Third hand smoke (THS) is the accumulation of second hand smoke (SHS) toxins on surfaces in homes, cars, clothing and hair of smokers. It is known that 88M US nonsmokers ≥3 years old living in homes of smokers are exposed to THS toxicants and show blood cotinine levels of ≥0.05 ng/ml, indicating that the toxins are circulating in their circulatory systems. The goal of the present study is to investigate the mechanisms by which THS causes impaired wound healing. We show that mice living under conditions that mimic THS exposure in humans display delayed wound closure, impaired collagen deposition, altered inflammatory response, decreased angiogenesis, microvessels with fibrin cuffs and a highly proteolytic wound environment. Moreover, THS-exposed mouse wounds have high levels of oxidative stress and significantly lower levels of antioxidant activity leading to molecular damage, including protein nitration, lipid peroxidation and DNA damage that contribute to tissue dysfunction. Furthermore, we show that elastase is elevated, suggesting that elastin is degraded and the plasticity of the wound tissue is decreased. Taken together, our results lead us to conclude that THS toxicants delay and impair wound healing by disrupting the sequential processes that lead to normal healing. In addition, the lack of elastin results in loss of wound plasticity, which may be responsible for reopening of wounds.

Wound healing is a complex process that involves sequential phases that overlap in time and space and affect each other dynamically at the gene and protein levels. We previously showed that insulin accelerates wound healing by stimulating faster and regenerative healing. One of the processes that insulin stimulates is an increase in monocyte/macrophage chemotaxis. In this study, we performed experiments in vivo and in vitro to elucidate the signaling transduction pathways that are involved in insulin-induced monocyte/macrophage chemotaxis. We found that insulin stimulates THP-1 cell chemotaxis in a dose-dependent and insulin receptor-dependent manner. We also show that the kinases PI3K-Akt, SPAK/JNK, and p38 MAPK are key molecules in the insulin-induced signaling pathways that lead to chemoattraction of the THP-1 cell. Furthermore, both PI3K-Akt and SPAK/JNK signaling involve Rac1 activation, an important molecule in regulating cell motility. Indeed, topical application of Rac1 inhibitor at an early stage during the healing process caused delayed and impaired healing even in the presence of insulin. These results delineate cell and molecular mechanisms involved in insulin-induced chemotaxis of monocyte/macrophage, cells that are critical for proper healing.