A new Drosophila melanogaster larval model recapitulates key aspects of tumor-induced cachexia, including muscle wasting, loss of tissue integrity and lipid mobilization, the authors of a multicenter Australian study reported in the September 1, 2021, online edition of Developmental Cell.

This model was used to show for the first time that tumors secrete a matrix metalloproteinase 1 (Mmp1) that disrupts extracellular matrix/basement membrane (ECM/BM) proteins and activates transforming growth factor-beta (TGF-beta) signaling in adipose tissue to cause fat and muscle wasting.

Fat-derived factors could therefore relay the metabolic status of adipose tissue to muscles, so in future it may be important to identify such inter-organ communication factors, targeted inhibition of which could be a promising treatment of cachexia by preserving muscle integrity.

Inter-organ communication ensures a synchronized organ response to physiological changes and is particularly relevant to cancer, as tumors cause loss of fat and skeletal muscle, and anorexia.

Cachexia is present in approximately 80% of advanced-stage cancers, and up to one-third of cancer deaths are due to cachexia-related complications, with treatment options being limited.

"The only treatment available for cachexia is dietary intervention, to which patients do not always respond," said study leader Louise Cheng, an associate professor and group leader of the Organogenesis and Cancer Program at Peter MacCallum Cancer Centre in Melbourne.

In addition, early cachexia detection is impeded by the lack of effective biomarkers, with cancer cachexia having been studied mainly in rodent models, in which genetic analyses are problematic.

"It is difficult to dissect out inter-organ communications using rodent models," said Chen, noting "this is time consuming, while genetic studies in flies, particularly Drosophila, are much more sophisticated."

Drosophila

Humans and D. melanogaster share important physiologic regulatory similarities, due to evolutionarily conserved enzymes and related organs regulating carbohydrate and lipid metabolism, making the flies invaluable for studying cachexia.

Moreover, they store excess fat as triglycerides, thus functioning as adipose tissues, while insect secretory cells called oenocytes can function as hepatocytes by mobilizing lipids during starvation.

Fly muscles are also critical mediators of systemic energy homeostasis that coordinate energy supply and utilization, while inducing metabolic changes in distant tissues.

Recently, cachexia models have been developed in Drosophila, in which several tumor-derived factors have been shown to modulate host physiology.

However, tumor signaling regulation driving tissue remodeling remains unclear, as does how remodeled host tissues communicate with one another.

In the new Developmental Cell study, the newly developed larval cachexia model was shown to enable visualization of the onset and progression of the disease, including associated lipid mobilization, muscle detachment, and tissue remodeling.

Taking a ride on the glass bottom boat

"Visualizing events leading to muscle detachment, including fat breakdown and lipid droplet accumulation, demonstrated the importance of fat metabolism in mediating cachexia, suggesting fat breakdown signalling may cause muscle detachment," said Cheng.

Among several cachectic factors, Mmp1 was found to alter availability of the ligand 'glass bottom boat' (Gbb) in the tumor and triggered TGF-beta activation in the fat body, resulting in disrupted BM/ECM proteins at the adipocyte cell-cell adhesion site.

"Initially, we observed TGF-beta activation in fat bodies of tumor-bearing animals, which led to the discovery that tumors secrete Gbb, which may be cleaved by Mmp1 to facilitate its release into the blood stream," explained Chen.

"We then found that TGF-beta caused BM/ECM to breakdown in adipose tissue, which is significant, as ECM disruption in both muscle and fat is seen in cachectic patients," she noted.

The researchers further demonstrated that the Mmp1 and Mmp2 enzymes both acted directly on the fat body and muscle cells to trigger their disruption.

"Mmp1 and 2 can cleave proteins directly besides the role of Mmp1 in the tumor, where it releases Gbb and activates the TGF-beta pathway, while directly cleaving ECM proteins in the fat body and muscle," said Chen.

Although the mechanisms underlying this are unclear, "we showed that direct Mmp1 and 2 inhibition in fat and muscle can prevent muscle detachment in the presence of tumor," she told BioWorld Science.

"This is important, since Mmp1 has previously only been implicated in tumor metastasis, so finding how to prevent cachexia in the presence of a tumor is a major advance in terms of improved cachexia treatment."

Interestingly, disassembly of fat-body cell-cell adhesion in the absence of a tumor was shown to be sufficient to induce muscle detachment, suggesting adipocyte breakdown releases signals culminating in that detachment.

"In the presence of tumor, the fat-body breaks down before muscle detachment, suggesting it may release a signal causing muscle breakdown," Chen said.

While the identity of that signal is unknown, "to show this might be the case, we disrupted fat-body adhesion, which was seen to result in muscle detachment, suggesting fat breakdown alone releases a signal causing muscle detachment."

Importantly, inhibition of TGF-beta signaling or matrix metalloproteinases in the fat body/muscle using QF2-QUAS, a binary system allowing expression of one set of genes in the tumor and a different set concurrently but independently in the fat body, was shown to rescue muscle wasting in the presence of tumor.

Thus, "in the presence of a tumor, we could theoretically prevent cachexia by inhibiting Mmps or TGF-beta in fat or muscle," Chen said.

"If we could use similar strategies to treat cachexia in the clinic, we could perhaps preserve muscle even when tumors are present. This is important, as cachexia is often associated with metastatic disease, so tumor removal may not be an option."

Collectively, these findings provide valuable insights into interactions between tumor, muscle, and adipose tissue during cachexia and establish tumor-derived Mmps as being central mediators of wasting in cancer cachexia.

Regarding treatment implications, "we are finding ways to prevent muscle wasting/detachment in the presence of tumor, with Mmps and TGF-beta being potential therapeutic targets," Chen said.

"Furthermore, finding new diagnostic markers of cachexia remains an unmet clinical need, in which regard Mmps seems to be a central player, as we detected high levels of Mmps in blood in our study, which could be an early indicator of cachexia."