Mycotoxins are the secondary metabolites from different fungi species, belonging to genera of Aspergillus, Fusarium and Penicillium are the most prominent, and they are widespread in the natural environment, while fumonisin B1 (FB1) and ochratoxin A (OTA) are belonging to Fusarium and Aspergillus, respectively [1,2]. These two mycotoxins are worldwide distributed with cytotoxic, immunosuppressive, estrogenic, neurotoxic, mutagenic and cancerogenic effects if contaminated food or feed is ingested [[3], [4], [5]]. Besides, OTA has strong nephrotoxicity and can cause severe kidney damage in pigs and humans [6], and FB1 could cause porcine pulmonary edema [7]. Although FB1 and OTA are widespread in many crops, the content of FB1 in crops such as maize, wheat, and barley is rarely, while the OTA level in crops is often causing toxicosis in animals [[8], [9], [10]], so whether the nontoxic doses of FB1 affect the OTA toxicity when they co-exist in crops is less known.

Several kinds of mycotoxins could cause a dysfunctional effect on mitochondrial respiration, accompanying the drastic structural alteration in mitochondria [[11], [12], [13]]. Mitochondria are increasingly recognized for their contributions to apoptosis, necrosis, inflammation, autophagy and so on, and mtROS is a key factor mediating these processes in epithelial cells in response to both physiological and pathophysiological stressors [[13], [14], [15]]. Mitochondrial membrane damage has also been observed in the toxicity tests in vivo. As we all know, the mitochondria is the critical organelle in cells. It is responsible for the production of cellular energy, but it can also be a source of fatigue and many chronic diseases [12,16]. Some pieces of evidence suggested that mitochondria are extremely important molecular targets for FB1, and it also disturbs mitochondria’ bio-functional and increases mtROS, which affects cell survival [17,18]. One recent study suggested that FB1 exposure could induce oxidative stress of mitochondria and apoptosis in porcine oocytes, thus affecting their maturation in vitro [19]. Other studies have demonstrated that exposure to OTA could disturb mitochondrial function, increase ROS, and cause cell apoptotic and death [[20], [21], [22]]. Besides, OTA exposure would destroy intestinal barrier function and mitochondrial integrity of ducks and decrease their performance [23]. Our previous study also suggested that the c-Jun N-terminal kinase (JNK)/Mitogen-activated protein kinase (MAPK) signaling pathway was activated by FB1 combined with OTA in vitro [24].

Heat shock proteins (HSPs) are serious molecular chaperones and are named as their molecular weight, such as Hsp10, Hsp40, Hsp60, Hsp70 and Hsp90 [25]. Hsp70 is the best-studied and ubiquitous heat shock protein and is low expressed in normal tissues or cells, while a high expression in the early stage of stress protects cells from adverse situations [26,27], but the Hsp70 protein would rapidly decrease after the stress and not enough to resist the damage [25,28]. Numerous findings proved that Hsp70 exhibits cell protection and apoptosis inhibition. Li et al. [29] reported that Hsp70 inhibited apoptosis via suppressing apoptosis downstream of Cyto C release and upstream of Casp3 activation. Another study has shown that Hsp70 and phosphatidylinositol 3-phosphate could protect Plasmodium falciparum from heat-induced cell death [30]. In animals, Hsp70 also displays protective effects. For example, overexpression of Hsp70 increased cardiac resistance to stressors [31]. The up-regulating Hsp70 levels could restrain oxidative damage and nuclear DNA damage, thereby providing protective effects for astrocytes in mice [32]. The inducible Hsp70 chaperone that was overexpressed could suppress neuropathology and improve motor function in SCA1 mice [33]. Hsp70 has a crucial role in conserving and repairing cellular homeostasis or disorders under thermal, I/R (ischemia/reperfusion), and oxidative stress [34,35]. Hsp70, as a chaperone protein, exhibits protective effects in vivo and in vitro. However, what is the relationship between Hsp70 and JNK/MAPK signaling pathway? For example, Hsp90 inhibited the MAPK signaling pathway by suppressing TLR2 and TLR4 in melanoma [36]. And overexpression of Hsp72 protects the liver against a variety of stressful situations by inhibiting the JNK/MAPK signaling pathway in human hepatocellular and Hsp72-overexpressing mice [37]. Otherwise, in a chronic ocular-hypertensive rat model, Hsp72 protects retinal ganglion cells and the lateral geniculate body from neuronal injury by reducing the activation of the SAPK/JNK apoptotic pathway [38].

Until now, the impacts of low doses of FB1 on OTA-induced renal injury and cell apoptosis are less explored. Besides, some articles have shown that Hsp70 plays a vital role in regulating mtROS, keeping cell function and mtDNA integrity [33,39], but whether Hsp70 could remission these damages caused by mycotoxins are still unknown. Therefore, the present study aimed to investigate the impacts of the nontoxic doses of FB1 on OTA-induced nephrotoxicity in vivo and in vitro, and evaluate the importance of Hsp70 in preserving the mitochondrial integrity, resisting cell apoptosis and renal injury induced by co-exposure to FB1 and OTA, further probe the underlying mechanisms. At the same time, we promote the possibility of targeting Hsp70 as a promising therapy for co-exposure to low doses of FB1 and OTA-induced nephrotoxicity.