Tumor-derived cytokines are known to drive the catabolism of host tissues, including skeletal muscle. However, our understanding of the specific cytokines that initiate this process remains incomplete. In the current study, we conducted multiplex analyte profiling of cytokines in conditioned medium (CM) collected from human pancreatic cancer (PC) cells, human tumor-associated stromal (TAS) cells, and their co-culture. Of the factors identified, interleukin-8 (IL-8) is released at high levels from PC cells and PC/TAS co-culture and has previously been associated with low muscle mass in cancer patients. We, therefore, treated C2C12 myotubes with IL-8 which led to the activation of ERK1/2, STAT, and Smad signaling, and induced myotube atrophy. Moreover, the treatment of mice with IL-8 also induced significant muscle wasting, confirming the in vivo relevance of IL-8 on muscle. Mechanistically, IL-8-induced myotube atrophy is inhibited by treatment with the CXCR2 antagonist, SB225002, or by treatment with the ERK1/2 inhibitor, U0126. We further demonstrate that this axis mediates muscle atrophy induced by pancreatic cancer cell CM, as neutralization of IL-8 or treatment with SB225002 or U0126 significantly inhibit CM-induced myotube atrophy. Thus, these data support a key role of IL-8 released from human PC cells in initiating atrophy of muscle cells via CXCR2-ERK1/2.

Skeletal muscle wasting is a devastating consequence of cancer that contributes to increased complications and poor survival, but is not well understood at the molecular level. Herein, we investigated the role of Myocilin (Myoc), a skeletal muscle hypertrophy-promoting protein that we showed is downregulated in multiple mouse models of cancer cachexia. Loss of Myoc alone was sufficient to induce phenotypes identified in mouse models of cancer cachexia, including muscle fiber atrophy, sarcolemmal fragility, and impaired muscle regeneration. By 18 months of age, mice deficient in Myoc showed significant skeletal muscle remodeling, characterized by increased fat and collagen deposition compared with wild-type mice, thus also supporting Myoc as a regulator of muscle quality. In cancer cachexia models, maintaining skeletal muscle expression of Myoc significantly attenuated muscle loss, while mice lacking Myoc showed enhanced muscle wasting. Furthermore, we identified the myocyte enhancer factor 2 C (MEF2C) transcription factor as a key upstream activator of Myoc whose gain of function significantly deterred cancer-induced muscle wasting and dysfunction in a preclinical model of pancreatic ductal adenocarcinoma (PDAC). Finally, compared with noncancer control patients, MYOC was significantly reduced in skeletal muscle of patients with PDAC defined as cachectic and correlated with MEF2c. These data therefore identify disruptions in MEF2c-dependent transcription of Myoc as a novel mechanism of cancer-associated muscle wasting that is similarly disrupted in muscle of patients with cachectic cancer. Significance: This work identifies a novel transcriptional mechanism that mediates skeletal muscle wasting in murine models of cancer cachexia that is disrupted in skeletal muscle of patients with cancer exhibiting cachexia.

Objective: Obesity alters protein metabolism in skeletal muscle, but consistent evidence is lacking. This study compared muscle protein synthesis in adults with obesity and in lean controls in the fasted state and during an amino acid infusion.Methods: Ten subjects with obesity (age: 36 ± 3 years; BMI: 34 ± 1 kg/m²) and ten controls (age: 35 ± 3 years; BMI: 23 ± 1 kg/m²) received an infusion of L‐[2,3,3,4,5,5,5,6,6,6‐²H₁₀]leucine (0.15 μmol/kg fat‐free mass/min) to measure muscle protein synthesis after an overnight fast and during amino acid infusion.Results: Despite greater muscle mammalian target of rapamycin phosphorylation (P ≤ 0.05), fasted‐state mixed‐muscle and mitochondrial protein synthesis were lower in subjects with obesity (P ≤ 0.05). However, the change in mixed‐muscle protein synthesis during the amino acid infusion was 2.7‐fold greater in subjects with obesity (P ≤ 0.05), accompanied by a greater change in S6 kinase‐1 phosphorylation (P ≤ 0.05). The change in mitochondrial protein synthesis did not differ between groups (P > 0.05).Conclusions: Adults with obesity have reduced muscle protein synthesis in the fasted state, but this response is compensated for by a greater change in overall muscle protein synthesis during amino acid infusion.

The addition of aerobic exercise (AE) to a resistance exercise (RE) program (concurrent exercise, CE) can interfere with maximum muscle fiber growth achieved with RE. Further, CE appears to markedly affect the growth of myosin heavy chain (MHC) I, but not MHC IIa fibers. The mechanism responsible for this “interference” is unclear. Satellite cell (SC) responsiveness to exercise appears to influence muscle adaptation but has not yet been examined following acute concurrent exercise. Thus, we assessed the fiber-type-specific SC response to RE, AE, and CE exercise. Eight college-aged males completed the following two exercise trials: the RE trial, which consisted of unilateral leg extensions and presses (4 sets ≥ 10 repetitions: 75% 1 repetition maximum, RM); and the AE/CE trial, which included an identical RE protocol with the opposite leg, immediately followed by subjects cycling for 90 min (60% Wmax). Muscle biopsies were obtained from the vastus lateralis before and 4 days after each session. Samples were cross-sectioned, stained with antibodies against NCAM, Ki-67, and MHC I, counterstained with DAPI, and analyzed for SC density (SC per fiber), SC activation, and fiber type. SC density increased to a greater extent following RE (38 ± 10%), compared with CE (−6 ± 8%). Similarly, MHC I muscle fiber SC density displayed a greater increase following RE (46 ± 14%), compared with AE (−7 ± 17%) and CE (−8 ± 8%). Our data indicate that the SC response to RE is blunted when immediately followed by AE, at least in MHC I muscle fibers, and possibly MHC II fibers. This suggests that the physiological environment evoked by AE might attenuate the eventual addition of myonuclei important for maximum muscle fiber growth and consequent force-producing capacity.

Immune cells can mount desirable anti-cancer immunity. However, some immune cells can support cancer disease progression. The presence of cancer can lead to production of immature myeloid cells from the bone marrow known as myeloid-derived suppressor cells (MDSCs). The immunosuppressive and pro-tumorigenic effects of MDSCs are well understood. Whether MDSCs are involved in promoting cancer cachexia is not well understood. We orthotopically injected the pancreas of mice with KPC cells or PBS. One group of tumor-bearing mice was treated with an anti-Ly6G antibody that depletes granulocytic MDSCs and neutrophils; the other received a control antibody. Anti-Ly6G treatment delayed body mass loss, reduced tibialis anterior (TA) muscle wasting, abolished TA muscle fiber atrophy, reduced diaphragm muscle fiber atrophy of type IIb and IIx fibers, and reduced atrophic gene expression in the TA muscles. Anti-ly6G treatment resulted in greater than 50% Ly6G+ cell depletion efficiency in the tumors and TA muscles. These data show that, in the orthotopic KPC model, anti-Ly6G treatment reduces the number of Ly6G+ cells in the tumor and skeletal muscle and reduces skeletal muscle atrophy. These data implicate Ly6G+ cells, including granulocytic MDSCs and neutrophils, as possible contributors to the development of pancreatic cancer-induced skeletal muscle wasting.