Lipid Phosphate Phosphatases In Skeletal Muscle: Spanning Myogenic Differentiation To Nutritional Stress

Abstract

Lipid phosphate phosphatase 3 (LPP3) has been identified as a key regulator of bioactive lipid signaling in cardiac muscle; however, the regulation and functional roles of its paralogs, including LPP1, LPP2, and LPP3, in skeletal muscle remain largely unexplored. To address this gap, our study aimed to examine gene and protein expression of the three LPP paralogs (LPP1, LPP2, and LPP3) and the role of LPP3 in mitochondrial homeostasis in skeletal muscle under physiological and pathophysiological conditions, specifically high-fat diet (HFD)-induced obesity, streptozotocin/HFD induced type 2 diabetes, ER stress, and exogenous LPA exposure. We used skeletal muscle cell lines (C2C12 and L6), mouse models, and targeted molecular and pharmacological interventions to characterize how these enzymes respond to developmental signals, metabolic challenges, and cellular stress. Our findings reveal that during skeletal muscle cell differentiation, LPP3 and LPP1 display reciprocal expression with LPP3 and LPP1 protein levels decreasing and increasing, respectively. These changes in LPP protein levels appear to be controlled at the post-translational level rather than through transcriptional mechanisms since mRNA levels for LPP1, LPP2, and LPP3 were comparable throughout differentiation. In gastrocnemius muscle from female mice with HFD-induced obesity (DIO) and impaired glucose homeostasis, LPP1 mRNA levels were increased when compared to lean low-fat diet (LFD) fed control mice, an effect that was not observed in male mice. Similarly, LPP3 mRNA levels trended to increase with HFD feeding in female but not male mice, while LPP2 mRNA levels remained unchanged across all groups. At the protein level, LPP3 abundance was influenced by fiber type composition in female mice with reduced LPP3 protein levels in oxidative soleus muscle when compared to glycolytic gastrocnemius muscle. In both male and female mice, HFD feeding did not result in altered LPP3 protein levels when compared to LFD fed mice. Interestingly, in male mice with type 2 diabetes LPP3 protein levels were reduced in glycolytic gastrocnemius, but not oxidative soleus muscle fibers. In differentiated C2C12 cells, induction of endoplasmic reticulum (ER) stress and incubation with exogenous lysophosphatidic acid (LPA), which mimic aspects of metabolic disease following DIO and type 2 diabetes, induced LPP3 protein upregulation. Consistent with prior data from our lab, LPA treatment suppressed mitochondrial respiration in C2C12 cells. Adenoviral LPP3 overexpression increased protein levels of Tfam, a marker of mitochondrial biogenesis, but paradoxically reduced mitochondrial pyruvate-linked respiration in C2C12 cells. Collectively, these findings show that all three LPP paralogs are expressed in murine skeletal muscle and that protein and/or mRNA levels of distinct LPP paralogs are altered during skeletal muscle cell differentiation and with fiber type composition and metabolic disease. Our data also show that LPP3 overexpression can influence mitochondrial homeostasis and respiration in skeletal muscle cells. These data provide a foundation for future studies investigating the role of LPPs in skeletal muscle function and energy metabolism.