung tumors in experimental mice led to rapid tumor regression. “Overall, our findings highlight a previously unknown dependence of several cancer cell types on CMA activation, which favors tumorigenesis,” the authors conclude.
“The marked effects of compromised CMA on tumor growth and, in particular, on the size of pre-existing tumors, support the feasibility of manipulating this autophagic pathway to prevent tumorigenesis and favor tumor regression. We propose that CMA could be a druggable target for anticancer therapy and that selective blockade of CMA in some types of cancer cells could be the basis for anticancer therapeutic intervention.
"Keeping in mind that a number of different human tumors were found to have up-regulated CMA, the future development of chemical modifiers capable of blocking CMA may be a broadly useful anti-oncogenic therapy.”
The team is now looking into the development of drugs that mimic the effects of genetic LAMP-2A inhibition and CMA blockade and is also investigating the use of genetic manipulation for treating different types of lung tumors. This approach may be relevant for cancers in which the LAMP-2A shRNA can be delivered directly to the tumor site.
They admit that while blockade of CMA in tumor cells probably can’t be sustained for long periods of time, most nontumor cells will respond to CMA inhibition by upregulating macroautophagy, which will allow cellular survival under basal conditions.